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Astronomy&Astrophysicsmanuscriptno.GRB110709B (cid:13)cESO2013 January28,2013 GRB 110709B in the Induced Gravitational Collapse paradigm A.V.Penacchioni1,3,R.Ruffini1,2,C.L.Bianco1,2,L.Izzo1,M.Muccino1,G.B.Pisani1,3,J.A.Rueda1,2 1 Dip. di Fisica, Sapienza Università di Roma and ICRA, Piazzale Aldo Moro 5, I-00185 Roma, Italy. E-mail: [ruffini;luca.izzo;marco.muccino;bianco;jorge.rueda]@icra.it 2 ICRANet,PiazzaledellaRepubblica10,I-65122Pescara,Italy. 3 UniversitédeNiceSophiaAntipolis,Nice,CEDEX2,GrandChateauParcValrose,E-mail:[email protected] January28,2013 3 1 ABSTRACT 0 2 Context. GRB110709B isthefirstsourceforwhichSwiftBATtriggeredtwice,withatimeseparationof ∼10minutes. Thefirst emission(Episode1)goesfrom40sbeforethefirsttriggerupto60safterit.Thesecondemission(Episode2)goesfrom35sbefore n thesecondtriggerto100safterit. [...] WithintheInduced GravitationalCollapse(IGC)model,weassumetheprogenitortobea a closebinarysystemcomposedofacoreofanevolvedstarandaNeutronStar(NS).TheevolvedstarexplodesasaSupernova(SN) J andejectsmaterialthatispartiallyaccretedbytheNS.WeidentifythisprocesswithEpisode1.TheaccretionprocessbringstheNS 5 overitscriticalmass,thusgravitationallycollapsingtoaBH.ThisprocessleadstotheGRBemission,Episode2.[...] 2 Aims.WeanalyzethespectraandtimevariabilityofEpisode1and2andcomputetherelevantparametersofthebinaryprogenitor andtheastrophysicalparametersbothintheSNandtheGRBphaseintheIGCparadigm. ] Methods.Weperformatime-resolvedspectralanalysisofEpisode1byfittingthespectrumwithablackbody(BB)plusapower-law E (PL)spectralmodel.WeanalyzeEpisode2withintheFireshellmodel,identifyingtheProper-GRB(P-GRB)andsimulatingthelight H curveandspectrum. Weestablishtheredshifttobez= 0.75,followingthephenomenologicalmethodsbyAmati,byYonetokuand . byGrupe,andouranalysisofthelateX-rayafterglow[...]. h Results. WefindforEpisode1atemperatureoftheBBcomponent thatevolveswithtimefollowingabrokenPL,withtheslope p ofthePLatearlyandlatetimestimesα = 0andβ = −4±2, respectively. Thebreakoccursat t = 41.21s. Thetotalenergyof - o Episode1isE(1) =1.42×1053erg.ThetotalenergyofEpisode2isE(2) =2.43×1052erg.WefindattransparencyaLorentzfactor iso iso r Γ∼1.73×102,laboratoryradiusof6.04×1013cm,P-GRBobservedtemperaturekT =12.36keV,baryonloadB=5.7×10−3 st andP-GRBenergyofEP−GRB =3.44×1050erg.[...] P−GRB a Conclusions. Weinterpret GRB 110709B as amember of the IGC sources, together withGRB 970828, GRB 090618 and GRB [ 101023.TheexistenceoftheXRTdataduringthepromptphaseoftheemissionofGRB110709B(Episode2)offersanunprecedented toolforimprovingthediagnosticofGRBsemission. 1 v Keywords. Gamma-rayburst:individual:GRB110709B—Blackholephysics 4 1 0 1. Introduction sion. Finally, a SN emission is either observed or expected in 6 association with the GRB, ∼ 10 days after the burst in the rest . 1Ofalltheastrophysicalprocessescurrentlybeinganalyzed,few frame. We aim to find sources in which the data are of such a 0are more fundamental than the one presenting the coincidence goodqualitytoallowtoseethiscompletesequence. 3of some Gamma-Ray Bursts (GRBs) with the explosion of a 1Supernova (SN). For this, the Induced Gravitational Collapse TheprototypefortheIGCparadigmhasrecentlybeengiven :(IGC) paradigm was first introduced by Ruffinietal. (2001b) in the analysis of GRB 090618 (Izzoetal. 2012), following v iandfurtheranalyzedinRuffinietal.(2007),Ruffinietal.(2008), theworksofRueda&Ruffini(2012)andIzzo,Rueda&Ruffini XRueda&Ruffini(2012) andIzzo,Rueda&Ruffini(2012). Re- (2012). In this work we follow the same line and identifyfour rcently,ithasbeenevidencedthatindeedthisprocesscanexplain different episodes in GRB 110709B. Episode 1 starts 40 s be- a the coincidencebetweenthe SN and GRB emission, bothfrom fore the first trigger and lasts up to 60 s after it and is well fit anobservationalandatheoreticalpointofview(Izzoetal.2012; by a blackbody (BB) plus a power-law (PL) spectral model . Penacchionietal.2012). It corresponds to the trigger of the SN explosion of the com- In the IGC paradigm (Ruffinietal. 2001b, 2007), a binary pact core and its accretion onto the NS companion. We notice systemformedbyanevolvedstarandaNeutronStar(NS)com- that the BB temperature decays with time following a broken panionisconsideredastheprogenitor. power-law(Ryde2004). Episode2starts35sbeforethesecond TheIGCparadigmimpliesawelldeterminedtimesequence. triggerandlastsupto100safterit. Itcorrespondstotheemis- Inaclosebinarysystemofamassivestarinthelatestphasesof sion of the canonical GRB emitted in the formation of a BH. its thermonuclear evolution and a NS companion, the massive Episode 3 starts at 800 s all the way to 106 s. It consists in a star undergoesa SN explosion. The accretion of the early-SN standardX-ray emission identified in all systems following the materialontotheNScompanionleadstheNStoitscriticalmass IGC paradigm (Pisani et al., in preparation). Episode 4 corre- and consequently to its gravitational collapse to form a black spondstotheobservationoftheopticalSNemission,observable hole(BH).TheemissionofacanonicalGRB inthecollapseto afterT =(1+z)T . Inthepresentcase,thereisnoevidence obs SN the BH takes place. A young NS is born out of the SN explo- ofanassociatedSNintheopticalband. Anexplanationforthis Articlenumber,page1of12 isgivenbyZaudereretal.(2012),whoclassifiedGRB110709B 1st trigger as dark and stated that its optical emission may have been ab- 2nd trigger sorbed by the host galaxy and/orthe interstellar medium. The 0.4 ensembleofthesefourepisodescharacterizetheIGCscenario. Asanoutcome,attheendpointoftheIGCscenario,abinary 0.3 sByHste(fmorrmeperdesaeftnetredthbeyGaRNBSe(xfpolromsieodnb)yshthoeulSdNbeexepxlpoescitoend).anda Counts/s 0.2 As in the case of GRB 101023, we do not know the cos- mological redshift of GRB 110709B due to the lack of optical 0.1 data. Therefore, we infer it from phenomenological methods: 1) The Amati relation (Amati 2006), 2) the Yonetoku relation 0 (Yonetoku2004,b),3)theworkofGrupeetal.(2007)and4)the workbyPenacchionietal.(2012),Ruffini(2012)andbyPisani 0 200 400 600 800 1000 Time (s) et al. (in preparation), which describe a scaling of the late X- Fig.1. BAT light curve of GRB 110709B, including both triggers. ray emission of GRB 090618. In the case of GRB 111228, Herewecanappreciatethetimeseparation(about10minutes)between whichwearecurrentlyanalyzing,wefindastrikingcoincidence thefirstandthesecondtrigger. Thelightcurveisinthe(15-150keV) between the values of the cosmologicalredshift determined by energy band. The time is relative to the first trigtime, of 331939966 thesemethodsforGRB110709B. s(inMETseconds). Thesecondtriggerwasat331940612 sinMET In section 2 we report the observations of the two compo- seconds. nents of GRB 110709B by the different instruments, in space andontheground.Insection3wereducetheSwiftdataandper- XRT data of GRB 110709B Cyan: WT setting; Blue: WT; Red: PC formadetailedspectralanalysisofbothEpisode1andEpisode 2. Insection4weinfertheredshiftofthesourceusingthefour s)−1 10−8 phenomenologicalmethods mentioned above. In section 5 we m −2 determinetheradiusoftheemittingregionfromtheknowledge g c 10−9 oftheredshiftandtheBBfluxofthefirstepisode. Insection6 x (er10−10 we give a brief description of the Fireshell model and we per- u form a deeper analysis of Episode 2 within this model, repro- eV fl10−11 ducing the light curve and the spectrum by a numerical simu- 0 k10−12 1 − lation. In section 7 we calculate the parameters of the binary 310−13 0. progenitorleadingtotheIGCoftheNStoaBHbytheSNex- plosion.DetailsontheaccretionrateontotheNS,totalaccreted 3 mass,SNejectadensity,NSmass,andbinaryorbitalperiodare Γ 2 obtainedforselectedvaluesoftheSNprogenitormass. Insec- tion 8 we comment on the radio emission detected by EVLA 1 (Zauderer&Berger2012). In section 9 we presentthe conclu- 100 1000 104 105 106 107 sions. Time since BAT trigger (s) Fig.2. CountlightcurveofGRB110709BobtainedfromtheSwift- 2. ObservationsofGRB110709B XRTdetector,inthe(0.3-10keV)energyband. Thetimeisrelativeto thefirsttrigtime,of331939966s(inMETseconds). GRB110709BhasbeendetectedbySuzaku(Ohmorietal.2011) and Swift (Cummingsetal. 2011) satellites, and by ground- based telescopeslike GROND(Updikeetal. 2011) and Gemini JHK, reveal two point sources within the 5” .3 XRT error cir- (Berger2011). cle reportedbyCummingsetal. (2011). Theysuggestthatone TheBurstAlertTelescope(BAT)onboardSwifttriggereda ofthemcouldbeanafterglowcandidateforGRB110709B,al- firsttimeat21:32:39UT(triggerN◦=456967). Thelocationof thoughitisveryfaint. thiseventisRA=164.6552,DEC=-23.4550.Thelightcurveis It has been suggested by Zaudereretal. (2012) that this composedofmultiplepeaks,withthewholeemissionextending sourceisan“opticallydark"GRB.Thepossiblereasonsforthis up to 60 s after the trigger (see Fig. 1). What is most inter- are: 1) dust obscuration, 2) intrinsically dim event, and/or 3) esting is that there was anothertriggerat 21:43:25UT (trigger highredshift(opticalemissionsuppressedbyLyαabsorptionat N◦=456969),∼11minutesafterthefirsttrigger. Theon-board λ ≤ 1216Å(1+z)). However, they rule out the possibility obs calculated location is RA=164.647, DEC=-23.464. This time ofa highredshifteventduetotheassociationwithanoptically Swift did not need to slew, because it was already pointing to detectedhostgalaxy. Furthermore,theyhave inferredthe opti- that position. This second emission shows a bump that begins cal brightness of the afterglow according to the standard after- 100sbeforethe secondtriggerandlastsaround50s, followed glow synchrotron model (Granot&Sari 2002; Sarietal. 1999), byseveraloverlappingpeakswithatotaldurationofabout40s, andfromthenon-detectionintheoptical-NIRwavelengthsthey andanotherisolatedpeakof10sofduration,200safterthesec- findaverylargerestframeextinctionforGRB 110709B.This ondtrigger. Fig. 1showsthecompleteBATlightcurveandFig. canexplainthelackofdetectionsintheopticalband. 2showsthelightcurvetakenbytheX-RayTelescope(XRT)in Therehavebeendetectionsintheradiobandonseveraloc- the0.3-10keVband. casionsbyEVLA(Zauderer&Berger2012),revealingasingle Therehavenotbeendetectionsinthe opticalbandby Swift unresolvedradio source within the XRT errorcircle, which re- UVOT,whichstartedtoobserve70safterthefirstBAT trigger brightened by a factor of 1.6 between 2.1 and 7 days after the (Holland2011). TheobservationswithGRONDatLaSillaOb- burst. The locationof the source is RA=10:58:37.114,DEC=- servatory (Updikeetal. 2011) simultaneously in the g’ r’ i ’z’ 23:27:16.760. Articlenumber,page2of12 A.V.Penacchionietal.:GRB110709B 0.4 GRB 110709B light curve (1st trigger) m s keV)−2−1−1 100 Unfolded Spectrum 0.3 keV (Photons c2 50 Counts/s 00..12 malized counts s keV cm−1−1−2 2100−3 0 nor 0 -40 -20 0 20 40 60 −10−3 Time (s) 20 50 100 Fig.3. CountlightcurveofEpisode1ofGRB110709Bobtainedfrom Energy (keV) theSwift-BATdetector,inthe(15-150keV)energyband. Thetimeis 3−Dec−2012 20:11 relativetothefirsttrigtime,of331939966s(inMETseconds). Fig.4. Fit of Episode 1 witha BB+PLmodel. The parameters of thefitare: kT = (22±5)keV,BBAmp= 0.2±0.1,γ =1.4±0.1,PL Amp=2.2±0.8,Red-χ2 =1.049(54DOF). 3. Dataanalysis In the following we refer to the emission that goes from 40 s beforethefirstBATtriggerto60safteritasEpisode1(seeFig. 3). Wecalltheemissiongoingfrom35sbeforethesecondBAT 20 triggerto100safteritasEpisode2. WemakeuseofSwiftBAT datatoperformthespectralanalysiswithXSPEC. 10 D V e k 3.1.Episode1 @ 5 T k We perform a time-integrated analysis to the whole Episode 1, using five different spectral models, namely BB, Band 2 (Bandetal.1993),BB+PL,PLandCutoffPL.Theresultsofthe fitsareshowninTable1andinFig. 4. TheBandfunctionisnot 1 well constrained,so we haveexcludeditin the followinganal- 1 2 5 10 20 50 ysis. A statistical test shows that the best models are BB+PL t @sD obs (χ2 = 56.65)and CutoffPL (χ2 = 54.45). Since the difference in the χ2 between these two models is 2.2, the two models are Fig.5. EvolutionofthekTcomponent oftheBB+PLmodelduring Episode1. Thefirstdatapointcorrespondsto5sbeforethefirstBAT statistically equivalent. So we discriminate between these two trigger. Thevertical linecorresponds tothetriggertime. Thetimeis models based on the physical grounds expected from the IGC in the observer frame. The temperature evolves in time following a scenario. In this scenario, we expect a thermal emission from brokenpower-lawfit. Thereisabreakat t = 41.21s. Theindicesof the expansion of the outer layers of the compact core SN pro- thePLareα=0(consistentwithaconstantfunction)andβ=−4±2, genitor. Thus,wehavechosentheBB+PLmodel. We obtaina respectively.ThepresenceoftheBB,althoughsmallerthaninprevious BB temperature kT = (22±5) keV, a PL index γ = 1.4±0.1 cases,isessentialtohaveanacceptablefit. and a χ2 = 56.65 (54 DOF). The flux of the BB component is ∼ 12% of the total flux. The total energy of Episode 1 is EEp1 =1.42×1053erg. TheresultsofthefitareshowninTable second trigger. We tried to fit the spectrum with the following iso 1.Thenweperformatime-resolvedspectralanalysiswithabin- spectralmodels: BB, PL, BB+PL, cutoffPL and Band (see Ta- ningof5sfittingthesamemodelandfindthatthetemperature ble2). WecaneasilydiscardtheBBandBandmodelsbecause oftheBBcomponentfollowsabrokenpower-law,asmentioned in one case the Red χ2 is too high and in the other case there in Ryde (2004), from5 s beforethe triggerto 55 s afterit (see is an unconstrained parameter. As the PL and the BB+PL are Fig. 5). The broken power-law is indeed a constant function nestedmodels,we performeda statistical testto see whichone plusasimplepower-lawfunction. Thisisthesamebehavioras isthebest. We obtainedaprobabilityProb=0.001thatthesim- forthepreviouslyanalyzedGRB 090618(Izzoetal. 2012)and plermodelisbetter,sotheBB+PLdominatesoverthePL.Then GRB101023(Penacchionietal.2012).However,wenoticethat wehavetocomparebetweentheBB+PLandtheCutoffPLmod- the temperaturesfor this GRB are lower. Nevertheless, the si- els. As they are notnested, we cannotapplythe same test. So multaneouspresenceofaBBandPLcomponentisnecessaryin wechosethemodelthatgivesthelowestχ2. Weconcludedthat ordertoobtainanacceptablefitofthedata(seeFig. 4). themodelthatbestfitsEpisode2isthecutoffPLmodel. It is clear from the analogies with GRB 090618 and GRB 101023thatEpisode2 hasallthe characteristicsofacanonical 3.2.Episode2 GRB.AdifferencebetweenGRB110790Bandthealreadyana- Wealsoperformedatime-integratedspectralanalysisofEpisode lyzedonesisthattheseparationbetweenEpisode1andEpisode 2, whose lightcurveisshownin Fig. 7. Thisepisodestarts35 2, ∼ 10 min, is much biggerthan previously, ∼ 50 s. This re- s before the second triggerand last 135 s, until 100 s after the markable time separation between the two episodes is an ad- Articlenumber,page3of12 Table1.FitresultsofEpisode1withfivespectralmodels:BB,Band,BB+PL,PLandCutoffPL.Thefluxisintheenergyband(15-150)keV,in unitsoferg/cm2/s. BB Band BB+PL PL CutoffPL kT =18.9±0.2 α=−1.2±0.1 kT =22±5 γ=1.37±0.02 γ=1.1±0.1 K =0.95±0.01 β=unconstr. K =0.2±0.1 K =2.0±0.2 E =196±68 BB BB PO 0 E =296±255 γ=1.4±0.1 K =0.8±0.2 0 K =2.2±0.8 PO Redχ2 =7.3 Redχ2 =1.031 Redχ2 =1.049 Redχ2 =1.14 Redχ2 =0.99 56DOF 54DOF 54DOF 56DOF 55DOF Flux=7.52×10−8 Flux=8.99×10−8 Flux=8.96×10−8 Flux=9.08×10−8 Flux=8.93×10−8 m s keV)−2−1−1 Unfolded Spectrum 4W.1e.fiNrsHtctroieludmtongdeetnasnityupperlimit for z following the work of keV (Photons c2 50 Gsgoirvrupepntieobenytca∆oll.Nu(Hm20n=0d7eN)n.Hs,iTftiyth−einyNecxHoc,ngeasslisdaoenfrdtahthereeglaraeltaidocstnhicibfcteotzwl,utemhernnoduthegnehsaitthbye-, equation m−2 20 malized counts s keV c−1−1 140×10−3 Lloagb(W1S+eurczva)el<cyu1wla.3eteb−dsiN0te.H51,[gblaolygfr(eo1nm+tert∆ihnNegHrat)hd].eiocmooaprdoinfattheesgoaflatxhyeiGnRt(h1Be) nor 2×10−3 (RA=164.64,DEC=-23.46). We obtained N = 10.5×1020 H,gal 0 cm−2. To obtain the value of N we took the XRT data from 20 50 100 H,fit Energy (keV) 2000 s to 106 s after the first BAT trigger and fitted the model 3−Dec−2012 20:26 phabs*po using the program XSPEC. The XRT data were re- Fig.6. FitofEpisode2withaCutoffplmodel. Thephotonindexis ducedbythexrtpipelinesoftware,version0.10.4,whichis part γ=1.0±0.1,thenormalizationconstantis0.5±0.1,thecutoffenergy isE =132±31andthereducedchisquaredofthefitisRed-χ2 =0.77 of the HEASOFT package, version 6.12. We use the standard 0 (55DOF). response matrix swxpc0to12s6_20010101v013.rmf for the PC mode data. The model phabs represents the photoelectric ab- sorption 0.5 GRB110709B light curve (2nd trigger) M(E)=e−nHσ(E), (2) 0.4 where n is the equivalent hydrogen column density (in units H of 1022 cm−2) and σ(E) is the photoelectric cross section, not 0.3 includingThompsonscattering. WeobtainedavalueofNH,fit = Counts/s 0.2 u7p1p.7e6rl×im1i0t2f0orcmth−e2r.edUshsiinftgofthzes<e1v.a3l5u.es in (1) we obtained an 4.2.AmatiRelation 0.1 WealsotriedtodeterminetheredshiftofEpisode2throughthe 0 Amati relation (Amati 2006), that relates the isotropic energy E oftheGRBtothepeakEnergyintherestframe E ofthe -100 -50 0 50 100 150 200 250 iso p,i Time (s) νF spectrum(Amatietal. 2009). Theanalyticalexpressionof ν Fig.7. CountlightcurveofEpisode2ofGRB110709Bobtainedfrom Eisois theSwift-BATdetector,inthe(15-150keV)energyband. Thetimeis relativetothesecondtrigtime,of331940612s(inMETseconds). 4πd2 E = L S , (3) iso (1+z) bol ditional new fact to propose a different astrophysical origin of whered2 istheluminositydistance,zistheredshiftandS is thesetwocomponents. L bol thebolometricfluence,relatedtotheobservedfluenceinagiven We turn nowto the crucialanalysis of the determinationof detectionband(E ,E )by thecosmologicalredshiftofEpisode2. min max 104/(1+z) Eφ(E)dE 4. Cosmologicalredshiftdetermination Sbol =SobsR1/(1E+mza)x . (4) Eφ(E)dE We used four phenomenologicalmethods to constrain the red- Emin R shiftofthesource,basedondifferentrelations,detailedbelow. 1 http://www.astro.uni-bonn.de/∼webaiub/english/tools_labsurvey.php Articlenumber,page4of12 A.V.Penacchionietal.:GRB110709B Table2.SpectralfitofthewholeEpisode2withdifferentmodels: BB,PL,BB+PL,cutoffPLandBand. Thefluxcorrespondstothe(15-150) keVenergyrange. ThemodelthatbestfitsthedataisthecutoffPL.WecaneasilyseethattheBBandBandmodelsarenotgood. AsthePLand BB+PLarenestedmodels,weperformedastatisticaltesttoseewhichwasthebestone. WeobtainedaprobabilityProb=0.001,indicatingthat theBB+PLdominatesoverthePLmodel.Then,betweenBB+PLandCutoffPL(thatarenotnested,thenwecannotapplythesametest),wetook theCutoffPLasthebestmodelbecauseitgivesthelowestχ2. NotethatthemodelswithwhichwefitthedataarethosedefinedintheXSPEC’s Manual:http://heasarc.nasa.gov/xanadu/xspec/xspec11/manual/manual.html BB PL BB+PL CUTOFFPL BAND kT [keV]=17.5±0.2 γ=1.46±0.02 kT [keV]=20±3 γ=1.0±0.1 α=−1.0±0.1 K =0.661±0.009 K =2.1±0.2 K =0.16±0.06 E =132±31 β=unc BB PO BB 0 γ=1.5±0.1 K =0.5±0.1 E =142±42 0 K =2.3±0.8 K =0.0048±0.0008 PO Redχ2 =7.16 Redχ2 =1.109 Redχ2 =0.78 Redχ2 =0.77 Redχ2 =0.79 DOF=56 DOF=56 DOF=54 DOF=55 DOF=54 Flux=5.2×10−8 Flux=6.52×10−8 Flux=6.35×10−8 Flux=2.43×10−8 Flux=6.36×10−8 Here, φ is the spectral model considered for the spectral data fit; in this case a Band model (Bandetal. 1993), composed of 700 twosmoothlyconnectedpower-laws. E isrelatedtothepeak TeIEtnhpce,eiarg=npyebEaEekppw(e1irnni+etttrehzgn)ey.aoissbstehreveernefrragmyeatbtyhe peapk,i ofthe νFν spectru(m5). @DEp,ikeV 530000 XXXXXXXXXXXXXXXXXXXXXXXXXX 200 X E =E (2+α), p 0 150 where E is the energy at which the two power-laws intersect 0 100 andαistheslopeofthelow-energypower-law,accordingtothe 1051 1052 1053 Bandmodel. Eiso@ergD Wecalculatetheluminositydistanced ,asgivenbythestan- L dardcosmologicalmodel Fig.8. Amatirelation(solidline)withits1−σuncertainties(doted lines) and peak energy E vs. E for GRB 110709B, for different p.i iso valuesoftheredshift,from0.1to3,atstepsof0.1.Wecanseethatthe c z dx datamatchesthetheoreticalfunctionwithin1−σforz>0.4. d = (1+z) , (6) L H0 Z0 Ωm(1+x)3+ΩΛ p around the most intense peak in order to better constrain the wheretheHubbleconstantisH =70kms−1Mpc−1,Ω =0.27, 0 m valueoftheparameters(i.e.,toincreasethenumberofphotons Ω =0.73andcisthespeedoflight. Λ in the spectrum and obtain an error which is smaller than the Following the same procedure as described in valueoftheparameters). Thepeakluminosityintherestframe (Penacchionietal. 2012), we calculated E and E for iso p,i (withtheproperK-correction)canbecalculatedas differentvaluesofz,from0.1to3,atstepsof0.1. Fig. 8shows that the relation is satisfied for values of z > 0.4. This puts a lowerlimittotheestimationoftheredshift. L=4πd2F , (8) L bol where 4.3.YonetokuRelation 10000/(1+z) We finally obtained a range of possible redshifts by using the EN(E)dE Yonetoku relation (Yonetoku 2004). This relation, also known Fbol = PobsR1/(1+zE)max N(E)dE (9) astheE -Luminosityrelation(E -L),connectstheobserved p p Emin isotropicluminosityLinunitsof1052 ergs−1 withthepeaken- R istheenergyfluxandP isthephotonflux. ergy E (1+z) in therestframeofthe GRB. Itisvalidforval- obs p Fig.9showstheYonetokurelation(solidline)withitsuncer- uesof E between50and2000keV,andaluminosityrangeof p tainties(dottedlines),andthevaluesofLandE foreachvalue 1050−1054ergs−1. p,i of z, from 0.1 to 3, at steps of 0.1. We see that the Yonetoku ThebestfitfunctionfortheE -Lrelationis p relation is satisfied within 1σ for values of the redshift > 0.7, consistentwiththeresultsobtainedwiththeAmatirelation. L E (1+z) 2.0±0.2 Inconclusion,ifweputtogetherthethreemethods,wehave =(2.34+2.29)×10−5 p (7) arangeofpossibleredshiftsof0.7<z<1.35. 1052ergs−1 −1.76 " 1keV # The peak luminosity and the peak energyare calculated by 4.4.EstimateoftheredshiftusingtheX-rayafterglow integratingwithina1sintervalaroundthemostintensepeakof the light curve, because this is a better distance indicator than We already presented in (Penacchionietal. 2012) a method to the burst average luminosity. However, we took a 10s interval estimate the redshift of GRB 101023 by comparing its X-ray Articlenumber,page5of12 10 cause GRB 110709B XRT light curve presents many spikes at thebeginning,whichaccordingtoourinterpretationcorrespond nottothesteepdecayoftheX-raylightcurvebuttotheprompt 1 emission. g InthecaseofGRB110709B,thankstothefactthatXRTwas 520er 0.1 alreadyactiveandcollectingdataatthetimeofthesecondBAT 1 (cid:144)L trigger,we were ableto followthebehaviorofthe wholeGRB emission of Episode 2. This is a key point to our understand- 0.01 ing of GRB 110709B,since onlyin veryfew cases XRT had a responseduringtheearlyemission. 0.001 In the big flare at ∼ 1000 s after the first BAT trigger we 100 150 200 300 500 700 1000 Epeak@keVD notice a strongcorrelationbetween the emission in X-raysand in γ-rays. We identifythis emission as the promptemission of Fig.9. Yonetokurelation(solidline)withits1−σuncertainties(doted Episode2. Afterthispromptphasethetraditionalplateauphase lines) and peak Luminosity vs. E for GRB 110709B, for different p.i isobserved.Aftertheplateauphase,thereisthelatedecayphase valuesoftheredshift,from0.1to3,atstepsof0.1.Wecanseethatthe datamatchesthetheoreticalfunctionwithin1−σforz>0.6. in the X-raylightcurvefollowinga power-lawbehaviorwhich has already been observedin othersources(i.e., GRB 101023, GRB 090618, GRB 111228). We study this decay in the IGC light curve to the one of GRB 090618, of known redshift(z = paradigm,andconsiderthepossibilitythatitisproducedbythe 0.54). HerewerescaletheX-raylightcurveofGRB090618as early emission of the newly-bornNS. It is interesting to notice ifitwasseenatdifferentredshiftsandplotittogetherwithGRB that in GRB 110709B the typical flare in X-rays just preced- 110709Blightcurve,lookingforthevaluesofzforwhichthese ingtheplateauphaseandfollowingthe promptemissionisnot lightcurvesoverlapatlatetimes. Wefindaremarkableconsis- observed.ThisX-rayemissionusuallyoccurswithoutanyasso- tency between this methodand the phenomenologicalmethods ciated γ-ray emission, since the data is usually below the BAT alreadymentioned. threshold. In the presentcase, it is conceivablethat the flaring Inodertocompareinacommonrestframethetwoemissions indeedoccurredduringsomeof the gapsof ∼ 4000s in which fromtheGRBs,weapplythefollowingoperationsonlytoGRB thereisnodataduetoEarthoccultation. 090618: We can then distinguish two types of flares in the X-ray 1)determinationofthe startingtime Tstart of thelate decay lightcurve. Thefirsttypeoccursatearlytimes, previousto the emission, steepdecay,andbelongstothepromptemission.Thisflarescan 2)spectralanalysisofthisemissionwithanabsorbedpower- be seen in X-rays only when XRT starts its detection at early lawmodel, enough times, e.g., when the satellite was already pointing at 3)extrapolationofthisspectralmodelinacommoncosmo- a regionnearthe burst position and did notneed much time to logicalrest-frame energyrangeand, consequently,rescaling of slew. The light curve in X-rays generally follows the trend of GRB090618lightcurveforthedifferentenergyranges, the light curve in γ-rays. The second type of flares occurs at 4)cosmologicalcorrectionforthearrivaltimebytakinginto latertimes,justprecedingtheplateauphase.Thisflaresareseen due account the different scaling due to cosmological redshift, only in X-rays since their photon flux is much lower than the and BATthreshold. IntheICGparadigm,weinterpretthisflaresas 5)correctionoftheobservedfluxbychangingtheredshiftof possibleindicatorsofthebreakoutoftheSN. GRB090618. Wearecurrentlyanalyzingmoresourcesinthecatalogueby Adetaileddescriptionofthemethodwillbegiveninaforth- Marguttietal.(2012)tolookforthesethreeverydistinctphases, comingpublication(Pisanietal.,inpreparation). i.e.,theflaresinthepromptemission,theflaresintheafterglow Inthiswaywecomparedirectlybothlightcurvesfordiffer- andthe late decayafterthe plateau, each of themhavinga dif- entredshiftsofGRB090618. Fig. 10showsGRB090618light ferentphysicaloriginwithintheIGCparadigm. curveseenasifthesourcewaslocatedatdifferentredshifts:0.2 (blue),0.4(green),0.7(grey),1.0(orange)and2.0(purple).The 5. Episode1: radiusoftheemittingregion red light curve correspondsto GRB 110709B.We can see that itliesbetweenthegreenandtheorangeones. Amoreaccurate Withtheknowledgeoftheredshiftandtheparametersofthefit scalingofthelateX-rayafterglowsuggestsaredshiftofz=0.75 withaBB+PLmodel,wecomputedtheisotropicenergyofthe forthissource. wholeEpisode1,E(1) =1.42×1053erg. iso Fig. 11showsthesuperpositionofGRB110709BandGRB WiththeenergyfluxoftheBBcomponentφ asafunction BB 090618lightcurvesintheobserverframe,asiftheywerelocated of time from the time-resolved spectral analysis and the lumi- ataredshiftz=0.75. nositydistanced ,wecancomputethevalueoftheradiusofthe L Thereishowevera secondaspectwhichisduetothepecu- emitterincm(wethenexpressitinkminFig. 12)through liarity of the turn-on T of the XRT detector. At the time BAT 0 triggeredforthe secondtime, XRT was alreadypointingat the φ d source and was able to detect the emission at very early times, r = BB L . (10) makingthisGRB probablythefirstforwhichXRThastheear- em rσT4(1+z)2 liestdetectionuptodate. WeneedtoshiftGRB 110709Blight Here φ is the BB flux in units of erg cm−2s−1, σ = curveinordertomaketheearlysteepdecays(originatingin the 5.6704eBrBgcm2s−1K−4 istheStefan-Boltzmannconstantandd promptin ourinterpretation)coincide. This is done by adding L istheluminositydistanceincm. a time T = +800s to GRB 090618lightcurve. The superpo- ∗ Thebestfitoftheexpandingradiusis sition is verygood. In thisway we also makethe early decays coincide. This factoris arbitrary, butwe need to include it be- r(t)=atb, (11) Articlenumber,page6of12 A.V.Penacchionietal.:GRB110709B 150000 10-8 100000 1-D 70000 s 2-m 10-10 DKm 50000 c @ g m er re 30000 @ Flux 10-12 20000 15000 10-14 10000 100 1000 104 105 106 1.0 1.5 2.0 3.0 5.0 7.0 10.0 15.020.0 30.0 time@sD Time@sD Fig.12. Radiusof theemittingregionasafunctionof time(inthe Fig.10. PlotofGRB090618seenasifitwasatdifferentredshifts: cosmological rest frame), corresponding toEpisode1. Theradius in- 0.2(blue), 0.4(green), 0.7(grey), 1.0(orange) and 2.0(purple). The creaseswithtimefollowingapower-lawatb,witha=(1.5±1.2)×104 red light curve corresponds to GRB 110709B. We can see that it lies km/sandb=0.32±0.27. betweenthegreenandtheorangelightcurves,indicatingthatthered- shiftmustbebetween0.4and1.0. The e± plasma reaches thermal equilibrium on a timescale of 10−12 s (Aksenov,Ruffini&Vereshchagin 2007). Being opti- 21--@DFluxergcms 111000---11208OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO O woTticaMenatphirilBlotevlliByncaobnacfptaol=uhlrrttyuiheycbmsMeoktsehn,,uoBiiarcutcctenhhk2tmeyee/(rfiREaprersettuele±oht±alffiets-ae.shprtlmnielhivBsislaoilesatdmscottneieonacudslmlfsc.epr-trpeax1libabacop9seccesa9mheedn9lldeesaeabssir,boysbeatynhn)ttte.hehge(sreaARu±mndl-udffpuatsffii1ehmel0rtonehte−atqioeno2nue,snitibieto-losaaibatbnrlhrai.ylrlneryeio2ytursewn0oemsri0xnnicps0pawpae)amlrl.inaattrahshsmTatimedtohiereitnaeer--. self-acceleratestoultrarelativisticvelocities,finallyreachingthe 10-14 transparencycondition.Aflashofradiationisthenemitted.This 100 1000 104 105 106 istheP-GRB(Ruffinietal.2001a). Theamountofenergyradi- Time@sD ated in the P-GRB is only a fraction of the initial energy Ee±. tot Theremainingenergyis storedin the kinetic energyof the op- Fig.11. GRB090618lightcurve(blue)asifitwasseenatz = 0.75 tically thin baryonic and leptonic matter fireshell that expands togetherwithGRB110709Blightcurve(red).Thereisanexcellentsu- ballistically and starts to slow down due to the inelastic col- perpositioninthelatedecayassumingatemporalshiftofGRB090618 ofT =800stotherighttohavethecoincidenceofthesteepdecaysof lisions with the Circumburst Medium (CBM). This interaction ∗ thelightcurves. gives rise to a multi-wavelength emission, the extended after- glow (Ruffinietal. 2001a). We can estimate the characteristic inhomogeneitiesoftheCBMbyfittingtheluminosityoftheX- wherea=(1.5±1.2)×104kms−bandb=0.32±0.27(seeFig raysourceandimposingthefullyradiativeconditioninthecol- 12). lisionbetweentheultrarelativisticbaryonicshellandtheclouds We associate the BB component to the expansion of the of the Interstellar Medium (ISM). The complete analytic solu- ejected material, while the power-law is associated (as we in- tion has been developed in Bianco&Ruffini (2004, 2005a,b), terpretfromtheIGCparadigm)withtheaccretionofpartofthis together with the analytic expression of the Surfaces of Equal materialontotheNScompanion. arrival Time of the photons at the detector (EQTS). The after- glowpresentsthreedifferentregimes:arisingpart,apeakanda decayingtail.Wethereforedefinea“canonicalGRB”lightcurve 6. AnalysisofEpisode2intheFireshellmodel withtwosharplydifferentcomponents:1)theP-GRBand2)the We recall that the Fireshell model (Damour&Ruffini 1975; extended afterglow. What is usually called “Promptemission” Ruffinietal. 2000; Ruffini 2001; Ruffinietal. 2010b) is an al- inthecurrentGRBliteraturemixestheP-GRBwiththeraising ternativetotheFireballmodel,firstproposedbyCavallo&Rees partandthepeakoftheextendedafterglow(Ruffinietal.2003). (1978), Goodman (1986) and Paczynsky (1986). We assume, Thespectrumoftheextendedafterglowisinitiallyassumedtobe within the Fireshell model, that all GRBs originate from the thermalinthecomovingframeoftheexpandingshell. Recently, gravitational collapse of a star approaching asymptotically the aftertheanalysisofsomehighlyenergeticsourcesobservedby formation of a Kerr-Newmann BH (Wiltshire,Visser&Scott Swift and Fermi satellites, this assumption of a pure comov- 2009). An electric field E is created just outside the collaps- ingthermalspectrumhasbeenrelaxedandaphenomenological ingcoreandinbetweentheexpandingoutershellsthatactas a modificationbyapower-lawofthelowenergyspectralslopehas capacitor(Preparata,Xue&Ruffini 1998). This electrical field been introduced (Patricelietal. 2012). The observed non ther- grows until it reaches a critical value, E = m2c3/~e. At this malspectrumshapeisduetoadoubleconvolutionofthousands c time, vacuum polarization occurs, leading to pair creation at of instantaneouscomovingspectra, with differenttemperatures the expensesof the gravitationalenergy. An optically thick e± and differentDoppler factors, over both the EQTS and the ob- plasmaformswithtotalenergyEe± intherange1049-1054erg. servationtime(Ruffinietal.2004b). tot Articlenumber,page7of12 Table3.FitoftheP-GRBandtheafterglowofGRB110709B,Episode 2.TheP-GRBiswellfitwithaBBmodel,whilethewholeEpisode2is 3.5 SimGuRlBat i1o1n0 o7f0 t9hBe lliigghhtt ccuurrvvee bestfitbyaCutoffPLmodel. Fromthisfitandthevalueoftheredshift 3 weareabletocalculateE andE . iso P−GRB 2.5 PkaTra[mkeeVte]r P1-4G±R1B P-GRB+Afterglow 2Counts/(cm s) 1. 52 BBAmp 0.30±0.02 1 γ 1.03±0.1 PLAmp 0.5±0.1 0.5 Redχ2 1.448(56DOF) 0.77(55DOF) 0 EnergyFlux 2.413×10−8 6.34×10−8 -40 -20 0 20 40 60 80 100 120 Time [s] (15−150keV) [ergcm−2s−1] 1 SSwwiifftt XBRASTTi mssppueelaccttterruudmm co oouffn GGt RRspBBe 11ct11ru00m7700 o99fBB G ((10R5.B3 - -11 1510007 kk0ee9VVB)) Energy[erg] 3.44×1050 2.43×1052 0.1 Havingfixedthevalueoftheredshifttoz=0.75,westarted 2m s keV) tlhoeokaendalfyosristhoefPE-GpiRsoBdedu2riwngiththinefithrestFbiuremspheollfEmpoidsoeld.eW2e(frfiormst Photons/(c 0.01 100 to 40 s before the second trigger) by fitting the data with 0.001 a BB + PL model. We selectedseveraltime intervalsas theP- GRB duringthe firstbumpof Episode2, butinsomecases the fits were notgood. In some othercases, to reproducethe ratio 1 10 100 Energy [keV] between the P-GRB energy and the total energy we needed to consider a baryon load B > 10−2 (which has no sense within Fig.13. Simulationof theBAT(a)light curveand(b) spectrumof Episode2.WeincludedXRTdatainthefitofthespectrumtoshowthat the Fireshell model) and, in other cases, there was a discrep- theslopepredictedinthefireshellmodelisinagreementwiththeslope ancybetweentheobservedtemperatureandtheonegivenbythe oftheX-rayspectrum. simulation. Thusweconcludedthatthisbumpshouldbelongto Episode1. Thereasonwhywedonotfindastrongthermalsig- natureinthisbumpisthatEpisode1starts∼ 10minutesbefore Density mask 100 thebeginningofthebumpandthetemperatureoftheBBcom- ponentdecreasesveryrapidlyfollowingapower-lawinthefirst 10 secondsofemission.Consequently,aftersuchalongtimewedo notWexepeficntatollyfinsdelaenctyedsigthneatuPr-eGoRfBaBasBthfreom9 sEpfriosomde315.to 26 s 3Particles/cm 1 beforethe second trigger,and the followingemission from-26 to100sastheafterglow.Table3showstheparametersofthefit. 0.1 WecalculatedaP-GRBenergyofE =3.44×1050ergand P−GRB anisotropicenergyofE =2.43×1052erg. iso 0.01 We inserted these valuesof the energiesinto ournumerical 1e+16 codeandcalculatedthevalueofthebaryonload,B=5.7×10−3. Radius [cm] Fig.14. ParticledensityoftheISMcloudsasafunctionofthedis- Wesimulatedthelightcurveandthespectrum,obtaining,atthe tance.Themeandensityis76part/cm3. transparencypoint, a LaboratoryRadius r = 6.04×1013 cm, tr agammaLorentzfactorΓ = 1.73×102 andaP-GRBobserved temperature(aftercosmologicalcorrection)kT =12.36keV. by the NS companionat times largerthan t , when the ma- 0,accr Figs. 13aand13bshowthesimulationofthelightcurveand terialreachestheNSgravitationalcaptureregion. Theemission the spectrum of Episode 2, respectively. The photon index of observedin Episode 1 is associated to this early-SN evolution, the XRT and BAT spectra are in agreementwith that predicted identified with the thermal component, and accretion process bythe simulation. Detailsofthiscalculationwillbe givenin a onto the NS, possibly related to the non-thermal component. forthcomingletter (Penacchioniet al., in preparation). Fig. 14 The NS reaches in a time t + ∆t the critical mass and 0,accr accr showsthedensitymaskoftheISM,i.e.,thedensityofparticles gravitationallycollapsestoablackhole,emittingtheGRBseen oftheinterstellarcloudsasafunctionofthedistancetothecen- inEpisode2. Weassumethecriticalmassofanon-rotatingNS terofthe BH. Thisdensityhastobe interpretedasaneffective M =2.67M asgivenbyBelvedereetal.(2012). crit ⊙ densitybecausefragmentationmayoccurintheexpandingshell The amount of material that reaches the NS gravitational (Ruffinietal.2007;Dainottietal.2007). captureregion 2GM (t) R (t)= NS (12) 7. NatureoftheProgenitor cap v2 (t) ej,rel Following the works of Rueda&Ruffini (2012) and per unit time is given by (see Rueda&Ruffini (2012) and Izzo,Rueda&Ruffini (2012), we suggest for the origin of Izzo,Rueda&Ruffini(2012)) GRB 110709B a binary system formed by a massive evolved star on the verge of a SN explosion and a NS. The early-SN materialexpandingatnon-relativisticvelocitiesisthenaccreted M˙(t)=πρ (t)v (t)R2 (t), (13) ej ej,rel cap Articlenumber,page8of12 A.V.Penacchionietal.:GRB110709B whereR ismeasuredfromtheNScenter. mass-radius relation of Belvedereetal. (2012). From the con- cap Intheseexpressions,ρ (t)=3M (t)/(4πr3(t))isthedensity straint given by Eq. (18) we fix the binary separation a. We ej ej ej thenproceedwiththenumericalintegrationoftheaccretionrate oftheejecta, equationsbyrequiringthat M (t)= M att=∆t ,givenby NS crit accr M (t)= M (0)−M(t) (14) Eq.(19),fromwhichweobtaintheefficiencyη . ej ej accr It is interesting to analyze how the NS can accrete such a isthetotalavailablemasstobeaccretedbytheNS,MNS(t)isthe large mass, in some cases of the orderof 47% of the early-SN NSmass,andvej,rel(t)isthevelocityoftheejectarelativetothe material (see column 5 of Table 4), since one could think that NS solid angles of ∼ 50% between the early-SN material and the accreting NS are hard to obtain. Indeed, during the accretion vej,rel(t)= v2orb,NS(t)+v2ej(t). (15) processtheNSismovingwithhighorbitalvelocitiesoftheorder q of 108 cm s−1 relative to the core progenitor(see column 7 of In Eq. (14), Mej(0) is the given initial mass of the ejecta (just Table 4), and consequently travels effective arc-lengths several at the beginningof the accretion process); we choose different timeslargerthanthecircumferenceoftheorbit(seecolumn8of valuesforitinTable4. M(t)isthemassoftheejectathatislost Table4). becauseitpassesthroughthecaptureregionoftheNS. Assumingthatthegainingravitationalenergyoftheaccreted TheactualmassaccretionrateontotheNS,M˙accr(t),isafrac- materialintotheNScanbereleasedfromthesystemleadstoan tionηaccr ≤1ofEq. (13),i.e. upperlimitoftheluminosity M˙accr(t)=ηaccrM˙(t), (16) GM˙ (t)M (t) |E˙ (t)|= accr NS , (20) b where η is the accretionefficiencyonto the NS. So, there is an RNS(t) amount of material per unit time M˙ (t) = (1−η )M˙(t) not out accr accretedbytheNS. wherewetakeintoaccountthedependenceoftheNSradiuswith time,duetotheincrementoftheNSmassbytheaccretionpro- InEq. (15),v (t)= G(M +M (t))/aistheorbital orb,NS prog NS cess. The self-consistentradiusis computedat each time from velocity relative to the SN cpore progenitor, a is the separation themass-radiusrelationofBelvedereetal.(2012). distancebetweentheNSandtheSNcoreprogenitor,and The actual luminosity depends on the efficiency η in rad dr (t) r (t) convertinggravitationalenergy into electromagnetic energy by ej ej v (t)= =b (17) ej somestillunknownprocess. Sinceinourmodelweassumethat dt t the BB componentof Episode 1 is due to the early-SN expan- is the expansion velocity of the early-SN material, where we sion, we estimate the efficiency η from the assumption that rad haveusedrej(t)=rem(t),givenbyEq.(11). |E˙b|isresponsibleforthepower-lawluminosityLPL,namely Wehavealreadymentionedthatthepower-lawcomponentin thespectrumofEpisode1mightbeduetotheaccretionontothe L NS companion. As this power-law componentis presentsince ηrad(t)= |E˙P(Lt)|. (21) thebeginningofEpisode1, we havefixedthevalueoft to b 0,accr beequaltothestartingtimeofEpisode1. Thisputsaconstraint InFig. 15weshowtheevolutionoftheefficiencyη inthe intheseparationdistanceaofthebinary,whichunderthesecon- rad firstsecondsofemissionforthebinarysystemsshowninTable ditionsisgivenby 4. Weassumeaconstantandisotropicpower-lawluminosityof Episode 1, L ≈ 1.8×1050 erg s−1 ≈ 10−4 M s−1, as found PL ⊙ fromthespectralanalysis. Forallthecases,weobtainthesame a=r +R (0), (18) 0 cap evolution of the efficiency with time, i.e. the curves overlap. where r = r (0) and R (0) are the radius of the early-SN Thisisduetothefactthatweconstrainedallthesystemstohave 0 ej cap ejectaandthecaptureradiusoftheNScompanionatthebegin- thesameinitialNSmassand∆taccr. ningofEpisode1. Inthiscase,r ≈1.75×109cm,seeFig. 12. 0 Theseparationaisa functionoftheinitialmassoftheNSand oftheSNcoreprogenitormass,aswellasoftheorbitalvelocity, 8. Radioobservations throughR . ItisclearthattheconstraintgivenbyEq. (18)isa cap Zauderer&Berger (2012) report observations with the EVLA lowerlimit, sincetheaccretionprocessontotheNScouldhave radio telescopes on several occasions between 11 and 16 July, beentriggeredbeforebylayersatlowerdensities(e.g. He). In at a frequency of 5.8 GHz. They found a radio source which suchacase,thebinaryseparationacouldbehigher. brightenedbyaboutafactorof1.6,between2.1and7daysafter Inadditiontothisconstraint,wemusttakeintoaccountthat theburst. ThecoincidencewiththeXRTpositionandtherising the NS must reach its critical mass M at the beginning of crit flux indicate that this is the radio afterglow of GRB 110709B. Episode2,sincebythattimetheNSmustcollapsetoaBHand The position of the source is RA = 10:58:37.114, DEC = - emitthecanonicalGRB.Thisimpliesthat 23:27:16.760. WeshowinFig. 16the5.8GHzlightcurvepre- 611 sentedinZaudereretal.(2012),wherethereisevidenceofara- ∆t ≈ ≈349 s. (19) accr (1+z) diobump.FollowingtheworkofChevalier&Soderberg(2010), wehavereproducedtheplotofthepeakspectralradioLuminos- WeshowinTable4theparametersofthebinarysystemlead- ityperunitfrequencyas a functionoftime(days)atwhichthe ingtoIGCoftheNSinatimeintervalequaltothedurationof peakisproducedfordifferentSNassociatedwithGRBs,includ- Episode1.WeadoptaninitialmassfortheNS,M (0)=1.4M ingGRB110709B(seeFig. 17).Wefindthattheradioemission NS ⊙ and,correspondingly,aNSradiusofR (0)=12.3kmfromthe ofthissourceishigherthantheonesassociatedwithtypicalSN. NS Articlenumber,page9of12 M /M M (0)/M ρ (0)(gcm−3) η ∆M /M (0) P(min) v (0)(kms−1) ∆t /P a/R prog ⊙ ej ⊙ ej accr accr ej orb,NS accr ⊙ 4 2.7 2.39×105 0.92 0.47 0.52 5.24×103 11.14 0.037 5 3.7 3.27×105 0.88 0.34 0.45 5.84×103 12.96 0.036 6 4.7 4.16×105 0.88 0.27 0.39 6.39×103 14.71 0.035 7 5.7 5.04×105 0.89 0.22 0.35 6.91×103 16.39 0.034 8 6.7 5.93×105 0.91 0.19 0.32 7.40×103 18.00 0.033 9 7.7 6.81×105 0.94 0.16 0.30 7.87×103 19.55 0.032 10 8.7 7.69×105 0.96 0.15 0.27 8.32×103 21.04 0.031 Table4.Themassivestar-neutronstarbinaryprogenitorofGRB110709B.M isthemassofthemassivestar(insolarmasses), M (0)isthe prog ej massoftheejectedmaterialintheearly-SNphase(insolarmasses),ρ (0)isthedensityoftheejectaatthebeginningoftheexpansion,η isthe ej accr efficiencyoftheaccretionprocessontotheNS,∆M = M −M (0)isthetotalmassaccretedbytheNSbeforethecollapse,P=2πa/v accr crit NS orb,NS istheperiodofthebinary,v (0)istheinitialorbitalvelocityoftheNSand∆t /Pisthearc-lengthtravelledbytheNSduringtheaccretion orb,NS accr processinunitsofthelengthofthewholeorbitanda/R isthebinaryseparation(inunitsofsolarradii). Wesupposethattheaccretionprocess ⊙ starts5sbeforethefirsttrigger,i.e. t coincideswiththetimecorrespondingtothefirstdatapointinFig.5. 0,accr 0.08 0.06 d a Ηr 0.04 0.02 0.00 10 20 30 40 50 tHsL Fig.15. Theoreticalestimationoftheefficiencyη givenbyEq.(21) rad oftheprocesstoconvertgravitationalenergyinradiationasafunction oftime.Forthisplot,wehaveassumedaconstantandisotropicpower- lawluminosityofEpisode1,L ≈1.8×1050ergs−1 ≈10−4M s−1.We PL ⊙ computedthevaluesoftheefficiencyforthebinarysystemsshownin Fig.16. X-Ray(black),radio(blue)andNIR(red,upperlimits)light Table4.Forallthecases,weobtainthesameevolutionoftheefficiency curvesofGRB110709B.TakenfromZaudereretal.(2012)withkind withtime,i.e.thecurvesoverlap.Thevaluesofη arealways<10%. permission. rad 9. Conclusions 1031 XX GRB 110709B is a very peculiar source, since it is the first 1030 fcourrvwehpircehseSnwtsiftwt BoAwTellhdaesfitnriegdgeepreidsotdweisc,eE.piIstosdSew1ifatnBdAETpisliogdhet 1-Lz 1029 æ æ 2. Episode 1 lasts 100 s and Episode 2 lasts 135 s. Particu- 1-sH 1028 æ ò à ò lwoafrelltylhibisnetfseoorrueesrcttiehnegshsiaesrcetohnsedimfatircliatgrgthecarh.tatrThahceteeXrli-isgrtahiyctscowubrsivethervaGantRidoBnssp0es9ct0atrr6ut1emd8 HLergp,Ν 1027 à ò òò àò ò ò (Izzo,Rueda&Ruffini 2012), GRB 101023 (Penacchionietal. 1026 2012) andGRB 970828(Izzoetal. 2012). Ithasbeenrecently 1025 ò shown that these GRBs which show such distinct emissions, 5 10 20 50 100 Episodes1and2,formanewfamilyofGRBsdescribedbythe IGC paradigm (Rueda&Ruffini 2012; Izzo,Rueda&Ruffini tpHdaysLHΝ(cid:144)5GHzL 2012). Within this scenario, the GRB originates in a binary Fig. 17. Plot of the peak spectral radio Luminosity per unit fre- system formed by a massive star on the verge of a SN and a quency versusthetimeatwhichthepeakoccurs, fordifferent SNas- NS close to its critical mass for the gravitational collapse to a sociatedtoGRBs. ThecirclesrepresenttheSNemissionassociatedto BH.ThecompactcoreSNprogenitorejectsmaterialinthevery SN2006aj(GRB060218),SN1998bw(GRB980425)andSN2003lw (GRB 031203). The triangles represent the SN Ib/c for which there early phases of the SN explosion, that is then accreted by the areradioobservations,namelySN2002ap,SN1990B,SN2008D,SN NS; this process is identified with Episode 1. The accretion 1994I, SN 2009bb and SN 2003L. The squares represent the SN IIb: process onto the NS brings it to the critical mass, leading to SN2008ax, SN 2001ig, SN1993J, SN 2001gd and SN2003bg. The its gravitational collapse to a BH and emitting the GRB, iden- red cross is the luminosity related to GRB 110709B afterglow. It is tified with Episode 2. Later on, we see a standard emission in higherthantheemissionsoftheotherSN,considered“standard”. X-rays, which we have called Episode 3. Several days after the burst, when it is present, we see an optical emission, asso- Articlenumber,page10of12

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Preview GRB 110709B in the Induced Gravitational Collapse paradigm

Astronomy&Astrophysicsmanuscriptno.GRB110709B (cid:13)cESO2013 January28,2013 GRB 110709B in the Induced Gravitational Collapse paradigm A.V.Penacchioni1,3,R.Ruffini1,2,C.L.Bianco1,2,L.Izzo1,M.Muccino1,G.B.Pisani1,3,J.A.Rueda1,2 1 Dip. di Fisica, Sapienza Università di Roma and ICRA, Piazzale Aldo Moro 5, I-00185 Roma, Italy. E-mail: [ruffini;luca.izzo;marco.muccino;bianco;jorge.rueda]@icra.it 2 ICRANet,PiazzaledellaRepubblica10,I-65122Pescara,Italy. 3 UniversitédeNiceSophiaAntipolis,Nice,CEDEX2,GrandChateauParcValrose,E-mail:[email protected] January28,2013 3 1 ABSTRACT 0 2 Context. GRB110709B isthefirstsourceforwhichSwiftBATtriggeredtwice,withatimeseparationof ∼10minutes. Thefirst emission(Episode1)goesfrom40sbeforethefirsttriggerupto60safterit.Thesecondemission(Episode2)goesfrom35sbefore n thesecondtriggerto100safterit. [...] WithintheInduced GravitationalCollapse(IGC)model,weassumetheprogenitortobea a closebinarysystemcomposedofacoreofanevolvedstarandaNeutronStar(NS).TheevolvedstarexplodesasaSupernova(SN) J andejectsmaterialthatispartiallyaccretedbytheNS.WeidentifythisprocesswithEpisode1.TheaccretionprocessbringstheNS 5 overitscriticalmass,thusgravitationallycollapsingtoaBH.ThisprocessleadstotheGRBemission,Episode2.[...] 2 Aims.WeanalyzethespectraandtimevariabilityofEpisode1and2andcomputetherelevantparametersofthebinaryprogenitor andtheastrophysicalparametersbothintheSNandtheGRBphaseintheIGCparadigm. ] Methods.Weperformatime-resolvedspectralanalysisofEpisode1byfittingthespectrumwithablackbody(BB)plusapower-law E (PL)spectralmodel.WeanalyzeEpisode2withintheFireshellmodel,identifyingtheProper-GRB(P-GRB)andsimulatingthelight H curveandspectrum. Weestablishtheredshifttobez= 0.75,followingthephenomenologicalmethodsbyAmati,byYonetokuand . byGrupe,andouranalysisofthelateX-rayafterglow[...]. h Results. WefindforEpisode1atemperatureoftheBBcomponent thatevolveswithtimefollowingabrokenPL,withtheslope p ofthePLatearlyandlatetimestimesα = 0andβ = −4±2, respectively. Thebreakoccursat t = 41.21s. Thetotalenergyof - o Episode1isE(1) =1.42×1053erg.ThetotalenergyofEpisode2isE(2) =2.43×1052erg.WefindattransparencyaLorentzfactor iso iso r Γ∼1.73×102,laboratoryradiusof6.04×1013cm,P-GRBobservedtemperaturekT =12.36keV,baryonloadB=5.7×10−3 st andP-GRBenergyofEP−GRB =3.44×1050erg.[...] P−GRB a Conclusions. Weinterpret GRB 110709B as amember of the IGC sources, together withGRB 970828, GRB 090618 and GRB [ 101023.TheexistenceoftheXRTdataduringthepromptphaseoftheemissionofGRB110709B(Episode2)offersanunprecedented toolforimprovingthediagnosticofGRBsemission. 1 v Keywords. Gamma-rayburst:individual:GRB110709B—Blackholephysics 4 1 0 1. Introduction sion. Finally, a SN emission is either observed or expected in 6 association with the GRB, ∼ 10 days after the burst in the rest . 1Ofalltheastrophysicalprocessescurrentlybeinganalyzed,few frame. We aim to find sources in which the data are of such a 0are more fundamental than the one presenting the coincidence goodqualitytoallowtoseethiscompletesequence. 3of some Gamma-Ray Bursts (GRBs) with the explosion of a 1Supernova (SN). For this, the Induced Gravitational Collapse TheprototypefortheIGCparadigmhasrecentlybeengiven :(IGC) paradigm was first introduced by Ruffinietal. (2001b) in the analysis of GRB 090618 (Izzoetal. 2012), following v iandfurtheranalyzedinRuffinietal.(2007),Ruffinietal.(2008), theworksofRueda&Ruffini(2012)andIzzo,Rueda&Ruffini XRueda&Ruffini(2012) andIzzo,Rueda&Ruffini(2012). Re- (2012). In this work we follow the same line and identifyfour rcently,ithasbeenevidencedthatindeedthisprocesscanexplain different episodes in GRB 110709B. Episode 1 starts 40 s be- a the coincidencebetweenthe SN and GRB emission, bothfrom fore the first trigger and lasts up to 60 s after it and is well fit anobservationalandatheoreticalpointofview(Izzoetal.2012; by a blackbody (BB) plus a power-law (PL) spectral model . Penacchionietal.2012). It corresponds to the trigger of the SN explosion of the com- In the IGC paradigm (Ruffinietal. 2001b, 2007), a binary pact core and its accretion onto the NS companion. We notice systemformedbyanevolvedstarandaNeutronStar(NS)com- that the BB temperature decays with time following a broken panionisconsideredastheprogenitor. power-law(Ryde2004). Episode2starts35sbeforethesecond TheIGCparadigmimpliesawelldeterminedtimesequence. triggerandlastsupto100safterit. Itcorrespondstotheemis- Inaclosebinarysystemofamassivestarinthelatestphasesof sion of the canonical GRB emitted in the formation of a BH. its thermonuclear evolution and a NS companion, the massive Episode 3 starts at 800 s all the way to 106 s. It consists in a star undergoesa SN explosion. The accretion of the early-SN standardX-ray emission identified in all systems following the materialontotheNScompanionleadstheNStoitscriticalmass IGC paradigm (Pisani et al., in preparation). Episode 4 corre- and consequently to its gravitational collapse to form a black spondstotheobservationoftheopticalSNemission,observable hole(BH).TheemissionofacanonicalGRB inthecollapseto afterT =(1+z)T . Inthepresentcase,thereisnoevidence obs SN the BH takes place. A young NS is born out of the SN explo- ofanassociatedSNintheopticalband. Anexplanationforthis Articlenumber,page1of12 isgivenbyZaudereretal.(2012),whoclassifiedGRB110709B 1st trigger as dark and stated that its optical emission may have been ab- 2nd trigger sorbed by the host galaxy and/orthe interstellar medium. The 0.4 ensembleofthesefourepisodescharacterizetheIGCscenario. Asanoutcome,attheendpointoftheIGCscenario,abinary 0.3 sByHste(fmorrmeperdesaeftnetredthbeyGaRNBSe(xfpolromsieodnb)yshthoeulSdNbeexepxlpoescitoend).anda Counts/s 0.2 As in the case of GRB 101023, we do not know the cos- mological redshift of GRB 110709B due to the lack of optical 0.1 data. Therefore, we infer it from phenomenological methods: 1) The Amati relation (Amati 2006), 2) the Yonetoku relation 0 (Yonetoku2004,b),3)theworkofGrupeetal.(2007)and4)the workbyPenacchionietal.(2012),Ruffini(2012)andbyPisani 0 200 400 600 800 1000 Time (s) et al. (in preparation), which describe a scaling of the late X- Fig.1. BAT light curve of GRB 110709B, including both triggers. ray emission of GRB 090618. In the case of GRB 111228, Herewecanappreciatethetimeseparation(about10minutes)between whichwearecurrentlyanalyzing,wefindastrikingcoincidence thefirstandthesecondtrigger. Thelightcurveisinthe(15-150keV) between the values of the cosmologicalredshift determined by energy band. The time is relative to the first trigtime, of 331939966 thesemethodsforGRB110709B. s(inMETseconds). Thesecondtriggerwasat331940612 sinMET In section 2 we report the observations of the two compo- seconds. nents of GRB 110709B by the different instruments, in space andontheground.Insection3wereducetheSwiftdataandper- XRT data of GRB 110709B Cyan: WT setting; Blue: WT; Red: PC formadetailedspectralanalysisofbothEpisode1andEpisode 2. Insection4weinfertheredshiftofthesourceusingthefour s)−1 10−8 phenomenologicalmethods mentioned above. In section 5 we m −2 determinetheradiusoftheemittingregionfromtheknowledge g c 10−9 oftheredshiftandtheBBfluxofthefirstepisode. Insection6 x (er10−10 we give a brief description of the Fireshell model and we per- u form a deeper analysis of Episode 2 within this model, repro- eV fl10−11 ducing the light curve and the spectrum by a numerical simu- 0 k10−12 1 − lation. In section 7 we calculate the parameters of the binary 310−13 0. progenitorleadingtotheIGCoftheNStoaBHbytheSNex- plosion.DetailsontheaccretionrateontotheNS,totalaccreted 3 mass,SNejectadensity,NSmass,andbinaryorbitalperiodare Γ 2 obtainedforselectedvaluesoftheSNprogenitormass. Insec- tion 8 we comment on the radio emission detected by EVLA 1 (Zauderer&Berger2012). In section 9 we presentthe conclu- 100 1000 104 105 106 107 sions. Time since BAT trigger (s) Fig.2. CountlightcurveofGRB110709BobtainedfromtheSwift- 2. ObservationsofGRB110709B XRTdetector,inthe(0.3-10keV)energyband. Thetimeisrelativeto thefirsttrigtime,of331939966s(inMETseconds). GRB110709BhasbeendetectedbySuzaku(Ohmorietal.2011) and Swift (Cummingsetal. 2011) satellites, and by ground- based telescopeslike GROND(Updikeetal. 2011) and Gemini JHK, reveal two point sources within the 5” .3 XRT error cir- (Berger2011). cle reportedbyCummingsetal. (2011). Theysuggestthatone TheBurstAlertTelescope(BAT)onboardSwifttriggereda ofthemcouldbeanafterglowcandidateforGRB110709B,al- firsttimeat21:32:39UT(triggerN◦=456967). Thelocationof thoughitisveryfaint. thiseventisRA=164.6552,DEC=-23.4550.Thelightcurveis It has been suggested by Zaudereretal. (2012) that this composedofmultiplepeaks,withthewholeemissionextending sourceisan“opticallydark"GRB.Thepossiblereasonsforthis up to 60 s after the trigger (see Fig. 1). What is most inter- are: 1) dust obscuration, 2) intrinsically dim event, and/or 3) esting is that there was anothertriggerat 21:43:25UT (trigger highredshift(opticalemissionsuppressedbyLyαabsorptionat N◦=456969),∼11minutesafterthefirsttrigger. Theon-board λ ≤ 1216Å(1+z)). However, they rule out the possibility obs calculated location is RA=164.647, DEC=-23.464. This time ofa highredshifteventduetotheassociationwithanoptically Swift did not need to slew, because it was already pointing to detectedhostgalaxy. Furthermore,theyhave inferredthe opti- that position. This second emission shows a bump that begins cal brightness of the afterglow according to the standard after- 100sbeforethe secondtriggerandlastsaround50s, followed glow synchrotron model (Granot&Sari 2002; Sarietal. 1999), byseveraloverlappingpeakswithatotaldurationofabout40s, andfromthenon-detectionintheoptical-NIRwavelengthsthey andanotherisolatedpeakof10sofduration,200safterthesec- findaverylargerestframeextinctionforGRB 110709B.This ondtrigger. Fig. 1showsthecompleteBATlightcurveandFig. canexplainthelackofdetectionsintheopticalband. 2showsthelightcurvetakenbytheX-RayTelescope(XRT)in Therehavebeendetectionsintheradiobandonseveraloc- the0.3-10keVband. casionsbyEVLA(Zauderer&Berger2012),revealingasingle Therehavenotbeendetectionsinthe opticalbandby Swift unresolvedradio source within the XRT errorcircle, which re- UVOT,whichstartedtoobserve70safterthefirstBAT trigger brightened by a factor of 1.6 between 2.1 and 7 days after the (Holland2011). TheobservationswithGRONDatLaSillaOb- burst. The locationof the source is RA=10:58:37.114,DEC=- servatory (Updikeetal. 2011) simultaneously in the g’ r’ i ’z’ 23:27:16.760. Articlenumber,page2of12 A.V.Penacchionietal.:GRB110709B 0.4 GRB 110709B light curve (1st trigger) m s keV)−2−1−1 100 Unfolded Spectrum 0.3 keV (Photons c2 50 Counts/s 00..12 malized counts s keV cm−1−1−2 2100−3 0 nor 0 -40 -20 0 20 40 60 −10−3 Time (s) 20 50 100 Fig.3. CountlightcurveofEpisode1ofGRB110709Bobtainedfrom Energy (keV) theSwift-BATdetector,inthe(15-150keV)energyband. Thetimeis 3−Dec−2012 20:11 relativetothefirsttrigtime,of331939966s(inMETseconds). Fig.4. Fit of Episode 1 witha BB+PLmodel. The parameters of thefitare: kT = (22±5)keV,BBAmp= 0.2±0.1,γ =1.4±0.1,PL Amp=2.2±0.8,Red-χ2 =1.049(54DOF). 3. Dataanalysis In the following we refer to the emission that goes from 40 s beforethefirstBATtriggerto60safteritasEpisode1(seeFig. 3). Wecalltheemissiongoingfrom35sbeforethesecondBAT 20 triggerto100safteritasEpisode2. WemakeuseofSwiftBAT datatoperformthespectralanalysiswithXSPEC. 10 D V e k 3.1.Episode1 @ 5 T k We perform a time-integrated analysis to the whole Episode 1, using five different spectral models, namely BB, Band 2 (Bandetal.1993),BB+PL,PLandCutoffPL.Theresultsofthe fitsareshowninTable1andinFig. 4. TheBandfunctionisnot 1 well constrained,so we haveexcludeditin the followinganal- 1 2 5 10 20 50 ysis. A statistical test shows that the best models are BB+PL t @sD obs (χ2 = 56.65)and CutoffPL (χ2 = 54.45). Since the difference in the χ2 between these two models is 2.2, the two models are Fig.5. EvolutionofthekTcomponent oftheBB+PLmodelduring Episode1. Thefirstdatapointcorrespondsto5sbeforethefirstBAT statistically equivalent. So we discriminate between these two trigger. Thevertical linecorresponds tothetriggertime. Thetimeis models based on the physical grounds expected from the IGC in the observer frame. The temperature evolves in time following a scenario. In this scenario, we expect a thermal emission from brokenpower-lawfit. Thereisabreakat t = 41.21s. Theindicesof the expansion of the outer layers of the compact core SN pro- thePLareα=0(consistentwithaconstantfunction)andβ=−4±2, genitor. Thus,wehavechosentheBB+PLmodel. We obtaina respectively.ThepresenceoftheBB,althoughsmallerthaninprevious BB temperature kT = (22±5) keV, a PL index γ = 1.4±0.1 cases,isessentialtohaveanacceptablefit. and a χ2 = 56.65 (54 DOF). The flux of the BB component is ∼ 12% of the total flux. The total energy of Episode 1 is EEp1 =1.42×1053erg. TheresultsofthefitareshowninTable second trigger. We tried to fit the spectrum with the following iso 1.Thenweperformatime-resolvedspectralanalysiswithabin- spectralmodels: BB, PL, BB+PL, cutoffPL and Band (see Ta- ningof5sfittingthesamemodelandfindthatthetemperature ble2). WecaneasilydiscardtheBBandBandmodelsbecause oftheBBcomponentfollowsabrokenpower-law,asmentioned in one case the Red χ2 is too high and in the other case there in Ryde (2004), from5 s beforethe triggerto 55 s afterit (see is an unconstrained parameter. As the PL and the BB+PL are Fig. 5). The broken power-law is indeed a constant function nestedmodels,we performeda statistical testto see whichone plusasimplepower-lawfunction. Thisisthesamebehavioras isthebest. We obtainedaprobabilityProb=0.001thatthesim- forthepreviouslyanalyzedGRB 090618(Izzoetal. 2012)and plermodelisbetter,sotheBB+PLdominatesoverthePL.Then GRB101023(Penacchionietal.2012).However,wenoticethat wehavetocomparebetweentheBB+PLandtheCutoffPLmod- the temperaturesfor this GRB are lower. Nevertheless, the si- els. As they are notnested, we cannotapplythe same test. So multaneouspresenceofaBBandPLcomponentisnecessaryin wechosethemodelthatgivesthelowestχ2. Weconcludedthat ordertoobtainanacceptablefitofthedata(seeFig. 4). themodelthatbestfitsEpisode2isthecutoffPLmodel. It is clear from the analogies with GRB 090618 and GRB 101023thatEpisode2 hasallthe characteristicsofacanonical 3.2.Episode2 GRB.AdifferencebetweenGRB110790Bandthealreadyana- Wealsoperformedatime-integratedspectralanalysisofEpisode lyzedonesisthattheseparationbetweenEpisode1andEpisode 2, whose lightcurveisshownin Fig. 7. Thisepisodestarts35 2, ∼ 10 min, is much biggerthan previously, ∼ 50 s. This re- s before the second triggerand last 135 s, until 100 s after the markable time separation between the two episodes is an ad- Articlenumber,page3of12 Table1.FitresultsofEpisode1withfivespectralmodels:BB,Band,BB+PL,PLandCutoffPL.Thefluxisintheenergyband(15-150)keV,in unitsoferg/cm2/s. BB Band BB+PL PL CutoffPL kT =18.9±0.2 α=−1.2±0.1 kT =22±5 γ=1.37±0.02 γ=1.1±0.1 K =0.95±0.01 β=unconstr. K =0.2±0.1 K =2.0±0.2 E =196±68 BB BB PO 0 E =296±255 γ=1.4±0.1 K =0.8±0.2 0 K =2.2±0.8 PO Redχ2 =7.3 Redχ2 =1.031 Redχ2 =1.049 Redχ2 =1.14 Redχ2 =0.99 56DOF 54DOF 54DOF 56DOF 55DOF Flux=7.52×10−8 Flux=8.99×10−8 Flux=8.96×10−8 Flux=9.08×10−8 Flux=8.93×10−8 m s keV)−2−1−1 Unfolded Spectrum 4W.1e.fiNrsHtctroieludmtongdeetnasnityupperlimit for z following the work of keV (Photons c2 50 Gsgoirvrupepntieobenytca∆oll.Nu(Hm20n=0d7eN)n.Hs,iTftiyth−einyNecxHoc,ngeasslisdaoenfrdtahthereeglaraeltaidocstnhicibfcteotzwl,utemhernnoduthegnehsaitthbye-, equation m−2 20 malized counts s keV c−1−1 140×10−3 Lloagb(W1S+eurczva)el<cyu1wla.3eteb−dsiN0te.H51,[gblaolygfr(eo1nm+tert∆ihnNegHrat)hd].eiocmooaprdoinfattheesgoaflatxhyeiGnRt(h1Be) nor 2×10−3 (RA=164.64,DEC=-23.46). We obtained N = 10.5×1020 H,gal 0 cm−2. To obtain the value of N we took the XRT data from 20 50 100 H,fit Energy (keV) 2000 s to 106 s after the first BAT trigger and fitted the model 3−Dec−2012 20:26 phabs*po using the program XSPEC. The XRT data were re- Fig.6. FitofEpisode2withaCutoffplmodel. Thephotonindexis ducedbythexrtpipelinesoftware,version0.10.4,whichis part γ=1.0±0.1,thenormalizationconstantis0.5±0.1,thecutoffenergy isE =132±31andthereducedchisquaredofthefitisRed-χ2 =0.77 of the HEASOFT package, version 6.12. We use the standard 0 (55DOF). response matrix swxpc0to12s6_20010101v013.rmf for the PC mode data. The model phabs represents the photoelectric ab- sorption 0.5 GRB110709B light curve (2nd trigger) M(E)=e−nHσ(E), (2) 0.4 where n is the equivalent hydrogen column density (in units H of 1022 cm−2) and σ(E) is the photoelectric cross section, not 0.3 includingThompsonscattering. WeobtainedavalueofNH,fit = Counts/s 0.2 u7p1p.7e6rl×im1i0t2f0orcmth−e2r.edUshsiinftgofthzes<e1v.a3l5u.es in (1) we obtained an 4.2.AmatiRelation 0.1 WealsotriedtodeterminetheredshiftofEpisode2throughthe 0 Amati relation (Amati 2006), that relates the isotropic energy E oftheGRBtothepeakEnergyintherestframe E ofthe -100 -50 0 50 100 150 200 250 iso p,i Time (s) νF spectrum(Amatietal. 2009). Theanalyticalexpressionof ν Fig.7. CountlightcurveofEpisode2ofGRB110709Bobtainedfrom Eisois theSwift-BATdetector,inthe(15-150keV)energyband. Thetimeis relativetothesecondtrigtime,of331940612s(inMETseconds). 4πd2 E = L S , (3) iso (1+z) bol ditional new fact to propose a different astrophysical origin of whered2 istheluminositydistance,zistheredshiftandS is thesetwocomponents. L bol thebolometricfluence,relatedtotheobservedfluenceinagiven We turn nowto the crucialanalysis of the determinationof detectionband(E ,E )by thecosmologicalredshiftofEpisode2. min max 104/(1+z) Eφ(E)dE 4. Cosmologicalredshiftdetermination Sbol =SobsR1/(1E+mza)x . (4) Eφ(E)dE We used four phenomenologicalmethods to constrain the red- Emin R shiftofthesource,basedondifferentrelations,detailedbelow. 1 http://www.astro.uni-bonn.de/∼webaiub/english/tools_labsurvey.php Articlenumber,page4of12 A.V.Penacchionietal.:GRB110709B Table2.SpectralfitofthewholeEpisode2withdifferentmodels: BB,PL,BB+PL,cutoffPLandBand. Thefluxcorrespondstothe(15-150) keVenergyrange. ThemodelthatbestfitsthedataisthecutoffPL.WecaneasilyseethattheBBandBandmodelsarenotgood. AsthePLand BB+PLarenestedmodels,weperformedastatisticaltesttoseewhichwasthebestone. WeobtainedaprobabilityProb=0.001,indicatingthat theBB+PLdominatesoverthePLmodel.Then,betweenBB+PLandCutoffPL(thatarenotnested,thenwecannotapplythesametest),wetook theCutoffPLasthebestmodelbecauseitgivesthelowestχ2. NotethatthemodelswithwhichwefitthedataarethosedefinedintheXSPEC’s Manual:http://heasarc.nasa.gov/xanadu/xspec/xspec11/manual/manual.html BB PL BB+PL CUTOFFPL BAND kT [keV]=17.5±0.2 γ=1.46±0.02 kT [keV]=20±3 γ=1.0±0.1 α=−1.0±0.1 K =0.661±0.009 K =2.1±0.2 K =0.16±0.06 E =132±31 β=unc BB PO BB 0 γ=1.5±0.1 K =0.5±0.1 E =142±42 0 K =2.3±0.8 K =0.0048±0.0008 PO Redχ2 =7.16 Redχ2 =1.109 Redχ2 =0.78 Redχ2 =0.77 Redχ2 =0.79 DOF=56 DOF=56 DOF=54 DOF=55 DOF=54 Flux=5.2×10−8 Flux=6.52×10−8 Flux=6.35×10−8 Flux=2.43×10−8 Flux=6.36×10−8 Here, φ is the spectral model considered for the spectral data fit; in this case a Band model (Bandetal. 1993), composed of 700 twosmoothlyconnectedpower-laws. E isrelatedtothepeak TeIEtnhpce,eiarg=npyebEaEekppw(e1irnni+etttrehzgn)ey.aoissbstehreveernefrragmyeatbtyhe peapk,i ofthe νFν spectru(m5). @DEp,ikeV 530000 XXXXXXXXXXXXXXXXXXXXXXXXXX 200 X E =E (2+α), p 0 150 where E is the energy at which the two power-laws intersect 0 100 andαistheslopeofthelow-energypower-law,accordingtothe 1051 1052 1053 Bandmodel. Eiso@ergD Wecalculatetheluminositydistanced ,asgivenbythestan- L dardcosmologicalmodel Fig.8. Amatirelation(solidline)withits1−σuncertainties(doted lines) and peak energy E vs. E for GRB 110709B, for different p.i iso valuesoftheredshift,from0.1to3,atstepsof0.1.Wecanseethatthe c z dx datamatchesthetheoreticalfunctionwithin1−σforz>0.4. d = (1+z) , (6) L H0 Z0 Ωm(1+x)3+ΩΛ p around the most intense peak in order to better constrain the wheretheHubbleconstantisH =70kms−1Mpc−1,Ω =0.27, 0 m valueoftheparameters(i.e.,toincreasethenumberofphotons Ω =0.73andcisthespeedoflight. Λ in the spectrum and obtain an error which is smaller than the Following the same procedure as described in valueoftheparameters). Thepeakluminosityintherestframe (Penacchionietal. 2012), we calculated E and E for iso p,i (withtheproperK-correction)canbecalculatedas differentvaluesofz,from0.1to3,atstepsof0.1. Fig. 8shows that the relation is satisfied for values of z > 0.4. This puts a lowerlimittotheestimationoftheredshift. L=4πd2F , (8) L bol where 4.3.YonetokuRelation 10000/(1+z) We finally obtained a range of possible redshifts by using the EN(E)dE Yonetoku relation (Yonetoku 2004). This relation, also known Fbol = PobsR1/(1+zE)max N(E)dE (9) astheE -Luminosityrelation(E -L),connectstheobserved p p Emin isotropicluminosityLinunitsof1052 ergs−1 withthepeaken- R istheenergyfluxandP isthephotonflux. ergy E (1+z) in therestframeofthe GRB. Itisvalidforval- obs p Fig.9showstheYonetokurelation(solidline)withitsuncer- uesof E between50and2000keV,andaluminosityrangeof p tainties(dottedlines),andthevaluesofLandE foreachvalue 1050−1054ergs−1. p,i of z, from 0.1 to 3, at steps of 0.1. We see that the Yonetoku ThebestfitfunctionfortheE -Lrelationis p relation is satisfied within 1σ for values of the redshift > 0.7, consistentwiththeresultsobtainedwiththeAmatirelation. L E (1+z) 2.0±0.2 Inconclusion,ifweputtogetherthethreemethods,wehave =(2.34+2.29)×10−5 p (7) arangeofpossibleredshiftsof0.7<z<1.35. 1052ergs−1 −1.76 " 1keV # The peak luminosity and the peak energyare calculated by 4.4.EstimateoftheredshiftusingtheX-rayafterglow integratingwithina1sintervalaroundthemostintensepeakof the light curve, because this is a better distance indicator than We already presented in (Penacchionietal. 2012) a method to the burst average luminosity. However, we took a 10s interval estimate the redshift of GRB 101023 by comparing its X-ray Articlenumber,page5of12 10 cause GRB 110709B XRT light curve presents many spikes at thebeginning,whichaccordingtoourinterpretationcorrespond nottothesteepdecayoftheX-raylightcurvebuttotheprompt 1 emission. g InthecaseofGRB110709B,thankstothefactthatXRTwas 520er 0.1 alreadyactiveandcollectingdataatthetimeofthesecondBAT 1 (cid:144)L trigger,we were ableto followthebehaviorofthe wholeGRB emission of Episode 2. This is a key point to our understand- 0.01 ing of GRB 110709B,since onlyin veryfew cases XRT had a responseduringtheearlyemission. 0.001 In the big flare at ∼ 1000 s after the first BAT trigger we 100 150 200 300 500 700 1000 Epeak@keVD notice a strongcorrelationbetween the emission in X-raysand in γ-rays. We identifythis emission as the promptemission of Fig.9. Yonetokurelation(solidline)withits1−σuncertainties(doted Episode2. Afterthispromptphasethetraditionalplateauphase lines) and peak Luminosity vs. E for GRB 110709B, for different p.i isobserved.Aftertheplateauphase,thereisthelatedecayphase valuesoftheredshift,from0.1to3,atstepsof0.1.Wecanseethatthe datamatchesthetheoreticalfunctionwithin1−σforz>0.6. in the X-raylightcurvefollowinga power-lawbehaviorwhich has already been observedin othersources(i.e., GRB 101023, GRB 090618, GRB 111228). We study this decay in the IGC light curve to the one of GRB 090618, of known redshift(z = paradigm,andconsiderthepossibilitythatitisproducedbythe 0.54). HerewerescaletheX-raylightcurveofGRB090618as early emission of the newly-bornNS. It is interesting to notice ifitwasseenatdifferentredshiftsandplotittogetherwithGRB that in GRB 110709B the typical flare in X-rays just preced- 110709Blightcurve,lookingforthevaluesofzforwhichthese ingtheplateauphaseandfollowingthe promptemissionisnot lightcurvesoverlapatlatetimes. Wefindaremarkableconsis- observed.ThisX-rayemissionusuallyoccurswithoutanyasso- tency between this methodand the phenomenologicalmethods ciated γ-ray emission, since the data is usually below the BAT alreadymentioned. threshold. In the presentcase, it is conceivablethat the flaring Inodertocompareinacommonrestframethetwoemissions indeedoccurredduringsomeof the gapsof ∼ 4000s in which fromtheGRBs,weapplythefollowingoperationsonlytoGRB thereisnodataduetoEarthoccultation. 090618: We can then distinguish two types of flares in the X-ray 1)determinationofthe startingtime Tstart of thelate decay lightcurve. Thefirsttypeoccursatearlytimes, previousto the emission, steepdecay,andbelongstothepromptemission.Thisflarescan 2)spectralanalysisofthisemissionwithanabsorbedpower- be seen in X-rays only when XRT starts its detection at early lawmodel, enough times, e.g., when the satellite was already pointing at 3)extrapolationofthisspectralmodelinacommoncosmo- a regionnearthe burst position and did notneed much time to logicalrest-frame energyrangeand, consequently,rescaling of slew. The light curve in X-rays generally follows the trend of GRB090618lightcurveforthedifferentenergyranges, the light curve in γ-rays. The second type of flares occurs at 4)cosmologicalcorrectionforthearrivaltimebytakinginto latertimes,justprecedingtheplateauphase.Thisflaresareseen due account the different scaling due to cosmological redshift, only in X-rays since their photon flux is much lower than the and BATthreshold. IntheICGparadigm,weinterpretthisflaresas 5)correctionoftheobservedfluxbychangingtheredshiftof possibleindicatorsofthebreakoutoftheSN. GRB090618. Wearecurrentlyanalyzingmoresourcesinthecatalogueby Adetaileddescriptionofthemethodwillbegiveninaforth- Marguttietal.(2012)tolookforthesethreeverydistinctphases, comingpublication(Pisanietal.,inpreparation). i.e.,theflaresinthepromptemission,theflaresintheafterglow Inthiswaywecomparedirectlybothlightcurvesfordiffer- andthe late decayafterthe plateau, each of themhavinga dif- entredshiftsofGRB090618. Fig. 10showsGRB090618light ferentphysicaloriginwithintheIGCparadigm. curveseenasifthesourcewaslocatedatdifferentredshifts:0.2 (blue),0.4(green),0.7(grey),1.0(orange)and2.0(purple).The 5. Episode1: radiusoftheemittingregion red light curve correspondsto GRB 110709B.We can see that itliesbetweenthegreenandtheorangeones. Amoreaccurate Withtheknowledgeoftheredshiftandtheparametersofthefit scalingofthelateX-rayafterglowsuggestsaredshiftofz=0.75 withaBB+PLmodel,wecomputedtheisotropicenergyofthe forthissource. wholeEpisode1,E(1) =1.42×1053erg. iso Fig. 11showsthesuperpositionofGRB110709BandGRB WiththeenergyfluxoftheBBcomponentφ asafunction BB 090618lightcurvesintheobserverframe,asiftheywerelocated of time from the time-resolved spectral analysis and the lumi- ataredshiftz=0.75. nositydistanced ,wecancomputethevalueoftheradiusofthe L Thereishowevera secondaspectwhichisduetothepecu- emitterincm(wethenexpressitinkminFig. 12)through liarity of the turn-on T of the XRT detector. At the time BAT 0 triggeredforthe secondtime, XRT was alreadypointingat the φ d source and was able to detect the emission at very early times, r = BB L . (10) makingthisGRB probablythefirstforwhichXRThastheear- em rσT4(1+z)2 liestdetectionuptodate. WeneedtoshiftGRB 110709Blight Here φ is the BB flux in units of erg cm−2s−1, σ = curveinordertomaketheearlysteepdecays(originatingin the 5.6704eBrBgcm2s−1K−4 istheStefan-Boltzmannconstantandd promptin ourinterpretation)coincide. This is done by adding L istheluminositydistanceincm. a time T = +800s to GRB 090618lightcurve. The superpo- ∗ Thebestfitoftheexpandingradiusis sition is verygood. In thisway we also makethe early decays coincide. This factoris arbitrary, butwe need to include it be- r(t)=atb, (11) Articlenumber,page6of12 A.V.Penacchionietal.:GRB110709B 150000 10-8 100000 1-D 70000 s 2-m 10-10 DKm 50000 c @ g m er re 30000 @ Flux 10-12 20000 15000 10-14 10000 100 1000 104 105 106 1.0 1.5 2.0 3.0 5.0 7.0 10.0 15.020.0 30.0 time@sD Time@sD Fig.12. Radiusof theemittingregionasafunctionof time(inthe Fig.10. PlotofGRB090618seenasifitwasatdifferentredshifts: cosmological rest frame), corresponding toEpisode1. Theradius in- 0.2(blue), 0.4(green), 0.7(grey), 1.0(orange) and 2.0(purple). The creaseswithtimefollowingapower-lawatb,witha=(1.5±1.2)×104 red light curve corresponds to GRB 110709B. We can see that it lies km/sandb=0.32±0.27. betweenthegreenandtheorangelightcurves,indicatingthatthered- shiftmustbebetween0.4and1.0. The e± plasma reaches thermal equilibrium on a timescale of 10−12 s (Aksenov,Ruffini&Vereshchagin 2007). Being opti- 21--@DFluxergcms 111000---11208OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO O woTticaMenatphirilBlotevlliByncaobnacfptaol=uhlrrttyuiheycbmsMeoktsehn,,uoBiiarcutcctenhhk2tmeyee/(rfiREaprersettuele±oht±alffiets-ae.shprtlmnielhivBsislaoilesatdmscottneieonacudslmlfsc.epr-trpeax1libabacop9seccesa9mheedn9lldeesaeabssir,boysbeatynhn)ttte.hehge(sreaARu±mndl-udffpuatsffii1ehmel0rtonehte−atqioeno2nue,snitibieto-losaaibatbnrlhrai.ylrlneryeio2ytursewn0oemsri0xnnicps0pawpae)amlrl.inaattrahshsmTatimedtohiereitnaeer--. self-acceleratestoultrarelativisticvelocities,finallyreachingthe 10-14 transparencycondition.Aflashofradiationisthenemitted.This 100 1000 104 105 106 istheP-GRB(Ruffinietal.2001a). Theamountofenergyradi- Time@sD ated in the P-GRB is only a fraction of the initial energy Ee±. tot Theremainingenergyis storedin the kinetic energyof the op- Fig.11. GRB090618lightcurve(blue)asifitwasseenatz = 0.75 tically thin baryonic and leptonic matter fireshell that expands togetherwithGRB110709Blightcurve(red).Thereisanexcellentsu- ballistically and starts to slow down due to the inelastic col- perpositioninthelatedecayassumingatemporalshiftofGRB090618 ofT =800stotherighttohavethecoincidenceofthesteepdecaysof lisions with the Circumburst Medium (CBM). This interaction ∗ thelightcurves. gives rise to a multi-wavelength emission, the extended after- glow (Ruffinietal. 2001a). We can estimate the characteristic inhomogeneitiesoftheCBMbyfittingtheluminosityoftheX- wherea=(1.5±1.2)×104kms−bandb=0.32±0.27(seeFig raysourceandimposingthefullyradiativeconditioninthecol- 12). lisionbetweentheultrarelativisticbaryonicshellandtheclouds We associate the BB component to the expansion of the of the Interstellar Medium (ISM). The complete analytic solu- ejected material, while the power-law is associated (as we in- tion has been developed in Bianco&Ruffini (2004, 2005a,b), terpretfromtheIGCparadigm)withtheaccretionofpartofthis together with the analytic expression of the Surfaces of Equal materialontotheNScompanion. arrival Time of the photons at the detector (EQTS). The after- glowpresentsthreedifferentregimes:arisingpart,apeakanda decayingtail.Wethereforedefinea“canonicalGRB”lightcurve 6. AnalysisofEpisode2intheFireshellmodel withtwosharplydifferentcomponents:1)theP-GRBand2)the We recall that the Fireshell model (Damour&Ruffini 1975; extended afterglow. What is usually called “Promptemission” Ruffinietal. 2000; Ruffini 2001; Ruffinietal. 2010b) is an al- inthecurrentGRBliteraturemixestheP-GRBwiththeraising ternativetotheFireballmodel,firstproposedbyCavallo&Rees partandthepeakoftheextendedafterglow(Ruffinietal.2003). (1978), Goodman (1986) and Paczynsky (1986). We assume, Thespectrumoftheextendedafterglowisinitiallyassumedtobe within the Fireshell model, that all GRBs originate from the thermalinthecomovingframeoftheexpandingshell. Recently, gravitational collapse of a star approaching asymptotically the aftertheanalysisofsomehighlyenergeticsourcesobservedby formation of a Kerr-Newmann BH (Wiltshire,Visser&Scott Swift and Fermi satellites, this assumption of a pure comov- 2009). An electric field E is created just outside the collaps- ingthermalspectrumhasbeenrelaxedandaphenomenological ingcoreandinbetweentheexpandingoutershellsthatactas a modificationbyapower-lawofthelowenergyspectralslopehas capacitor(Preparata,Xue&Ruffini 1998). This electrical field been introduced (Patricelietal. 2012). The observed non ther- grows until it reaches a critical value, E = m2c3/~e. At this malspectrumshapeisduetoadoubleconvolutionofthousands c time, vacuum polarization occurs, leading to pair creation at of instantaneouscomovingspectra, with differenttemperatures the expensesof the gravitationalenergy. An optically thick e± and differentDoppler factors, over both the EQTS and the ob- plasmaformswithtotalenergyEe± intherange1049-1054erg. servationtime(Ruffinietal.2004b). tot Articlenumber,page7of12 Table3.FitoftheP-GRBandtheafterglowofGRB110709B,Episode 2.TheP-GRBiswellfitwithaBBmodel,whilethewholeEpisode2is 3.5 SimGuRlBat i1o1n0 o7f0 t9hBe lliigghhtt ccuurrvvee bestfitbyaCutoffPLmodel. Fromthisfitandthevalueoftheredshift 3 weareabletocalculateE andE . iso P−GRB 2.5 PkaTra[mkeeVte]r P1-4G±R1B P-GRB+Afterglow 2Counts/(cm s) 1. 52 BBAmp 0.30±0.02 1 γ 1.03±0.1 PLAmp 0.5±0.1 0.5 Redχ2 1.448(56DOF) 0.77(55DOF) 0 EnergyFlux 2.413×10−8 6.34×10−8 -40 -20 0 20 40 60 80 100 120 Time [s] (15−150keV) [ergcm−2s−1] 1 SSwwiifftt XBRASTTi mssppueelaccttterruudmm co oouffn GGt RRspBBe 11ct11ru00m7700 o99fBB G ((10R5.B3 - -11 1510007 kk0ee9VVB)) Energy[erg] 3.44×1050 2.43×1052 0.1 Havingfixedthevalueoftheredshifttoz=0.75,westarted 2m s keV) tlhoeokaendalfyosristhoefPE-GpiRsoBdedu2riwngiththinefithrestFbiuremspheollfEmpoidsoeld.eW2e(frfiormst Photons/(c 0.01 100 to 40 s before the second trigger) by fitting the data with 0.001 a BB + PL model. We selectedseveraltime intervalsas theP- GRB duringthe firstbumpof Episode2, butinsomecases the fits were notgood. In some othercases, to reproducethe ratio 1 10 100 Energy [keV] between the P-GRB energy and the total energy we needed to consider a baryon load B > 10−2 (which has no sense within Fig.13. Simulationof theBAT(a)light curveand(b) spectrumof Episode2.WeincludedXRTdatainthefitofthespectrumtoshowthat the Fireshell model) and, in other cases, there was a discrep- theslopepredictedinthefireshellmodelisinagreementwiththeslope ancybetweentheobservedtemperatureandtheonegivenbythe oftheX-rayspectrum. simulation. Thusweconcludedthatthisbumpshouldbelongto Episode1. Thereasonwhywedonotfindastrongthermalsig- natureinthisbumpisthatEpisode1starts∼ 10minutesbefore Density mask 100 thebeginningofthebumpandthetemperatureoftheBBcom- ponentdecreasesveryrapidlyfollowingapower-lawinthefirst 10 secondsofemission.Consequently,aftersuchalongtimewedo notWexepeficntatollyfinsdelaenctyedsigthneatuPr-eGoRfBaBasBthfreom9 sEpfriosomde315.to 26 s 3Particles/cm 1 beforethe second trigger,and the followingemission from-26 to100sastheafterglow.Table3showstheparametersofthefit. 0.1 WecalculatedaP-GRBenergyofE =3.44×1050ergand P−GRB anisotropicenergyofE =2.43×1052erg. iso 0.01 We inserted these valuesof the energiesinto ournumerical 1e+16 codeandcalculatedthevalueofthebaryonload,B=5.7×10−3. Radius [cm] Fig.14. ParticledensityoftheISMcloudsasafunctionofthedis- Wesimulatedthelightcurveandthespectrum,obtaining,atthe tance.Themeandensityis76part/cm3. transparencypoint, a LaboratoryRadius r = 6.04×1013 cm, tr agammaLorentzfactorΓ = 1.73×102 andaP-GRBobserved temperature(aftercosmologicalcorrection)kT =12.36keV. by the NS companionat times largerthan t , when the ma- 0,accr Figs. 13aand13bshowthesimulationofthelightcurveand terialreachestheNSgravitationalcaptureregion. Theemission the spectrum of Episode 2, respectively. The photon index of observedin Episode 1 is associated to this early-SN evolution, the XRT and BAT spectra are in agreementwith that predicted identified with the thermal component, and accretion process bythe simulation. Detailsofthiscalculationwillbe givenin a onto the NS, possibly related to the non-thermal component. forthcomingletter (Penacchioniet al., in preparation). Fig. 14 The NS reaches in a time t + ∆t the critical mass and 0,accr accr showsthedensitymaskoftheISM,i.e.,thedensityofparticles gravitationallycollapsestoablackhole,emittingtheGRBseen oftheinterstellarcloudsasafunctionofthedistancetothecen- inEpisode2. Weassumethecriticalmassofanon-rotatingNS terofthe BH. Thisdensityhastobe interpretedasaneffective M =2.67M asgivenbyBelvedereetal.(2012). crit ⊙ densitybecausefragmentationmayoccurintheexpandingshell The amount of material that reaches the NS gravitational (Ruffinietal.2007;Dainottietal.2007). captureregion 2GM (t) R (t)= NS (12) 7. NatureoftheProgenitor cap v2 (t) ej,rel Following the works of Rueda&Ruffini (2012) and per unit time is given by (see Rueda&Ruffini (2012) and Izzo,Rueda&Ruffini (2012), we suggest for the origin of Izzo,Rueda&Ruffini(2012)) GRB 110709B a binary system formed by a massive evolved star on the verge of a SN explosion and a NS. The early-SN materialexpandingatnon-relativisticvelocitiesisthenaccreted M˙(t)=πρ (t)v (t)R2 (t), (13) ej ej,rel cap Articlenumber,page8of12 A.V.Penacchionietal.:GRB110709B whereR ismeasuredfromtheNScenter. mass-radius relation of Belvedereetal. (2012). From the con- cap Intheseexpressions,ρ (t)=3M (t)/(4πr3(t))isthedensity straint given by Eq. (18) we fix the binary separation a. We ej ej ej thenproceedwiththenumericalintegrationoftheaccretionrate oftheejecta, equationsbyrequiringthat M (t)= M att=∆t ,givenby NS crit accr M (t)= M (0)−M(t) (14) Eq.(19),fromwhichweobtaintheefficiencyη . ej ej accr It is interesting to analyze how the NS can accrete such a isthetotalavailablemasstobeaccretedbytheNS,MNS(t)isthe large mass, in some cases of the orderof 47% of the early-SN NSmass,andvej,rel(t)isthevelocityoftheejectarelativetothe material (see column 5 of Table 4), since one could think that NS solid angles of ∼ 50% between the early-SN material and the accreting NS are hard to obtain. Indeed, during the accretion vej,rel(t)= v2orb,NS(t)+v2ej(t). (15) processtheNSismovingwithhighorbitalvelocitiesoftheorder q of 108 cm s−1 relative to the core progenitor(see column 7 of In Eq. (14), Mej(0) is the given initial mass of the ejecta (just Table 4), and consequently travels effective arc-lengths several at the beginningof the accretion process); we choose different timeslargerthanthecircumferenceoftheorbit(seecolumn8of valuesforitinTable4. M(t)isthemassoftheejectathatislost Table4). becauseitpassesthroughthecaptureregionoftheNS. Assumingthatthegainingravitationalenergyoftheaccreted TheactualmassaccretionrateontotheNS,M˙accr(t),isafrac- materialintotheNScanbereleasedfromthesystemleadstoan tionηaccr ≤1ofEq. (13),i.e. upperlimitoftheluminosity M˙accr(t)=ηaccrM˙(t), (16) GM˙ (t)M (t) |E˙ (t)|= accr NS , (20) b where η is the accretionefficiencyonto the NS. So, there is an RNS(t) amount of material per unit time M˙ (t) = (1−η )M˙(t) not out accr accretedbytheNS. wherewetakeintoaccountthedependenceoftheNSradiuswith time,duetotheincrementoftheNSmassbytheaccretionpro- InEq. (15),v (t)= G(M +M (t))/aistheorbital orb,NS prog NS cess. The self-consistentradiusis computedat each time from velocity relative to the SN cpore progenitor, a is the separation themass-radiusrelationofBelvedereetal.(2012). distancebetweentheNSandtheSNcoreprogenitor,and The actual luminosity depends on the efficiency η in rad dr (t) r (t) convertinggravitationalenergy into electromagnetic energy by ej ej v (t)= =b (17) ej somestillunknownprocess. Sinceinourmodelweassumethat dt t the BB componentof Episode 1 is due to the early-SN expan- is the expansion velocity of the early-SN material, where we sion, we estimate the efficiency η from the assumption that rad haveusedrej(t)=rem(t),givenbyEq.(11). |E˙b|isresponsibleforthepower-lawluminosityLPL,namely Wehavealreadymentionedthatthepower-lawcomponentin thespectrumofEpisode1mightbeduetotheaccretionontothe L NS companion. As this power-law componentis presentsince ηrad(t)= |E˙P(Lt)|. (21) thebeginningofEpisode1, we havefixedthevalueoft to b 0,accr beequaltothestartingtimeofEpisode1. Thisputsaconstraint InFig. 15weshowtheevolutionoftheefficiencyη inthe intheseparationdistanceaofthebinary,whichunderthesecon- rad firstsecondsofemissionforthebinarysystemsshowninTable ditionsisgivenby 4. Weassumeaconstantandisotropicpower-lawluminosityof Episode 1, L ≈ 1.8×1050 erg s−1 ≈ 10−4 M s−1, as found PL ⊙ fromthespectralanalysis. Forallthecases,weobtainthesame a=r +R (0), (18) 0 cap evolution of the efficiency with time, i.e. the curves overlap. where r = r (0) and R (0) are the radius of the early-SN Thisisduetothefactthatweconstrainedallthesystemstohave 0 ej cap ejectaandthecaptureradiusoftheNScompanionatthebegin- thesameinitialNSmassand∆taccr. ningofEpisode1. Inthiscase,r ≈1.75×109cm,seeFig. 12. 0 Theseparationaisa functionoftheinitialmassoftheNSand oftheSNcoreprogenitormass,aswellasoftheorbitalvelocity, 8. Radioobservations throughR . ItisclearthattheconstraintgivenbyEq. (18)isa cap Zauderer&Berger (2012) report observations with the EVLA lowerlimit, sincetheaccretionprocessontotheNScouldhave radio telescopes on several occasions between 11 and 16 July, beentriggeredbeforebylayersatlowerdensities(e.g. He). In at a frequency of 5.8 GHz. They found a radio source which suchacase,thebinaryseparationacouldbehigher. brightenedbyaboutafactorof1.6,between2.1and7daysafter Inadditiontothisconstraint,wemusttakeintoaccountthat theburst. ThecoincidencewiththeXRTpositionandtherising the NS must reach its critical mass M at the beginning of crit flux indicate that this is the radio afterglow of GRB 110709B. Episode2,sincebythattimetheNSmustcollapsetoaBHand The position of the source is RA = 10:58:37.114, DEC = - emitthecanonicalGRB.Thisimpliesthat 23:27:16.760. WeshowinFig. 16the5.8GHzlightcurvepre- 611 sentedinZaudereretal.(2012),wherethereisevidenceofara- ∆t ≈ ≈349 s. (19) accr (1+z) diobump.FollowingtheworkofChevalier&Soderberg(2010), wehavereproducedtheplotofthepeakspectralradioLuminos- WeshowinTable4theparametersofthebinarysystemlead- ityperunitfrequencyas a functionoftime(days)atwhichthe ingtoIGCoftheNSinatimeintervalequaltothedurationof peakisproducedfordifferentSNassociatedwithGRBs,includ- Episode1.WeadoptaninitialmassfortheNS,M (0)=1.4M ingGRB110709B(seeFig. 17).Wefindthattheradioemission NS ⊙ and,correspondingly,aNSradiusofR (0)=12.3kmfromthe ofthissourceishigherthantheonesassociatedwithtypicalSN. NS Articlenumber,page9of12 M /M M (0)/M ρ (0)(gcm−3) η ∆M /M (0) P(min) v (0)(kms−1) ∆t /P a/R prog ⊙ ej ⊙ ej accr accr ej orb,NS accr ⊙ 4 2.7 2.39×105 0.92 0.47 0.52 5.24×103 11.14 0.037 5 3.7 3.27×105 0.88 0.34 0.45 5.84×103 12.96 0.036 6 4.7 4.16×105 0.88 0.27 0.39 6.39×103 14.71 0.035 7 5.7 5.04×105 0.89 0.22 0.35 6.91×103 16.39 0.034 8 6.7 5.93×105 0.91 0.19 0.32 7.40×103 18.00 0.033 9 7.7 6.81×105 0.94 0.16 0.30 7.87×103 19.55 0.032 10 8.7 7.69×105 0.96 0.15 0.27 8.32×103 21.04 0.031 Table4.Themassivestar-neutronstarbinaryprogenitorofGRB110709B.M isthemassofthemassivestar(insolarmasses), M (0)isthe prog ej massoftheejectedmaterialintheearly-SNphase(insolarmasses),ρ (0)isthedensityoftheejectaatthebeginningoftheexpansion,η isthe ej accr efficiencyoftheaccretionprocessontotheNS,∆M = M −M (0)isthetotalmassaccretedbytheNSbeforethecollapse,P=2πa/v accr crit NS orb,NS istheperiodofthebinary,v (0)istheinitialorbitalvelocityoftheNSand∆t /Pisthearc-lengthtravelledbytheNSduringtheaccretion orb,NS accr processinunitsofthelengthofthewholeorbitanda/R isthebinaryseparation(inunitsofsolarradii). Wesupposethattheaccretionprocess ⊙ starts5sbeforethefirsttrigger,i.e. t coincideswiththetimecorrespondingtothefirstdatapointinFig.5. 0,accr 0.08 0.06 d a Ηr 0.04 0.02 0.00 10 20 30 40 50 tHsL Fig.15. Theoreticalestimationoftheefficiencyη givenbyEq.(21) rad oftheprocesstoconvertgravitationalenergyinradiationasafunction oftime.Forthisplot,wehaveassumedaconstantandisotropicpower- lawluminosityofEpisode1,L ≈1.8×1050ergs−1 ≈10−4M s−1.We PL ⊙ computedthevaluesoftheefficiencyforthebinarysystemsshownin Fig.16. X-Ray(black),radio(blue)andNIR(red,upperlimits)light Table4.Forallthecases,weobtainthesameevolutionoftheefficiency curvesofGRB110709B.TakenfromZaudereretal.(2012)withkind withtime,i.e.thecurvesoverlap.Thevaluesofη arealways<10%. permission. rad 9. Conclusions 1031 XX GRB 110709B is a very peculiar source, since it is the first 1030 fcourrvwehpircehseSnwtsiftwt BoAwTellhdaesfitnriegdgeepreidsotdweisc,eE.piIstosdSew1ifatnBdAETpisliogdhet 1-Lz 1029 æ æ 2. Episode 1 lasts 100 s and Episode 2 lasts 135 s. Particu- 1-sH 1028 æ ò à ò lwoafrelltylhibisnetfseoorrueesrcttiehnegshsiaesrcetohnsedimfatircliatgrgthecarh.tatrThahceteeXrli-isgrtahiyctscowubrsivethervaGantRidoBnssp0es9ct0atrr6ut1emd8 HLergp,Ν 1027 à ò òò àò ò ò (Izzo,Rueda&Ruffini 2012), GRB 101023 (Penacchionietal. 1026 2012) andGRB 970828(Izzoetal. 2012). Ithasbeenrecently 1025 ò shown that these GRBs which show such distinct emissions, 5 10 20 50 100 Episodes1and2,formanewfamilyofGRBsdescribedbythe IGC paradigm (Rueda&Ruffini 2012; Izzo,Rueda&Ruffini tpHdaysLHΝ(cid:144)5GHzL 2012). Within this scenario, the GRB originates in a binary Fig. 17. Plot of the peak spectral radio Luminosity per unit fre- system formed by a massive star on the verge of a SN and a quency versusthetimeatwhichthepeakoccurs, fordifferent SNas- NS close to its critical mass for the gravitational collapse to a sociatedtoGRBs. ThecirclesrepresenttheSNemissionassociatedto BH.ThecompactcoreSNprogenitorejectsmaterialinthevery SN2006aj(GRB060218),SN1998bw(GRB980425)andSN2003lw (GRB 031203). The triangles represent the SN Ib/c for which there early phases of the SN explosion, that is then accreted by the areradioobservations,namelySN2002ap,SN1990B,SN2008D,SN NS; this process is identified with Episode 1. The accretion 1994I, SN 2009bb and SN 2003L. The squares represent the SN IIb: process onto the NS brings it to the critical mass, leading to SN2008ax, SN 2001ig, SN1993J, SN 2001gd and SN2003bg. The its gravitational collapse to a BH and emitting the GRB, iden- red cross is the luminosity related to GRB 110709B afterglow. It is tified with Episode 2. Later on, we see a standard emission in higherthantheemissionsoftheotherSN,considered“standard”. X-rays, which we have called Episode 3. Several days after the burst, when it is present, we see an optical emission, asso- Articlenumber,page10of12

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