Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(2)

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.