Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(25)

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

However,itconveysusefulinformationabouttheexplosion.Shockbreakout asheswerepredictedbyColgate(1968)asasourcefor(thethenundetected)gamma-raybursts.TheexplosionofSN1987Astimulatedareanalysisofsupernovabreakout ashesbyEnsman&Burrows(1992)and,morerecently,byBlinnikovetal.(1998)andBlinnikovetal.(2000).Thesestudiesrepresentanincreaseinsophisticationtowardthefullnumericaltreatmentofthiscomplicated,radiation-hydrodynamicproblem.Inprinciple,theXRTdatacouldconstraintheexistenceofashockbreakoutproducedbybotharedsupergiantexplosionlikeSN1993J(VanDyketal.2002)andabluesupergiantexplosionanalogtoSN1987A,buttheX-rayluminosityissensitivetotheuncertaindistributionofextragalacticgascolumnandthespeci cXRTobservingepochs.

UsingourESIopticalimaging,wecanalsolimitthepresenceofbrighteningduetoasupernovaorsupernova-likeemissionat8.17daftertheGRBtoRc≈25.0mag.Anormal,unextinguishedTypeIa(thermonuclear)supernovaatz=0.22wouldhaveR≈22mag,around6.7dafterexplosion(t=8.17dintheobserver’sframe).AverysubluminousSNIalikeSN1991bg(Filippenkoetal.1992)wouldhaveR≈24mag,stillsomewhatbrighterthanourlimit.ExtinctionwouldobviouslymaketheSNfainter,buttheMilkyWaycontributionissmall(AV≈0.06mag;Schlegeletal.1998),andtheoutskirtsofanellipticalgalaxyinaclustershouldhaveessentiallynodust.Whilesomecore-collapsesupernovaecouldbeasfaintas(orfainterthan)ourlimit,thepresenceofsuchasupernovaintheoutskirtsofanellipticalgalaxywouldbetrulyextraordinary(seevandenBerghetal.2005).OthershavealsoreportednoevidenceforaSNatlater-times(Hjorthetal.2005;Bersieretal.2005).

Thelocationofthis(andfuture)shortburstprovidesausefuldiscriminantfordistinguishingbetweendi erentprogenitormodelsofshortbursts.Simplistically,wewouldexpectevaporatingblackholestooccurnearthecenterofdeeppotentialwells(asdiscussedinthecontextofGalacticBHs;Clineetal.1999);thus,theo setfromG1seemstodisfavorthishypothesis.Agiant arefromamagnetarwouldneedtohaveaisotropicluminosity(Lγ,iso)largerbyafactorof～103andanEγ,isolargerbyafactorof～102comparedthetheinitialspikeofthe27December2004giant arefromSGR1806-20(thedi erenceinthefactorbetweenthetwoquantitiesarisessinceGRB050509blastedonly～30ms,whichis～10timesshorterthantheinitialspikeofthegiant arefromSGR1806-20).Burstsfrommagnetarsmightbeexpectedfromlater-typegalaxiesthanG1whereneutronstarswouldbeformedcopiously:magnetic elddecaywouldcuttheactivelifetimeformega areactivityafter～104yr.

8.Conclusions

WehavemonitoredthelocationofGRB050509batopticalandinfraredwavelengthsfrom8minutesto8daysafterthetriggerandfoundnoindicationofvariabilityatthelocationofthefadingX-raysource,the rstsolidX-raydetectionofanafterglowofashort-hardburst.Nearthelocationofthissourceweandothershavefoundanapparentgroupoffaintbluegalaxiesatredshifts 1.3.Whileitisindeedplausiblethatthisshortburstarosefromaprogenitorconnectedwiththosegalaxies,wefound—basedonapositionalargument—plausibleevidencethattheprogenitorislikelyassociatedwithG1,abrightellipticalgalaxyatz=0.2248.Wehavearguedthattheobservations ndnaturalexplanationwithacompactmergersystemprogenitor.Ifso,thenshort-hardGRBsprovideabridgefromelectromagnetictogravitationalwaveastronomy:indeedhasGRB050509boccurredafactorof～3closerinluminositydistanceismighthaveproducedadetectablechirpsignalwiththenextgenerationLIGO-IIgravitationalwavefacility9.