Probing the emission physics and weak/soft population of Gamma-Ray Bursts with LOFT

1.3 MB·English

by  L. Amati

Probing the emission physics and weak/soft
population of Gamma-Ray Bursts with
LOFT
5
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a
White Paper in Support of the Mission Concept of the
J

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Large Observatory for X-ray Timing
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]
E
H
.
h Authors
p
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o L.Amati1,G.Stratta2,J-L.Atteia3,M.DePasquale4,E.DelMonte5,B.Gendre6,
tr D.Götz7,C.Guidorzi8,L.Izzo9,C.Kouveliotou10,J.Osborne11,A.V.Penacchioni9,
s
a P.Romano4,T.Sakamoto12,R.Salvaterra13,S.Schanne7,J.J.M.in’tZand14,
[ L.A.Antonelli5,J.Braga15,S.Brandt16,N.Bucciantini17,A.Castro-Tirado18,

1 V.D’Elia19,M.Feroci5,F.Fuschino5,D.Guetta20,F.Longo21,M.Lyutikov22,
v T.Maccarone23,V.Mangano24,M.Marisaldi1,S.Mereghetti13,P.O’Brien11,
2
7 E.M.Rossi25,F.Ryde26,P.Soffitta5,E.Troja10,R.A.M.J.Wijers27,B.Zhang28
7
2
0
. 1 IASFBologna,Italy 15INPE,SaoJosédosCampos,Brazil
1
2 UniversityofUrbino,Urbino,Italy 16TechnicalUniversityofDenmark,Lyngby,Denmark
0
5 3 IRAPToulouse,France 17OsservatorioAstrofisicodiArcetri,Firenze,Italy
1 4 IASFPalermo,Italy 18IAA,Spain
: 5 IASFRoma,Italy 19ASIScienceDataCenter,Italy
v
i 6 ARTEMIS,Nice,France 20ObservatoryofRome,Rome,Italy
X
7 IRFU/CEASaclay,91191GifsurYvette,France 21UniversityofTrieste,Trieste,Italy
r 8 UniversityofFerrara,Ferrara,Italy 22PurdueUniversity,WestLafayette,IN,USA
a
9 ICRANet,Italy 23TexasTechUniversity,Lubbock,TX,USA
10NASAMarshallSpaceFlightCenter,Huntsville,AL, 24PennStateUniversity,PA,USA
USA 25UniversityofLeiden,Leiden,Netherlands
11UniversityofLeicester,Leicester,Italy 26UniversityofStockholm,Stockholm,Sweden
12AoyamaGakuinUniversity,Tokyo,Japan 27UniversityofAmsterdam,Amsterdam,Netherlands
13IASFMilano,Italy 28UniversityofNevada,LasVegas,NV,USA
14SRON,Utrecht,TheNetherlands
Gamma-RayBurstswithLOFT
Preamble
TheLargeObservatoryforX-rayTiming,LOFT,isdesignedtoperformfastX-raytimingandspectroscopy
with uniquely large throughput (Feroci et al., 2014a). LOFT focuses on two fundamental questions of
ESA’s Cosmic Vision Theme “Matter under extreme conditions”: what is the equation of state of ultra-
dense matter in neutron stars? Does matter orbiting close to the event horizon follow the predictions of
generalrelativity? ThesegoalsareelaboratedinthemissionYellowBook(http://sci.esa.int/loft/
53447-loft-yellow-book/)describingtheLOFT missionasproposedinM3,whichcloselyresembles
theLOFT missionnowbeingproposedforM4.
TheextensiveassessmentstudyofLOFT asESA’sM3missioncandidatedemonstratesthehighlevel
ofmaturityandthetechnicalfeasibilityofthemission,aswellasthescientificimportanceofitsunique
core science goals. For this reason, the LOFT development has been continued, aiming at the new M4
launch opportunity, for which the M3 science goals have been confirmed. The unprecedentedly large
effectivearea,largegrasp,andspectroscopiccapabilitiesofLOFT’sinstrumentsmakethemissioncapable
of state-of-the-art science not only for its core science case, but also for many other open questions in
astrophysics.
LOFT’s primary instrument is the Large Area Detector (LAD), a 8.5m2 instrument operating in the
2–30keVenergyrange,whichwillrevolutionisestudiesofGalacticandextragalacticX-raysourcesdown
totheirfundamentaltimescales. ThemissionalsofeaturesaWideFieldMonitor(WFM),whichinthe
2–50keVrangesimultaneouslyobservesmorethanathirdoftheskyatanytime,detectingobjectsdownto
mCrabfluxesandprovidingdatawithexcellenttimingandspectralresolution. Additionally,themissionis
equippedwithanon-boardalertsystemforthedetectionandrapidbroadcastingtothegroundofcelestial
brightandfastoutburstsofX-rays(particularly,Gamma-rayBursts).
This paper is one of twelve White Papers that illustrate the unique potential of LOFT as an X-ray
observatoryinavarietyofastrophysicalfieldsinadditiontothecorescience.
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Gamma-RayBurstswithLOFT
1 Summary
TheLargeObservatoryForX-rayTiming(LOFT,Ferocietal.2012,2014b)isaspacemissionconceptstudied
by ESA during an extended assessment phase in 2011–2013 as a candidate for an M3 mission within the
CosmicVisionProgramme,andproposedasaM4mission. LOFT aimsatprobinggravityinthestrongfield
environment of black holes and other compact objects, and investigating the state of matter at supra-nuclear
densitiesinneutronstarsthroughX-raytimingwithunprecedentedsensitivity. Thepayloadiscomposedofthe
LargeAreaDetector(LAD),inthe2–30keVenergyband,apeakeffectiveareaofabout8.5m2 andanenergy
resolutionbetterthan260eV,andtheWideFieldMonitor(WFM),acodedmaskimagerwithafieldofview
of4.1steradians,anenergyresolutionofabout300eVandapointsourcelocationaccuracyof1arcmininthe
2–50keVenergyrange(seeTable1). TheWFM(Brandtetal.,2012)willmonitorapproximately1/3ofthesky
atanytime.
Asdetailedinthenextsections,thankstoitsuniquecombinationoffieldofview(FOV),energyband,energy
resolutionandsourcelocationaccuracy,combinedwiththecapabilityofpromptlytransmittingGammaRay
Burst(GRB)positionstotheground,theLOFT/WFMwillachievescientificgoalsoffundamentalimportance
andnotfulfilledbyGRBexperimentspresentlyflying(e.g., Swift, Konus/WIND,Fermi/GBM,MAXI)and
futureapprovedmissions(seeTable2). Thesegoalscanbesummarizedasfollows:
substantialincrease(withrespecttothepastandcurrentmissions)inthedetectionrateofX-RayFlashes
•
(XRF),asub-classofsofteventswhichconstitutesthebulkoftheGRBpopulationandstilllackinga
goodbodyofobservationaldata. Withrespectto HETE-2/WXM(2–25keV),themostefficientXRF
detectorflownsofar,LOFT/WFMwillincreasetheXRFdetectionrate7-fold,withanexpectedrateof
30–40XRFperyear;
measurementoftheGRBspectraandtheirevolutiondownto2keVasanindispensabletoolfortesting
•
modelsofGRBpromptemission;
detectionandstudyoftherarelydetectedtransientX-rayabsorptionfeaturesinmedium/brightGRBs,
•
particularlyduringtheirinitialstage. Thesemeasurementsareofparamountimportancefortheunder-
standingofthepropertiesoftheCircum-BurstMatter(CBM)andhencethenatureofGRBprogenitors,
whichisstillamajoropenissueinthefield. Thedetectionoftransientfeaturescanenableustodetermine
theGRBredshift;
extensionofGRBdetectionuptoveryhighredshift(z > 6),whichisoffundamentalimportanceforthe
•
studyofluminosityevolutionaryeffects,themeasurementofthestarformationrate,theevolutionofthe
propertiesoftheinterstellarmedium,andpossibleforthediscoveryofpopulationiiistars;
fast and accurate localization of GRBs to allow prompt multi-wavelength follow-up with ground and
•
spacetelescopes. Thiswillinturnleadtotheidentificationoftheopticalcounterpartsand/orhostgalaxies
andtheredshift. GRBcosmologicalredshiftisafundamentalmeasurementforthescientificgoalslisted
above.
2 Introduction
Discoveredinthelate1960sbymilitarysatellitesandrevealedtothescientificcommunityin1973,Gamma-ray
Bursts(GRBs)areoneofthemostintriguing“mysteries”ofmodernscience(Mészáros,2006;Gehrelsetal.,
2009). Although being very bright (with fluences up to more than 10 4ergcm 2 released in a few tens of s)
− −
andveryfrequent(about0.8/dayasmeasuredbylowearthorbitsatellites),theiroriginandphysicsremained
obscureformorethan20years. Hugeobservationaleffortsinthelastdecadeshaveprovided,amongothers,1)a
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Gamma-RayBurstswithLOFT
Table1: MaincharacteristicsoftheLOFT instruments(basedonFerocietal.,2014b;Zaneetal.,2014;Brandtetal.,
2014).
LAD WFM
Energyband(keV) 2–30(80) 2–50
Effectivearea 8.5m2 (at6keV) 90cm2 (peak)
FOV 1deg 4.1sr
Sensitivity(5σ) 0.1mCrabin100s 0.6Crabin1s
2.1mCrabin50ks
Energyresolution 180–240eV 300eV
Timingresolution 10µs 10µs
Sourcelocation – 0.5–1arcmin
Table2: CharacteristicsoftheLOFT/WFMcomparedwiththepastandpresentexperimentswithmajorimpactinGRB
science,aswellwithfutureGRBexperimentsunderdevelopmentoradvancedstudy: theFrench-ChinesemissionSVOM
(Godetetal.,2012),Lomonosov/UFFO–p,anexperimentbyaninternationalcollaborationledbyKoreaandTaiwanto
belaunched onboard theRussian Lomonosov mission(Kim etal., 2012), theGRB Monitorof theJapanese CALET
experimentontheInternationalSpaceStation(ISS)(Yamaokaetal.,2013). SeeSect.3.
EnergyBand FOV Energyresolution Peakeff.area Sourcelocation Operation
sr keV cm2 accuracy
CGRO/BATSE 20keV–2MeV 4π 10(100) 1700 >1.7 ended
◦
∼
BeppoSAX/WFC 2–28keV 0.25 1.2(6) 140 1 ended
0
HETE-2/WXM 2–25keV 0.8 1.7(6) 350 1–3 ended
0
Swift/BAT 15–150keV 1.4 7(60) 2000 1–4 active
0
∼
Fermi/GBM 8keV–40MeV 4π 10(100) 126 >3 active
◦
Konus–WIND 20keV–15MeV 4π 10at100keV 120 – active
SVOM 4keV–5MeV 2 2(60) 400 2–10 2021-2025
0
Lomonosov/UFFO–p 5–100keV 1.5 2(60) 191 5–10 2015–2020?
0
CALET/GBM 7keV–20MeV 3 5(60) 68 – 2015–2018?
LOFT/WFM 2–50keV 4.1 0.3(6) 90 0.5–1 2025–2028
0
goodcharacterizationoftheburst-temporalandspectralproperties,2)theaccuratelocalizationanddiscoveryof
theirmulti-wavelengthafterglowemissionbyBeppoSAX andSwift,3)thedeterminationoftheircosmological
distance scale, and 4) the evidence of a connection of long GRBs (i.e. GRBs with burst duration > 2 s,
Kouveliotouetal.1993)withtypeIb/csupenovae(SNe). However,ourunderstandingoftheGRBphenomenon
isstillincompleteduetoseveralopenissues,bothobservationalandtheoretical. Theseinclude: identification
andunderstandingofsub-classesofGRBs(short/long, X-rayFlashes, sub-energetic), physicsandgeometry
of the prompt emission, unexpected early afterglow phenomenology (e.g. plateau and flares), the GRB/SN
connection,thecosmologicaluseofGRBs,VHEemission,natureoftheinnerengine,andmore.
InthisdocumentwebrieflyreviewinSect.3themainGRBscientifictopicsthatLOFT/WFMcanaccomplish.
InSect.4wediscussX-rayafterglowfollow-upwiththeLADandGRBobservationsthroughtheLADcollimator.
TheLOFT BurstAlertSystemandtherolethatLOFT couldplayinthemultiwavelengthandmultimessenger
astronomyisdescribedinSect.5.
3 GRB Science with the WFM
ManyissuesarestillopenonGRBs. InthissectionwebrieflydescribethemostrelevantGRBsciencethatcan
beaddressedwiththeLOFT/WFMobservations.
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Gamma-RayBurstswithLOFT
Figure1: Examplesofmeasurementswith1–1000keVcoverage. Left: Swift UVOT,XRT,BATandKonuslightcurves
ofGRB060124. Thecountrateshavebeennormalizedtothepeakofeachlightcurveandoffsetverticallyforclarity
(Romanoetal.,2006). Right: ExtrapolationtoX-raysofthebestfitmodels(Bandfunction,cut-offpowerlawandBB+
power-law)oftheBATSEspectrumofGRB980329(Ghirlandaetal.,2007). Apersistent“quasithermal”modelcouldbe
ruledoutbasedonlowenergy(2–28keV)WFCdata.
3.1 Physics of the prompt emission
LongGRBsaremostprobablyconnectedtothecollapseofmassivefastrotatingstars(collapsars,e.g.,Woosley,
1993;Paczyn´ski,1998). Inthemostextensivelydiscussedscenario,thecollapsarmodel,amassivestarexplodes
asasupernova(SN)and thecollapsingcoreproducesablack hole: theGRBoriginatesfroma jetemerging
alongtherotationaxis. ThebasicpropertiesoftheGRBafterglowemission(fluxandspectralshape)canbe
satisfactorilyexplainedintheframeworkofthe"standard"fireballmodelplustheexternalshockscenario(Rees
& Meszaros, 1992; Sari et al., 1998), where shocks are produced by the interaction of the GRB collimated,
relativisticejectawiththesurroundinginterstellarmatter. Onthecontrary,thephysicsunderlyingthecomplex
light curves and the fast spectral evolution of the "prompt" emission (i.e., the burst itself) is still far from
understood. Presently,thereisaforestofmodelsinvokingdifferentoriginsoffireball(kineticenergydominated,
Poyntingfluxdriven),ofshocks(internal,external)andofemissionmechanisms(synchrotronand/orInverse
Comptonoriginatedintheshocks,directorComptonizedthermalemissionfromthefireballphotosphere,and
mixturesofthese). ThesemodelscouldnotbedisentangledwithrecentFermiobservations(Zhang,2014).
A simple analysis of the morphology of the light curve as a function of energy shows that a wealth of
informationiscontainedinthepromptX-rayemissionofGRBs. Forinstance,thebroadeningandincreasing
timelagofthepulsestowardslowerphotonenergiesisalsoacrucialtestfortheemissionmodels(Fig.1,left).
PastobservationswithBeppoSAX/WFC(2-28keV)havedemostratedthatthemeasurementsofthelowenergy
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Gamma-RayBurstswithLOFT
Figure2: X-rayflasheswillbeoneoftheGRBsciencemain
targetsofLOFT/WFM.Left: 1sbackgroundsubtractedlight
curveoftheX-rayFlashXRF020427intheBeppoSAX/WFC
2–28keVenergyband(Amatietal.,2004). Top: Spectralpeak
energyE asafunctionofpeakflux,showingtheexistence
peak
of a sub-class of weak GRBs with very low values of E
peak
(i.e.,XRFevents)whichcanbesensitivelyinvestigatedonly
bydetectorswithenergythresholddownto1–2keV.
(<10keV) light curves and spectra are of key importance to discriminate among different emission models.
AnimportantcircumstanceisthatforalargefractionofGRBs,theslopeofthetimeintegratedpromptX-ray
(<10keV)emissionspectrumisinconsistentwiththepredictionsofopticallythinsynchrotronshockmodels
(Preeceetal.,2000;Frontera&etal.,2000). InconsistencywasfoundalsowithComptonizedthermalemission
ofthephotosphere(Ghirlandaetal.,2007)asshowninFig.1(right),pointingoutthat,ifpresent,thiscomponent
shouldrapidlyevolvewithtime(seealsoe.g. Fronteraetal.2013).
AnotherintriguingcaseisthedetectionbyBeppoSAXWFCsplusGRBMofatransientemissionfeatureinthe
promptemissionofGRB990712(Fronteraetal.,2001)thatwasinterpretedasevidenceofblackbodyemission
fromthefireballphotosphere. Evidenceofanevolvingthermalblackbodyspectrum(withtypicaltemperatures
of5keV)wasdiscoveredalsointhelongGRB060218,forwhichSwift/XRTobservationsduringtheprompt
emissionwereavailable(Campanaetal.,2006),andinterpretedasduetothesupernovashockbreakingoutof
thestellarenvelope.
3.2 X-ray flashes
Since 2000, anewclassoffasttransients(X-RayFlashes, XRFs)hasbeendiscoveredandstudied, firstby
∼
BeppoSAX (Heiseetal.,2001)andthenbyHETE-2 (Sakamotoetal.,2005;Pélangeonetal.,2008). XRFsshow
similardurationsaslongGRBs,areisotropicallydistributedinthesky,haveasimilarrateofoccurrenceandare
associatedwithverysimilartypeIb/cSNethatarefoundtoaccompanystandardlongGRBs(Soderbergetal.,
2005). TheirmainpropertyishoweverthatmostofthepromptemissionoccursintheX-rayband(2–20keV),
withnegligibleemissionathigherenergy(Fig.2). AnempiricaldefinitionofanXRFwasgivenasaGRBwitha
2–30keVover30-400keVfluxratiogreaterthanunity(Sakamotoetal.,2004).
As shown by Kippen et al. (2003), XRFs fall naturally in the low energy wing of the GRB peak energy
distribution. ThisclassislikelyalowerenergyextensionofGRBs. XRFssatisfythe‘Amati’relationbetween
redshift-correctedpeakenergy E intheνF promptemissionspectrumandreleasedenergy E (Amatietal.,
p ν iso
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Gamma-RayBurstswithLOFT
2002;Amati,2006). AnalysisbasedonthecompleteHETE-2spectralcatalog(Pélangeonetal.,2008)showed
thatmostXRFslieatlow/moderateredshiftand,importantly,theyarelikelypredominantintheGRBpopulation
andcloselylinkedtothe‘classical’GRBs(Fig.2,right).
InthefireballmodelparadigmforGRBs,XRFscouldbecharacterizedbyalowerbulkLorenzfactorthan
GRBs,possiblyduetoahighbaryonloadofthefireball(‘dirtyfireball’). A‘cleanfireball’mayalsoproduce
XRFs, because a gas outflow with higher Lorentz factor would cause internal shocks at larger radii where
magneticfielddensity(orequivalentlyinternalenergy)issmaller(Mochkovitchetal.,2004). XRFsmaybethe
missinglinkbetweenthemorenumerouspopulationofhard/luminousGRBsandtheclassofrelativelysoft/low
luminosityGRBsassociatedwithaSNevent. Inaddition,verysoftXRFs(expecteddetectionrate 30–50yr 1)
−
∼
mayformasub-classofsoft/ultrasofteventswhichmayconstitutethebulkoftheGRBpopulation(Pélangeon
etal.,2008). ItisthusimportanttoextendtheXRFsampletoconfirmthesepredictions. Thiscanbedoneonly
byextendingthebanddowntosoftX-rays.
3.3 X-ray spectral features
Several long GRBs were associated with supenova emission (Stanek et al., 2003). However there are a few
caseswithnoSN.Inaddition,therearecasesinwhichSNewithextremeexpansionvelocityoftheirremnants,
thatisthemaincharacteristicofGRB-SN,havenosimultaneousGRBeventsassociatedwiththem,asinthe
cases of SN2002ap (Wang et al., 2003), 2009bb (Soderberg et al., 2010), and that of 2012ap (Chakraborti
etal.,2014). SeveralGRBprogenitorscenariosaredebated(e.g. collapsar,supranova,inducedgravitational
collapse). ProbingthechemicalabundanceinthecircumburstenvironmentofGRBsisapossiblewaytotest
progenitormodels(Levesqueetal.,2010). Thistaskisextremelychallengingintheoptical,however,andX-ray
spectroscopyoftheearlyphasesoftheGRBpromptemissionprovidesacomplementary,and,inmanyrespects,
auniquetoolforthisgoal.
PastobservationswithBeppoSAX showedvariableabsorptionfeaturesintheX-rayspectrumoftheprompt
emission of a few GRBs, namely GRB000528 (Frontera et al., 2004), GRB010214 (Guidorzi et al., 2003),
GRB990705(Amatietal.,2000). Thesefeatureswerepresentinthefirstpart(rise)oftheburstandfadedaway
soonafter. Thisisinlinewiththeoreticalexpectationsbecausemostoftheopacityevolution,whichisprincipally
due to photoionization of gas-phase ions and of dust grain destruction, takes place in the early phase of the
event(Böttcheretal.,1999;Lazzati&Perna,2003). Theobservedfeaturespointtothepresenceofaniron-rich
circumburstenvironment,butnoconclusiveconstraintsontheprogenitormodelwereachievedgiventhelow
statisticalquality. TheSwift mission(Gehrelsetal.,2004),inspiteoftheenormousefforttopromptlyfollow-up
(slewtime 1min)inthe0.3–10keVbandtheGRBsdetectedwithSwift/BATinthehardX-rays(15-150keV),
∼
hasbeenunabletostudytheearlyphasesofthepromptemissiondownto1keV,whentheabsorptionfeatures
areexpected(e.g.,inthecaseofGRB990705,theabsorptionfeaturewasvisibleonlyinthefirst13s).
3.4 High-z GRBs
OurobservationalwindowontheUniverseextendsuptoz 10,wherethemostdistantobjectsdiscoveredso
∼
farreside,andthenrecoversatz = 1000,theepochofprimordialfluctuationsmeasuredbyBOOMERANG,
WMAP, and Planck. The formation of the first objects, stars, and protogalaxies, should have taken place at
epochscorrespondingtoz = 10–30, certainlybeyondz = 7. Discoveringandstudyingthefirst“light”from
primordialgravitationallyboundedobjectsintheUniverseatsuchredshiftsisthusaprimarygoalofcosmology
andastrophysics. FarinfraredandX-rayradiationaretheonlytwowavelengthregimesinwhichthesestudies
canbeattempted,withtheX-rayrevealingthemostenergeticpartofthephenomenon.
TheincreaseofthenumberofhighredshiftGRBsandtheirfew-arcminlocalization,neededtoallowoptical
follow-up and hence redshift determination, is of fundamental importance not only for GRB physics and
progenitors,butalsoforthestudyofthestarformationrateevolution,ofpopulationiiistarsand,moreingeneral,
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Gamma-RayBurstswithLOFT
1000
15
LOFT 3years
Swift-BAT
1s]− 10 BATSE z) 1year
2− Fermi-GBM > 100
m (
c SVOM N
h n
p o
[V uti
ke 5 rib
000 dist 10
ak,1–1 ulative
e m
p
F u
c 1
2
1 10 100 0 2 4 6 8 10
Epeak [keV] redshiftz
Figure3: IllustrativeplotsofLOFT/WFMcapabilitiesandoftheirimpactinGRBscience. Left: GRBdetectionsensitivity
intermsofpeakfluxsensitivityasafunctionofthespectralpeakenergyE (Band,2003)oftheLOFT/WFM(red)forM4
p
configurationcomparedtothoseofCGRO/BATSE(green),Swift/BAT(blue),Fermi/GBM(brown),andSVOM/ECLAIRs
(cyan). Right: CumulativeredshiftdistributionoflongGRBspredictedforLOFT/WFMandcomputedassumingabroken
power-lawGRBluminosityfunctionandapuredensityevolution(seeSalvaterraetal.,2012,forthedetailsofthemodel).
AtthefluxlimitofLOFT/WFMwepredictabout1–2GRBsyr 1atz>6,correspondingto3–6high-zGRBsin3years.
−
ofthepropertiesoftheuniverseattheionizationepoch. Severalrecentstudies(Salvaterraetal.,2008,2009)
showthatloweringthethresholdoftheGRBdetectorenergybandcanincreasesubstantiallythesensitivityto
high-zGRB(seealsoFig.3,right).
3.5 SN shock breakout
Wide-fieldsoftX-raysurveysarepredictedtodetecteachyearhundredsofsupernovaeintheactofexploding
throughtheSNshockbreakoutphenomenology. AccordingtorecentSNrateestimates(Horiuchietal.,2011),
abouttwoSNebreakoutsperyearshouldbedetectedwiththeLOFT widefieldmonitorwithin20Mpc.
SNshockbreakouteventsoccurinX-raysattheverybeginningofthesupernovaexplosions,allowingfor
follow-upatotherwavelengthstostartextremelyearlyoninthesupernova,andpossiblysheddinglightonthe
natureoftheprogenitorofsometypesofSNe. Todate,onlyoneSNshockbreakouthasbeenunambiguously
detected,inSwift observationsofNGC2770. Ithasafast-rise,exponentialdecaylightcurvewitharisetimeof
72seconds,andanexponentialdecaytimescaleof129seconds,andapeakX-rayluminosityof6.1 1043ergss 1
−
×
(Soderberg et al., 2008). More uncertain is the case of GRB 060218, the X-ray prompt emission of which
(observedwithSwift/XRT)wasinterpretedastheevidenceofaSNshockbreakout(Campanaetal.,2006).
If we take the case of NGC 2770 as a template, the optimum integration time to detect such events out to
20Mpcis240s(i.e.,WFMsensitivity 30mCrab).
∼
3.6 Expected performance of the LOFT/WFM
LOFT/WFMaddressesmostoftheGRBscientificopenissuesdiscussedintheprevioussection. TheWFM
outperformsinspectralresolution,sourcelocationaccuracy,andfieldofviewthetwoearliermaininstruments
capableofmeasuringGRBpromptemissiondownto2keV(BeppoSAX/WFCandHETE-2/WXM)aswellas
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Gamma-RayBurstswithLOFT
GRB 090618 − WFM+GBM: BB+PL (Black, Blue) vs. Band(Red) 0
0
1
m s keV)−2−1−1 100 mskeV−2 −1 −1 10
keV (Photons c2 10 counts c
2 5 10 20
10 100
Energy (keV) Energy (keV)
Figure4: ExamplesofLOFT/WFMsimulationsoftheGRBpromptemission(burst)spectrafrom. Left: SimulatedWFM
spectraofthefirst 50sofGRB090618(fromIzzoetal.2012)obtainedbyassumingeithertheBandfunction(black)or
∼
thepower-lawplusblack-bodymodel(red)whichequallyfittheFermi/GBMmeasuredspectrum,whichisalsoshown
(blue). Theblack–bodypluspower-lawmodelcomponentsbest–fittingtheFermi/GBMspectrumarealsoshown(black
dashedlines). Right: SimulationofthetransientabsorptionfeatureintheX-rayenergybanddetectedbyBeppoSAX/WFC
inthefirst8sofGRB990705aswouldbemeasuredbytheLOFT/WFM(Amatietal.,2000).
otherpast,presentandfuturehighenergyinstruments(Table2). ThiswillallowWFM,possiblyincombination
withotherGRBexperimentsflyingatthesameepoch,toprovideauniquecontributiontothefulfilmentofthe
scientificgoalsmentionedabove.
Morespecifically,theWFMlowenergythresholdisfundamentalforthedetectionandstudyofXRFs,high-z
GRBs,andsofttransientX-rayabsorptionfeatures,forwhichenergyresolutionof300eVatenergies<10keV
is mandatory. A FOV of about 4sr combined with a sensitivity of 0.5Crab (in 1s integration time) in the
∼
2–50keVenergybandisrequiredtodetectandlocalize 120GRBsperyear(including 1–2eventsatz > 6,
∼ ∼
Fig.3)andperformtimeresolvedspectroscopyof 2/3ofthem.
∼
AsestimatedwiththeBeppoSAX/WFCandHETE-2/WXMlogN-logS and E distributions(Vetereetal.,
p
2007; Sakamoto et al., 2005), about 2/3 of GRBs are X-Ray Flashes (XRFs) or X-Ray Rich (XRRs) GRBs.
LOFT/WFMhasamuchbetterefficiencyforthedetectionofXRFswithrespecttomainGRBdetectorslike
CGRO/BATSE,Swift/BATandFermi/GBM.InFigure 3(left)weshowtheexpectedpeakfluxsensitivityasa
functionofthespectralpeakenergyE (followingthemethodproposedbyBand,2003).
p
Starting from the logN-logP from 2–25keV HETE-2/WXM data and: 1) taking into account the FOV
differencebetweenHETE-2/WXMandLOFT/WFM;2)assuming80%observationefficiency;3)assuminga
totalof120LOFT/WFMGRBperyear,theLOFT/WFMwillincreasetheXRFdetectionperyearbyafactorof
7withrespecttoHETE-2/WXM(Sakamotoetal.,2005). Thisresultsinanestimatedrateof36LOFT XRFs
∼
peryear,oratotalnumberof108LOFT XRFdetectionsin3years(180for5years),tocomparedforexample
withthe19XRFin3yearsdetectedwithHETE-2/WXM(Sakamotoetal.,2005).
Theextensionoftheenergybanddownto 2keVoftheLOFT/WFMwillmakeitmoresensitivetoverysoft
∼
XRFs. EventhemainGRBdetectoroftheChinese-FrenchSVOMmission,currentlyunderstudy,willnotbe
abletoexplorebelow4keV.
SimulatedWFMspectraofthefirst 50sofGRB090618wereobtainedbyassumingfirstaBandfunctionand
∼
thenapower-lawplusblack-bodymodel,whichdescribetheFermi/GBMmeasuredspectrumequallywell(Izzo
etal.,2012). AsclearlyshowninFig.4,theextensionofthespectralmeasurementbelow10keVallowedbythe
LOFT/WFMwillmakeitpossibletodiscriminatebetweentheBandandblack-bodypluspower-lawmodels.
An energy resolution better than 15% (FWHM) for photon energies above 50keV is enough for accurate
measurementofthepeakenergyoftheνF spectrum. Asimulationofthetransientabsorptionfeatureinthe
ν
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Gamma-RayBurstswithLOFT
X-ray energy band detected in the first 8s of GRB 990705 is plotted in Fig. 4, together with a simulation
of how LOFT/WFM would detect the transient blackbody component in the X-ray energy band detected by
BeppoSAX/WFCinGRB990712.
Sourcelocationaccuracyofafewarcminutesisrequiredtotriggerfollow-upobservationsofthedetected
transients by other telescopes, but it is also essential to achieve the GRB scientific objectives by means of
multi-wavelengthandmulti-messengerstudies. ForthefurtheradvancementoftheGRBscienceitisessentialto
haveinthe>2025time-frameaspaceinstrumentcapabletocontinueprovidingalertservice,presentlyprovided
bytheSwiftsatellite,tospaceandgroundmulti-wavelengthtelescopes. Onboarddatahandlingisrequiredto
allowpromptfollow-upobservations(seeSect.5).
4 Perspective GRB Science with the LAD
WhiletheGRBsciencewiththeWFMwillcome‘forfree’,thecontributionfromtheLADconsistingpossibly
ofthespectralandtimingcharacterizationofX-rayafterglowemissionupto 30keV,willcriticallydependon
∼
thefollow-upcapabilitiesandpoliciesofthemission,anditwillbestronglydependentonthetimethatwillbe
requiredtostartaToOobservation.
GRB science that could be performed by the LAD includes (see also Amati et al., Exp. Astr., submitted)
theX-rayafterglowspectralandtemporalanalysis,andthesearchforemission/absorptionlines,predictedby
theoreticalmodelsifthecircum-burstenvironmentwerehighlymetal-enriched. Spectralemissionandabsorption
linesweredetectedintheX-rayafterglowsofonlyfewGRBs,namelyGRB000214byBeppoSAX(Antonelli
etal.,2000),ofGRB991216byChandra (Piroetal.,2000)andmorerecentlyofGRB130925AbyNuSTAR
(Bellmetal.,2014). However,nosuchfeaturesweredetectedsofarbySwift/XRT(Hurkettetal.,2008). Further
observationsarestronglydesirableinordertoconfirmtheirexistenceandultimatelyexploitalltheirpotential
informationontheGRBprogenitor,surroundingenvironmentandradiationmechanisms.
SimulationshaveshownthatLOFT/LADcapabilitieswillenabletodetectemissionlineswithcharacteristics
similartothoseobservedinthepast. Forexample,Figure6showssimulationsofanemissionlineat4.7keV
astheonedetectedbyBeppoSAX/NFI12hoursafterthetriggerofGRB000214. Thecontinuumfluxusedin
thesimulationwasfixedafactor2.6higherwithrespecttothe2-10keVfluxmeasuredforGRB000214,i.e. at
2 10 12 ergcm 2 s 1,correspondingtotheexpectedfluxat6hoursafterthetrigger(insteadof12hours),by
− − −
×
assumingthemeasuredfluxdecayindexof1.4(Antonellietal. 2000),whilethelinefluxwasleftunaltered.
Thelineisdetectedwithhighstatisticalconfidence.
Providingfastfollow-up,e.g.,within5–10hoursfromthebursttrigger,asignificantfractionofGRBX-ray
afterglows( 20%)havefluxesabovetheestimatedsensitivitylimitforLOFT/LAD(Fig.5). Simulationsshow
∼
thatwithinabout10hoursoftrigger,X-rayafterglowfluxeswillbedetectedbyLOFT/LADwithmoreefficiency
andlessintegrationtimethanforSwift/XRT.X-rayafterglowstobefollowedwithLADcanbeselectedamong
thebrightestGRBs,fromtheburstfluencethatcorrelateswithX-rayafterglowfluxes(Gehrelsetal.,2008). For
theseGRBs,LOFT/LADwillenableinvestigationsoftheplateauphaseanditstransitiontothe“normaldecay”,
whichmaymarktheenergyinjectionbyamagnetizedneutronstar(Nouseketal.,2006;Dall’Ossoetal.,2011).
TheLADwillalsobesensitivetopromptGRBemission, evenwhentheGRBisoutsidethefieldofview,
that is able to penetrate the LAD collimator. Preliminary Monte Carlo simulations show that, by combining
thespectralefficiencyofthesilicondetectorswiththeLADcollimatortransparency,acollectingareaofafew
hundredcm2 willbeavailable,wheretheexactvaluedependsontheoff-axisangleandthephotonenergy(see
Marisaldietal.,LOFT WPonTGFs). Asaconsequence,GRBscanbedetectedthroughtheLADcollimator,
enablingustoextendtheenergybandofthepromptemissionupto80keVandtoperformtiminganalysisinthe
2–80keVenergyband.
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