The localization of the short-duration, hard-spectrum GRB 050509b was a watershed event. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. No
ClosinginonaShort-HardBurstProgenitor:ConstraintsfromEarly-TimeOpticalImagingandSpectroscopyofaPossibleHostGalaxyofGRB050509bJ.S.Bloom1,J.X.Prochaska2,D.Pooley1,3,C.H.Blake4,R.J.Foley1,S.Jha1,E.Ramirez-Ruiz5,2,3,J.Granot6,5,A.V.Filippenko1,S.Sigurdsson7,A.J.Barth8,H.-W.Chen9,M.C.Cooper1,E.E.Falco4,R.R.Gal10,B.F.Gerke11,M.D.Gladders12,J.E.Greene4,J.Hennanwi1,13,L.C.Ho12,K.Hurley14,B.P.Koester15,W.Li1,L.Lubin10,J.Newman16,13,D.A.Perley1,G.K.Squires17,andW.M.Wood-Vasey4
arXiv:astro-ph/0505480v2 5 Sep 200512DepartmentofAstronomy,601CampbellHall,UniversityofCalifornia,Berkeley,CA94720-3411.UniversityofCaliforniaObservatories/LickObservatory,UniversityofCalifornia,SantaCruz,CA95064.34ChandraFellow.Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138.56InstituteforAdvancedStudy,OldenLane,Princeton,NJ08540.KIPAC,StanfordUniversity,P.O.Box20450,MailStop29,Stanford,CA94309.7DepartmentofAstronomy&Astrophysics,525DaveyLaboratory,PennsylvaniaStateUniversity,UniversityPark,PA16802.8DepartmentofPhysics&Astronomy,4129FrederickReinesHall,UniversityofCalifornia,Irvine,CA92697-4575.9CenterforSpaceResearch,MassachusettsInstituteofTechnology,Cambridge,MA02139-430710DepartmentofPhysics,OneShieldsAve.,UniversityofCalifornia,Davis,CA95616-8677.11DepartmentofPhysics,366LeConteHall,UniversityofCalifornia,Berkeley,CA94720-7300.12CarnegieObservatories,813SantaBarbaraStreet,Pasadena,CA91101.13HubbleFellow.14UCBerkeley,SpaceSciencesLaboratory,7GaussWay,Berkeley,CA94720-7450.
15DepartmentofPhysics,UniversityofMichigan,AnnArbor,MI48109-1090.
16InstituteforNuclearandParticleAstrophysics,LawrenceBerkeleyNationalLaboratory,Berkeley,CA
94720.
17SpitzerScienceCenter,CaliforniaInstituteofTechnology314-6,Pasadena,CA91125.
ABSTRACT
Thelocalizationoftheshort-duration,hard-spectrumgamma-rayburstGRB050509bbythe
Swiftsatellitewasawatershedevent.NeverbeforehadamemberofthismysterioussubclassofclassicGRBsbeenrapidlyandpreciselypositionedinaskyaccessibletothebevyofground-basedfollow-upfacilities.ThankstothenearlyimmediaterelayoftheGRBpositionbySwift,webeganimagingtheGRB eld8minutesaftertheburstandcontinuedforthefollowing8days.ThoughtheSwiftX-rayTelescope(XRT)discoveredanX-rayafterglowofGRB050509b,the rsteverof
The localization of the short-duration, hard-spectrum GRB 050509b was a watershed event. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. No
ashort-hardburst,noconvincingoptical/infraredcandidateaftergloworsupernovawasfound
fortheobject.Wepresentare-analysisoftheXRTafterglowand ndanabsolutepositionof
R.A.=12h36m13.s59,Decl.=+28 59′04′′.9(J2000),witha1σuncertaintyof3′′.68inR.A.,3′′.52
inDecl.;thisisabout4′′tothewestoftheXRTpositionreportedpreviously.Closetothis
positionisabrightellipticalgalaxywithredshiftz=0.2248±0.0002,about1′fromthecenter
ofarichclusterofgalaxies.Thisclusterhasdetectabledi useemission,withatemperatureof
.36kT=5.25+3 1.68keV.Wealso ndseveral(～11)muchfaintergalaxiesconsistentwiththeXRT
positionfromdeepKeckimagingandhaveobtainedGeminispectraofseveralofthesesources.
Neverthelessweargue,basedonpositionalcoincidences,thattheGRBandthebrightelliptical
arelikelytobephysicallyrelated.Wethushavediscoveredevidencethatsupportsthenotion
thatatleastsomeshort-duration,hard-spectraGRBsareatcosmologicaldistances.
Wealsoexploretheconnectionofthepropertiesoftheburstandtheafterglow, ndingthat
GRB050509bwasunderluminousinbothoftheserelativetolong-durationGRBs.However,we
alsodemonstratethattheratiooftheblast-waveenergytotheγ-rayenergyisconsistentwith
thatoflong-durationGRBs.Thissuggestsacomparablyhighe ciencyofγ-rayconversionas
inlongGRBsasmightbeexpectedifthesameemissionmechanismisatworkinshortandlong
GRBs.Basedonthisanalysis,onthelocationoftheGRB(40±13kpcfromabrightgalaxy),on
thegalaxytype(elliptical),andthelackofacoincidentsupernova,wesuggestthatthereisnow
observationalconsistencywiththehypothesisthatshort-hardburstsariseduringthemergerofa
compactbinary(twoneutronstars,oraneutronstarandablackhole).Inthiscontext,welimit
thepropertiesofaLi-Paczy´nski”mini-supernova”thatispredictedtoariseon～daytimescales.
Otherprogenitormodelsarestillviable,andadditionalrapidlylocalizedburstsfromtheSwift
missionwillundoubtedlyhelptofurtherclarifytheprogenitorpicture.
Subjectheadings:gammarays:bursts,gamma-raybursts:individual:050509b
1.Introduction
Thedistributioninduration(Mazetsetal.1981;Norrisetal.1984)andhardness(Kouveliotouetal.1993)revealsevidencefortwodistinctpopulationsofclassicgamma-raybursts(GRBs):long-durationbursts,withtypicaldurationsaround30sandpeakenergiesat～200keV,andtheminorityshort-durationbursts,withdurationsofafewhundredmilliseconds(ms)andharderspectra.Despiteremarkableprogressinunderstandingthenatureandprogenitorsoflong-durationGRBs,comparativelylittlehasbeenlearnedabouttheoriginofshort-hardbursts,primarilybecauseveryfewsuchburstshavehadrapidandpreciselocalizations.
Themodeledburstingrateatredshiftz=0oflong-softburstsoutnumbersshort-hardburstsbyaboutafactorof3.5intheBATSEcatalog(Schmidt2001);thisassumesthesameburstingrateasafunctionofredshiftanddoesnotincludethee ectofbeaming,which,ifdi erentforlongandshortbursts,wouldimplythattheintrinsicrelativeratesdi erfromthoseobserved.WhileanumberofburstshavebeentriangulatedthroughtheInterplanetaryNetwork(seeHurleyetal.2005b)onroughlyday-longtimescales,therehasonlybeenonepreciselylocalizedshort-hardburstrelayedtogroundobserversinlessthan1hr(GRB050202/Swift:Tuelleretal.2005)1;owingtoitsproximitytotheSunattimeoflocalization,sparsegroundbasedfollowup
The localization of the short-duration, hard-spectrum GRB 050509b was a watershed event. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. No
wasundertaken.IncludingGRB050509b,thiscorrespondstoaratioof1:18forshort-hardtolong-softburstdetectionswithSwift,muchsmallerthantheBATSEresult.2
Aswithlong-durationbursts,thedistributionofshortburstsappearsverynearlyisotropic(Kouveliotouetal.1993;Briggsetal.1996),andtheirbrightnessdistribution(<V/Vmax>≈0.35)isconsistentwithbeingacosmologicalpopulation.Still,thereisnostrongevidencetosupporttheideathatshortburstsarepreferentiallyseenfromz<～0.37richAbellclusters(Hurleyetal.1997),noraretheyclearlyconnectedwithstarformationwithin～100Mpc(Nakaretal.2005).
Withoutpreciseandrapidlocalizations,thepopulationstatisticsdonotprovideastrongconstraintontheshort-burstprogenitors.Still,ithasbeenlargelyreckonedthattheleadingcandidatesforshortburstsarethemergerofaneutronstarbinary(NS–NS;Blinnikovetal.1984;Paczy´nski1986,1991;Narayanetal.1992;Katz&Canel1996;Ru ert&Janka1999;Rosswog&Ramirez-Ruiz2002;Rosswogetal.2003)orablackhole–neutronstarbinary(BH–NS;Lattimer&Schramm1976;Eichleretal.1989;Mochkovitchetal.1993;Kluzniak&Lee1998;Bethe&Brown1999;Pophametal.1999;Fryeretal.1999).Thesesystemsholdseveralparticularattractions.First,althoughuncertain,theestimatedrateofmergers(between1.5–20per106yrpergalaxy;Belczynskietal.2002;Sipior&Sigurdsson2002;Rosswogetal.2003)iscomparabletotheshort-burstrate(Schmidt2001).Second,thedynamicaltimescaleofsuchmergersisseveralmillisecondsandthesound-crossingtimesareofordertenmilliseconds,comparabletotheshortestobservedbursts(Miller2005).Third,compactmergersystemsarelikelytocontainenoughmass-energyinatransienttorustopowershort-burst uencesaswouldbeobservedifatcosmologicaldistances(Rosswogetal.2003;Leeetal.2004;Rosswog2005).Thetypicaldynamicaltimescaleinsuchbinariesimmediatelypriortocoalescence(ms)ismuchshorterthantheobservedburstduration,andsoitrequiresthecentralenginetoevolveintoacon gurationthatisstable,whileretainingasu cientamountofenergytopowertheburst(Leeetal.2004).
Mergersofsuchcompactremnantsarebynomeanstheonlypossiblechanneltoproduceshortbursts.Evaporatingprimordialblackholesmayproduceshort(<100ms)GRBs(Clineetal.1999),thoughbasicenergeticsargumentssuggestthatitwouldbedi culttoseesuchsourcesfromdistanceswellbeyondtheGalaxy.Therecentdiscoveryofamega arefromSGR1806 20(Mereghettietal.2005;Hurleyetal.2005a;Palmeretal.2005;Terasawaetal.2005)ledtoplausiblesuggestionsthatasubstantialfraction(≈40%)ofshortburstscouldbeproducedbyextragalacticmagnetars(Hurleyetal.2005a).However,positional(Palmeretal.2005;Nakaretal.2005)andspectral(Lazzatietal.2005)argumentshaveledotherworkerstosuggestthatatmostafewpercentoftheBATSEcatalogcouldconsistofshort-burstmagnetars.Notethatnotallcompactmergerscreatefertileconditions(atransienttorusaroundaBH)formakingashortburst(e.g.,Janka&Ru ert2001;Rosswogetal.2004).Thedurationoftheburstinacompactbinarymergerisdeterminedbytheviscoustimescaleoftheaccretinggas,whichissigni cantlylongerthanthedynamicaltimescale,thusaccountingnaturallyforthelargedi erencebetweenthedurationsofburstsandtheirfastvariability(Leeetal.2004).Inthecollapsarscenarioforlong-durationbursts,ontheotherhand,theburstdurationisgivenbythefall-backtimeofthegas(Woosley1993;MacFadyen&Woosley1999),whichistypicallygreaterthanafewseconds.However,amodi edcollapsarscenarioinwhichtheburstdurationisdeterminednotbyfall-backbutratherbythedynamicaltimescalesassociatedwiththeexpandingout ow
table.html.
The localization of the short-duration, hard-spectrum GRB 050509b was a watershed event. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. No
mightstillmeettheconstraintsofshortGRBs(Woosley2001).
ThetheoreticalpredictionsfortheafterglowsofshortGRBshavebeenconsideredbyPanaitescuetal.(2001).Sincethepeak uxofthepromptemissioniscomparableforshortandlongGRBs,iftheirdistancescalesaresimilartheisotropicequivalentenergyoutputingammarays(Eγ,iso)wouldbeproportionaltothedurationoftheGRB,whichis～10 100timeslargerforlongGRBs.Ifthee ciencyforproducingthegammaraysiscomparable,thentheisotropicequivalentkineticenergyintheafterglowshock(Ek,iso)wouldhaveasimilarratiobetweenlongandshortGRBs.ThiswouldimplytheafterglowofshortGRBstobeonaverage～10 40timesdimmerin uxthanthatoflongGRBs.TheafterglowsofshortGRBswouldbeevenmuchdimmerthanthisiftheyencounteramuchsmallerexternaldensitycomparedtolongGRBs;thisistheexpectationfromshort-burstsfrombinarymergersoutsideofthehostgalaxy.Panaitescuetal.(2001)arguedthatalowexternaldensitywouldnota ecttheX-rayband,asthelatterwasassumedtolieabovethecooling-breakfrequency,νc.We ndthatforaverylowexternaldensitytheelectroncoolingbecomesveryslowsothatνccanlieabovetheX-raybandforthe rstfewdays,thusreducingtheX-ray uxcomparedtothatforahigherexternaldensitytypicaloftheinterstellarmedium(ISM)foundnearstar-formingregionsoflong-durationGRBs.
Todatethedeepestearly-timeobservations( t<～1hr)yieldedupperlimitsVlim≈14magfromthe0.3mROTSE-Iexperiment(Kehoeetal.2001).Hurleyetal.(2002)compileddeepernon-detectionsatopticalandradiowavelengthsattimesfromdaystoweeksafterfourshortbursts,withthefaintestnon-detectionofR≈22.3magat t=20hr(seealsoGandol etal.2000).Clearly,deepandearlyobservationsinsearchofashort-burstafterglowwouldrequirearapidlocalizationtoanuncertaintycomparabletothe eldofviewofmeter-class(andlarger)telescopes.
GRB050509b(Gehrelsetal.2005)triggeredtheBATcoded-maskimageron-boardSwifton9May200504:00:19.23(UTdatesandtimesareusedthroughoutthispaper;Hurkettetal.2005).ThepositionofGRB050509b,withanuncertaintyof4′radius,wasrelayedtothegroundwithinafewseconds.TheinitiallocalizationwaslaterrevisedtoapositionR.A.=12h36m18s,Decl.=+28 59′28′′,witha95%con denceerrorradiusof2.8′(Barthelmyetal.2005a).Barthelmyetal.(2005a)describetheburstasasingle-peakedsourcewithdurationof～30ms,peak uxof2100countss 1(15–350keV),andahardnessratioconsistentwiththatoftheshort-hardpopulation.At06:29:23,afadingX-raysourcewasreportedwitha6′′localization(Kenneaetal.2005)andlaterupdatedtoan8′′uncertaintyradiusatpositionR.A.=12h36m13.9s,Decl.=+28 59′01′′(Roletal.2005).
GRB050509bthusrepresentsthe rstshort-hardburstlocalizedinrealtimetoapositionsuitableforimmediatefollow-upobservationsfromasuiteofground-basedfacilities.Inthispaperwedescribetheresultsofourobservationsofthe eldofGRB050509bandwhatbearingthesedatahaveonthenatureofshortburstsandthephysicsofshort-burstafterglows.In§2wedescribeimagingandspectroscopyofthe eld.OuranalysisoftheX-rayafterglowofGRB050509bisgivenin§3,leadingtoalocalizationnearanellipticalgalaxy(§4).In§5wepresentaspectrumofthatgalaxy,itsredshift,andinferredproperties.Wethenargue,onstatisticalgrounds,foraplausibleassociationofthisgalaxyandtheGRB.Wedemonstratein§6howGRB050509bappearstobeasubluminousburstrelativetolong-durationGRBs,butwitharatioofblast-waveenergytogamma-rayenergythatisconsistentwiththelong-durationpopulation.Intheremainingsectionswedescribenewconstraintsonthenatureofshort-burstprogenitors.Throughout,weassumeaconcordancecosmologywithH0=70kms 1Mpc 1, Λ=0.7,and m=0.3.Alloftheresultspresentedherein,thoughgenerallyconsistentwithourpreviousresultsinGCNCirculars,supersedethem.
The localization of the short-duration, hard-spectrum GRB 050509b was a watershed event. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. No
2.ObservationsandReduction
Initially,severalgroupsreported(Ryko etal.2005;Ugarteetal.2005;Bloometal.2005a;Torii2005)nonewoptical/infraredsourcethatwasconsistentwiththeXRTpositionofGRB050509b(Kenneaetal.2005).At07:21:27wehighlightedtheproximityoftheXRTtoabrightredgalaxy(hereafterG1=2MASXJ12361286+2858580)andsuggestedaplausiblephysicalassociation(Bloometal.2005a)basedonitspresumedmembershipinaz≈0.22cluster(Barthelmyetal.2005a).WelaterreportedthedeterminationoftheredshiftinProchaskaetal.(2005)andProchaskaetal.(2005).At08:44:13wenotedthepresenceofafaint,compactsource(hereafterS1;see2)intheoutskirtsofG1,whichwedeemedaplausiblecandidatecounterpart(Bloometal.2005b).Averysimilarsuggestionwasmadeat09:36:49byCenkoetal.(2005c);inaddition,theynotedapparentvariabilityofthecandidate(laterretractingthevariabilityclaiminCenkoetal.2005b)anddetectionofthreeotherfaintsources(S2–S4)consistentwiththeXRTposition(seealsoCenkoetal.2005a).Twoadditionalsources(S5andS6)intheXRTlocationweresubsequentlynotedfromVeryLargeTelescope(VLT)imagingbyHjorthetal.(2005),followedbyanother5sources(J1–J5)reportedbyBloometal.(2005).Noradioemission(Parkinson2005b;vanderHorstetal.2005)orGeV/TeVemission(Parkinson2005a)isconsistentwiththeXRTerrorlocalization.Belowwediscusstheobservations,andfurtherinterpretation,leadingtothesereports.
2.1.OpticalandInfraredImaging
Weobservedthe eldofGRB050509bonMay9withtheWIYN3.5mtelescopeandtheOPTICCCDimagerwitha9.6′×9.6′ eldofviewandaplatescaleof0.14′′/pixel.Underpoor(～2′′)seeingconditions,twoexposurestotaling360swereobtainedinthei′bandbeginningat04.344hr.Inaddition,weobtained2400sofintegrationinther′bandunderimprovedseeingconditions(～1′′)beginningat06.088hr.
Thedatawerereducedintheusualmannerusing at- eldsfromboththeilluminateddomeandthetwilightsky.TheastrometricsolutionstotheindividualimageswerecalculatedbycomparisontotheUSNOB-1.0catalogwitharoot-mean-square(rms)residualof0.1′′.Thephotometriczero-pointsoftheimageswerecalculatedbycomparisontomorethan50starsintheSloanDigitalSkySurvey(SDSS)pho-tometryprovidedbyEisensteinetal.(2005).Thezero-pointsoftheWIYNimagesareuncertainataboutthe3%level.Limitingmagnitudeswereestimatedfromthehistogramof uxesin104seeing-matchedaperturesplacedrandomlywithinthe eld.Thedispersion(σ)ofaGaussian ttedtothisdistributionwasusedtoestimatethe5σlimiting uxineachimage,whichwasconvertedtoamagnitudeusingtheknownzero-point.
ThebrightgalaxyG1tothewestoftheXRTpositioncontaminatesasigni cantportionofthe8′′radiusXRTerrorcircle.Weusedgal t(Pengetal.2002)to tasmoothS´ersicpro letothisgalaxyinordertoremovemostofthecontaminantlightpriortoexaminingtheXRTerrorcircle.Aseriesof1000seeing-matchedaperturesplacedrandomlywithintheXRTerrorcircleidenti ednonewsources.ThefaintgalaxyS1wasdetectedatthe>5σlevelinourdeeperr′images.
Near-infraredimageswereobtainedwiththe1.3mPAIRITELintheJ,H,andKsbands(seeBlakeetal.2005).Observationsconsistedofa1130sintegrationcomprisedof7.8sditheredexposuresbeginningat04.1375hr.Thesedatawerereducedbymedian-combiningsetsofindividualexposureswithinamoving5-minutewindow.Theresultingmedianwasusedtosubtractthebrightskyfromtheindividualimages.Finally,alloftheindividualimageswerecombinedtomakehigh-resolutionmosaicsusingamodi edversionofdrizzle(Fruchter&Hook1997).Zero-pointsweredetermined2MASSstarsinthe eld.UpperlimitsintheJ,H,KsmosaicswereestimatedusingthesametechniqueasfortheWIYNdata.TheWIYNand
The localization of the short-duration, hard-spectrum GRB 050509b was a watershed event. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. No
PAIRITELupperlimits,aswellaslimitsreportedintheliterature,areshowninFigure1.
Welaterimagedthe eldofGRB050509bwiththeKeckI10mtelescopeandtheLRIS-Binstrument(Okeetal.1995)usingthedichroicD560(50%transmissionpointat5696 A)withGandR lters.Startingat11.25May2005,beginningnearastronomicaltwilight,wetook5ditheredimagesineachbandforatotalof1660sand1620sexposuresinGandR,respectively.Thedatawerereducedintheusualmannerandcombined,weightedbyexposuretimes.
On17May2005,08:05.5,8.17daftertheGRB,weobtaineddeepR(Ellis)(whichissimilartoHarrisR;Baconetal.2003)imagingontheEchelleteSpectrographandImager(ESI;Sheinisetal.2002)ontheKeckII10mtelescope.InthepresenceofbrightglarefromtheMoon,wecombinedseveralreducedimagesforane ectiveexposuretimeof960s.SincethereisanegligiblysmallcolorterminconvertingHarrisRtoRc,3wefoundazero-pointrelativetotheLRISRimage.TherearenonewsourcestoRc≈25.0mag(5σ),norsigni cantvariationsofthefaintsourcesintheXRTerrorcircle.
3.TheX-rayEmission
TheSwiftXRT(Burrowsetal.2000)beganobservationsofGRB050509bon2005May9at04:00:56,approximately61saftertheBATtrigger.Theobservationsconsistedofelevenblocks,eachabout2.5ksinduration(exceptthe rstobservationof1.6ksandthelastobservationof1.8ks),spreadoveraperiodof～21hr.TheXRToperatedinanumberofdi erentmodesthroughouttheobservations.Themostcommon(32.3ksofexposure)andmostusefulmodeforthisobjectwasthe“PhotonCounting”mode,whichretainsthefullimagingandspectroscopicresolutionoftheinstrument.Theimagesare480×480pixels,withascaleof2′′.36perpixel.TheXRTpoint-spreadfunctionisenergydependent,withahalf-powerdiameterof′′18at1.5keV.Theenergyresolutionisalsoafunctionofenergy,varyingfromabout50eVat0.1keVtoabout190eVat10keV.
The rstPhotonCountingobservationbeganat04:01:20(Kenneaetal.2005)andlasted1640s.AsnotedinKenneaetal.(2005)andRoletal.(2005),afaintX-raysourceisdetectedinthis rstoftheelevenobservations,butitfadedquicklybelowthebackground.WehaveobtainedtheXRTdatafromtheSwiftarchive,andhaveanalyzedthemtodeterminethepositionofthisX-rayafterglowcandidateaswellastoexamineitsvariability.Webrie yreviewthedatareduction,andthenwediscussthelocalizationoftheafterglowcandidateandattempttoquantifythedecay.
3.1.SwiftDataReduction
UsingtheLevel1datafromtheSwiftarchive,weranthexrtpipelinescriptpackagedwiththeHEAsoft6.0softwaresuppliedbytheNASAHighEnergyAstrophysicsScienceArchiveResearchCenter4.Weusedthedefaultgradeselection(grades0to12)andscreeningparameterstoproduceaLevel2event lere-calibratedaccordingtothemostcurrent(asof2005May15)calibration lesintheSwiftdatabase5.Toproduceimagesforsourcedetection,weusedthexselectsoftware(alsopartofHEAsoft6.0),witha lterto
The localization of the short-duration, hard-spectrum GRB 050509b was a watershed event. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. No
includeonlycountsinPIchannels30–1000(correspondingtophotonenergiesof0.3–10keV).ThePIchanneltophotonenergyconversionwasaccomplishedwiththeredistribution leswxpc0to12
2seriesstartingat4pixels(4,5.657,8,11.314,16),andthesigni cancethresholdwassetat4×10 6,correspondingtoa～1falsepositivedetectionofapointsourceintheimage.Wedetect22compactsourcesintheentire32.3ksdataset.
Tostudythepropertiesoftheafterglowcandidate,weextractedthealltheeventswithinanareaofradius10pixelsaroundthenominalwavdetectposition.Inthe rstobservationof1.6ks,thereare14countsinthisregion.Whenexaminingaplotofthecumulativedistributionversustime,wenoticedthatthemajorityofthecountsfromthisregionoccurredinthe rst300s.Wethereforefurtherinvestigatedthisbriefinterval.
Inthe rst300softhe rstPhotonCountingobservation,theXRTdetected92countsontheentirechip,with73ofthemoutsideofthe22sourceregions.Withinany10-pixelradiussourceregion,wethereforeexpectanaverageof0.1backgroundcounts.Wedetect9countsinthisregionoftheX-rayafterglow,ingthemeanlocationofjustthese9counts,wecanobtainarelativelyuncontaminatedestimateofthesourceposition.Wecalculatethe68%√con denceintervalineachdirectionasTσj/
isdistributedbytheNationalOpticalAstronomyObservatory,whichisoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Inc.,undercooperativeagreementwiththeNationalScienceFoundation.6IRAF
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