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GRB 081029: Understanding Multiple Afterglow Components S. T. Holland∗,∗∗, M. De Pasquale‡, J. Mao§, T. Sakamoto∗,∗∗, P. Schadyk,‡, S. 1 Covino§, P. D’Avanzo§, A. Antonelli††, V. D’Elia††, G. Chincarini§, F. Fiore†† and S. 1 B. Pandey‡‡ 0 2 n ∗CentreforResearchandExplorationinSpaceScienceandTechnology,NASA/GSFC,Greenbelt,MD20771, a USA J †UniversitiesSpaceResearchAssociation,10211WinicopinCircle,Columbia,MD21044,USA 1 ∗∗Code660.1,NASA/GSFC,Greenbelt,MD20771,USA 3 ‡MullardSpaceScienceLaboratory,UniversityCollegeLondon,Holmbury,StMary,Dorking,Surrey,RH56NT, UK ] §INAF-OsservatorioAstronomicodiBrera,ViaEmilioBianchi46,I–23807Merate(LC),Italy O ¶JointCentreforAstrophysics,UniversityofMaryland,BaltimoreCounty,1000HilltopCircle,Baltimore,MD C 21250,USA . kMax-PlanckInstitutfürExtraterrestrischePhysik,Giessenbachstraße,85748Garching,Germany h ††INAF-OsservatorioAstronomicodiRoma,ViadeFrascati33,I–00040MonteporzioCatone(Roma),Italy p - ‡‡RandallLaboratoryofPhysics,UniversityofMichigan,450ChurchSt,AnnArbor,MI48109–1040,USA o r t Abstract. Wepresentananalysisoftheunusualopticallightcurveofthegamma-rayburstGRB081029,whichoccurredata s redshiftofz=3.8479.WecombineX-rayandopticalobservationsfromtheSwiftX-RayTelescopeandtheSwiftUltraViolet a OpticalTelescopewithopticalandinfrareddataobtainedusingtheREMandROTSEtelescopestoconstructadetaileddata [ setextendingfrom86sto∼100000saftertheBATtrigger.Ourdataalsocoverawideenergyrange,from10keVto0.77eV 1 (1.24Åto16000Å).TheX-rayafterglowshowsashallowinitialdecayfollowedbyarapiddecaystartingatabout18000s. v Theopticalandinfraredafterglow,however,showsanuncharacteristicriseatabout5000sthatdoesnotcorrespondtoany 2 featureintheX-raylightcurve.Ourdataarenotconsistentwithsynchrotronradiationfromasingle-componentjetinteracting 5 withanexternalmedium.Wedo,however,findthattheobservedlightcurvecanbeexplainedusingmulti-componentmodel 9 forthejet. 5 Keywords: gamma-rayburst:individual:GRB081029 . 1 PACS: 98.70s.Rz 0 1 1 INTRODUCTION : v i Thereisgrowingevidencethattheclassicalpictureofasingleuniformjetcannotexplainthespectralenergydistribu- X tionsandlightcurvesofsomegamma-rayburst(GRB)afterglows.Forexample,theunusuallybrightopticalafterglow r of the “naked-eye”burst GRB 080319B was best explained using a two-componentjet [6] while GRB 030329 [1] a appearstorequireanarrow,ultra-relativisticinnerjetandawide,mildlyrelativisticouterjettoexplainitslightcurves. This is in agreementwith results from magneto-hydrodynamicmodelling that show complex structure in GRB jets [e.g.,9].GRBafterglowsappeartobemorecomplexthanoriginallythought. An example of a GRB afterglow that appears to require a multi-component jet is GRB 081029. This burst was detectedbySwift/BATat01:43:56UTon2008Oct29[8].ROTSE-IIIcidentifiedtheopticalafterglowat86s[7],and theREMtelescopestartedobservingtheopticalafterglowat154s[2],sothereisawell-sampledR-bandlightcurve startinglessthan90saftertheBATtrigger.DuetoanobservingconstraingSwiftwasunabletoslewtothisburstas soonasitwasdetected.XRTandUVOTobservationsbeganabout45minuteaftertheBATtriggerandcontinuedfor approximately10days. TheVLT/UVESandGemini-Southmeasuredaredshiftofz=3.8479[4,3],whichcorrespondstoalookbacktime of11.9Gyr.TheGemini-SouthGMOSspectrumshowsevidenceforadampedLyman-alphasystemaswellasseveral metalabsorptionfeaturesinthehostgalaxyofGRB081029. OBSERVATIONS BAT:Prompt Emission The Swift/BAT discovered and observed GRB 081029. The burst duration was T =280±50 s, the peak flux 90 was (2.8±1.3)×10−8 erg cm−2 s−1, and the spectrum was best fit by a simple power law with a photonindex of G =1.5±0.2.TheBATlightcurvewassomewhatsmootherandweakerthanatypicalBAT-detectedGRB.Figure1 showstheBATlightcuvrvefortheprompeemissionfromGRB081029. 0.01 15−25 keV 5×10−3 0 0.01 25−50 keV 5×10−3 0 0.01 50−100 keV 5×10−3 0 4×10−3 100−150 keV 2×10−3 0 −2×10−3 0.03 0.02 15−150 keV 0.01 0 −0.01 −100 0 100 200 300 400 Time since the BAT trigger [sec] FIGURE1. TheBATenergyresolvedlightcurvesofGRB081029with10sbinning. XRT:X-Ray LightCurve and Spectrum The Swift/XRT observed GRB 081029 from 41.4 minutes to approximately 10 days after the BAT trigger. The X-raylightcurve(seeFigure2)iswellfitbya brokenpowerlaw withindices(fn (cid:181) t−a )ofa 1 =0.56±0.03until 18230±346s,anda =2.56±0.09afterthat.Thereissomeevidenceforflaringbetweenapproximately2500sand 2 5000s.ThetimescalesoftheseflaresareconsistentwithD t/t<1.TheX-raydataarenotunusual,andareconsistent withthecanonicalX-raylightcurveforGRBafterglowsdescribedby[5]and[10]. FIGURE2. TheX-ray,optical,andinfrareddatapresentedasfluxdensities. TheSwift/XRTspectrumcanbefitbyasinglepowerlaw(fn (cid:181) n −b )withanindexofb X =0.98±0.08.Thereis noevidenceforanyevolutioninthepowerlawindexatX-rayenergies.TheestimatedGalacticcolumndensityinthe directionoftheburstisN =2.8×10−20cm−2,andtheabsorptioninthehostisN =4.9×10−21cm−2. H H FIGURE3. Atwo-componentjetmodelprovidesareasonablefittotheoptical,infrared,andX-raylightcurvesoftheafterglow ofGRB081029. OpticalandInfrared Observations TheSwiftUVOTbeganobservingtheafterglowofGRB081029at2689saftertheBATtrigger.Theafterglowwas detectedintheUVOTv,b,andwhitebands,consistentwiththereportedredshiftofz=3.8479.Ground-baseddata was obtained using REM and ROTSE. ROTSE began observations 86 s after the burst in the R band. REM began observingGRB081029156saftertheBATtriggerintheR,J,andH bands.Theresultinglightcurvesarecomplex, instarkcontrasttothesimpleX-raylightcurve.ThecombinedopticalandinfraredobservationsareshowninFigure2 alongwiththeSwift/XRTlightcurve.Theopticalandinfrareddatashowajumpinthefluxdensityofapproximately afactoroftenatapproximately5000s.ThereisnocorrespondingincreaseintheX-rayfluxdensityatthattime. DISCUSSION The X-ray light curve is consistent with energy injection from ongoing central engine activity until about 15000 s followed by a jet break at 18230 s. However, this scenario cannot explain the jump in the flux seen at optical and infraredwavelengthsatabout5000s. Therefore,we donotthinkthata changein the energyinjectioniscapableof explainingthelightcurvesforthisafterglow.Similarly,thejumpcannotbemodelledbyinvokingthepassageofthe synchrotronpeakfrequencythroughtheopticalregime,orastheriseoftheforwardshockduetointeractionwiththe circumburstmedium. We also examined the possibility that the jump is due to density structure in the surrounding environment,butthisisunabletoreproducethespeedorthemagnitudeoftheincreaseinluminosity. Ingeneralwefindthataone-componentjetcannotexplaintheobservedlightcurvesandspectralenergydistribution oftheX-ray,optical,andinfraredafterglowsofGRB081029.However,atwo-componentjetmodel,similartowhatis seeninsomeotherGRBafterglows,doesprovideareasonablefittothedataOurtwo-componentjetmodelisshown inFigure3,andtheparametersofeachjetarelistedinTable1.Thehalf-openingangleofthejetisdenotedbyq ,G j 0 istheLorentzfactor,E istheisotropicequivalentkineticenergyinthejet, p istheelectronindex,e ande are K,iso e B thefractionsoftheenergyinelectronsandmagneticfieldsrespectively,nisthedensityofthecircumburstmedium, andzistheredshift. Thenarrow,innerjethasahalf-openingangleofq =0.01radandaLorentzfactorof500.Thiscomponentgives j,n TABLE 1. Model parameters for the best-fitting two-component jet model for GRB081029. Parameter NarrowJet WideJet q j(rad) 0.01 0.02 G 500 60 0 EK,iso(erg) 2.5×1054 2.0×1054 p 2.2 2.2 ee 0.02 1/3 eB 0.0002 0.0002 n(cm−3) 10 10 z 3.8479 3.8479 rise totheX-rayfluxandthe pre-jumpopticalflux.Thewider,outerjethasq =0.02radanda Lorentzfactorof j,w 60.Thiscomponentdominatestheafterglowafterabout10000s.Thetotalelectromagneticenergyintheafterglowis approximatelyequallydividedbetweenthetwojets. CONCLUSIONS GRB 081029 was a long–soft GRB with a redshift of z=3.8479. It had a smooth gamma-ray light curve and did notappearto haveanyunusualgamma-rayproperties.Neitherthe gamma-raynortheX-raypropertiesofthis burst showed any sign of strange behaviour.The optical and infraredlight curves, on the other hand, were not typical of GRBafterglows.Thereisabrighteningintheopticalandinfraredlightcurvesatabout5000sthatcannotbeexplained usingasingle-componentjetmodel.However,wefindthatatwo-componentjetmodelfitsthedatareasonablywell. WeconcludethattheafterglowofGRB081029wasprobablypoweredbyatwo-componentjetwiththeenergysplit approximatelyequallybetweenanarrow(q =0.01rad)innerjetandawider(q =0.02rad)outerjet.Theinner j,n j,w jet has a Lorentz factor of G =500 while the outer jet has G =60. This result providesevidence that some (and n w perhapsall)GRBjetshavecomplexinternalstructure. ACKNOWLEDGMENTS WeacknowledgetheuseofpublicdatafromtheSwiftDataArchive.Thisworkisbasedinpartonobservationstaken with the ROTSE-IIIctelescope in Namibia,the REM telescope atla Silla Observatory,andwith ESO Telescopesat theParanalObservatories. REFERENCES 1. Berger,E.,etal.,2003,Nature,426,154 2. Covino,S.,etal.,2008,GCNC8441 3. Cucchiara,A.,etal.,2008,GCNC8448 4. D’Elia,V.,etal.,2008,GCNC8438 5. Nousek,J.A.,etal.,2006,ApJ,642,389 6. Racusin,J.L.,etal.,2008,Nature,455,183 7. Rykoff,E.S.,2008,GCNC8436 8. Sakamoto,T.,etal.,2008,GCNC8435 9. Tchekhovskoy,A.,etal.,2010,NewAstronomy,15,749 10. Zhang,B.,etal.,2006,ApJ,642,354