X-Ray Emission of Gamma-Ray Bursts

arXiv:astro-ph/9610237v129Oct1996X-RayEmissionofGamma-RayBurstsM.TAVANIColumbiaAstrophysicsLaboratory,ColumbiaUniversityNewYork,NY10027SubmittedtotheAstrophysicalJournal:August30,1996Accepted:October28,1996ToappearintheApril10,1997issue,Vol.474.–2–ABSTRACTX-rayemissioncanprovideacrucialdiagnosticofgamma-raybursts(GRBs).WecalculatetheX-rayandgamma-rayspectraofimpulsiveaccelerationepisodesrelatedtoGRBpulses.Weusethesynchrotronshockmodel(SSM)asabasisofourcalculations.Weshowthatthecurrentdataonsoft-to-hardemissionratiosofGRBpulseemissionareinagreementwiththeSSM.Inparticular,GRBpulseemissiondetectedbyGINGAisinagreementwiththeSSMlow-energyspectra.WededucethatGINGAdetectedthemajorityofbrightGRBsdetectablebyBATSE.TheseresultsindicatethatthephysicalenvironmentsurroundingtheGRBemissionsiteisopticallythintoX-rayphotonenergies.WealsocalculateemissionratiosintheEinstein,ROSAT,SAXandHETEenergybands,anddiscusshowfutureinformationonsimultaneoussoft/hardGRBemissioncancontributeindistinguishingdifferentemissionmodels.TwodifferentcomponentsofX-rayemissionmaysimultaneouslyexistinafractionofGRBs.OnecomponentisclearlyassociatedwiththeindividualGRBpulses,andanadditionalcomponentmayberelatedtothepulseX-rayspectralupturnsand/ortheprecursors/tailsoccasionallyobserved.WealsoshowthatameaningfulsearchofGRB-drivenX-rayflashesinAndromeda(M31)canbecarriedoutwithexistingROSATPSPCdataandfutureSAXWFCobservations.Subjectheadings:Gamma-raybursts–Relativisticshocktheory–3–1.IntroductionGRBsarecharacterizedbyarelativelylowfluenceintheX-rayenergybandascomparedtothehardX-ray/gamma-rayband(Trombkaetal.1974,Wheatonetal.1973,Katohetal.1984,Larosetal.1984,Yoshidaetal.1989,hereafterY89,Murakamietal.1991).Thesoft/hardenergyratio(e.g.,1-10keV/30-1000keV)ofGRBfluencesisoforderoffewpercent.The“X-raypaucity”featureisafundamentalcharacteristicofGRBs,andawellestablishedobservationalfact.WecalculateinthispapertheX-rayspectrumofGRBsusingthesynchrotronshockmodel(SSM)whichhasbeenrecentlyshowntosuccessfullyreproducethebroad-bandspectraofbrightGRBs(Tavani1995;Tavani1996a,b,c,hereafterT96a,b,c).Weshowthatintheabsenceofabsorptionprocesses(duetoopacityand/orsynchrotronself-absorption)intheX-raybandorspectraldistorsionsduetoinverseComptonscattering,theX-ray/gamma-rayemissionratioscanbereliablycomputed.Theobservedemissionratiosaredependentontheunderlyingparticleenergydistributionfunction,andwecalculatetheX-ray/gamma-rayemissionratiosfordifferentspectralassumptions.WecompareourresultswiththepreviouslydeterminedsimultaneousX-ray/gamma-rayemissionratiosfromthejointdataoftheXMOSP78-1andthePioneerVenusSatellite(PVO)instruments(Larosetal.1984),theGINGAGRBmonitor(Y89),andtheWATCHinstrument(Castro-Tirado1994).Wealsocalculateobservableemissionratiosappropriatetocurrenthigh-energymissions,BATSE,ROSAT,SAXandHETE.Thispaperisorganizedasfollows.Sect.2providesasummaryoftheGRBemissionmodelusedhere,andadiscussionofthemostrelevanttheoreticalpointsaddressedintheanalysis.Afirstgoalofourpaperistousesimultaneoussoft/hardemissionratiostoconstraintheGRBemissionmechanism.WeshowinSect.3thatadefinitecorrespondencebetweenX-ray/gamma-rayemissionratiosandpeakenergyoftheνFνspectrum(Ep)canbeestablished.Simultaneousbroad-bandspectroscopyofGRBsdetectedbydifferentinstrumentscanprovideusefulinformationtotesttheSSM.–4–Oursecondgoalistodiscussthepossibleexistenceofanadditionallow-energycomponentintheGRBspectrum(Sect.4).Thisextracomponent,mostlikelyobservableintheX-rayenergyrange,mightbeeasilydetectableasalow-energyexcessduringtheGRBpulseemission,orasacomponentprecedingorfollowingGRBmainpulses.OuranalysisallowstoeasilyidentifyspectralcomponentsadditionaltotheSSMpulseemission.WediscussinSect.5attenuationandabsorptionprocessespossiblyaffectingthedetectionofX-rayfromGRBs.WefinallydiscussinSect.6thefeasibilityofasearchforGRB-drivenX-rayflashesfromothergalaxiesandinparticularfromAndromeda.WeconsiderROSATarchivaldataandfutureSAXobservationsofAndromedaasexamplesofdatausableforthissearch.2.TheSSMmodelTheSSMisbasedonopticallythinsynchrotronemissionofrapidlyacceleratedrelativisticparticles(electronsand/ore±-pairs)radiatinginthepresenceofaweaktomoderatemagneticfield(toavoidmagneticabsorptionprocesses)(Tavani1995,T96a,b).Atarget‘nebular’medium,abletoreprocesstherelativisticenergyoftheflowandtotriggerrapidaccelerationprocesses,isnecessary.TheultimateoriginofGRBmagnetizedrelativisticparticleflowscanbecompactstarcoalescencesatcosmologicaldistancesorcompactstaroutburstsinanextendedGalactichalo(forarecentreview,seeFishman1996).Thetargetmediumcanbetheinterstellarmedium,gaseouscircumstellarmaterialorself-generatedgaseousenvironments.SSMcanbeappliedinitsgeneralitytoboththecosmologicalandGalacticinterpretationsofGRBs,eventhoughimportantdifferencesbetweenthesetwomodelsariseintheradiationprocessesandoveralldynamics(Tavani1996d,hereafterT96d).AnMHDwindisassumedtointeractinanoptically-thinenvironmentwithmagneticturbulenceorhydromagneticshocksleadingtorapidparticleaccelerationandtotheformationofaprominentsupra-thermalcomponent.TheSSMrelevantphysical