Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(15)

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

expectedtobesmall;forexample,Grossman&Nowak1994).AreasonableestimateofthecoveringfractionofclustersontheskyisgivenbythetheDPOSSNorthernSkyOpticalClusterSurvey(Galetal.2003).Althoughthissurveyisnotverydeep(zlim～0.3),low-redshiftclustersshoulddominatetheskydensity.Galetal.(2003) ndacoveringfractionof～0.03assumingatypicalclusterradiusof1Mpc,whichsuggestsachancealignmentisimprobablebutnotimpossible.Moreover,theXRTlocalizationofGRB050509btowithin45′′ofthecenterofsuchaclusterwouldoccurbychancewithaprobabilityofjust～7×10 4,butitisdi culttoestimateaposteriorihowlargeadistancefromaclustercenteronewouldhaveconsidered“signi cant.”

Whilethegasfromtheclusterenvironmentmayenhancetheprobabilityoflocalizingshort-burstafter-glows(seebelow),ourexpectationisthatshortGRBprogenitorsarecausedbythedeathofstarsofsomesort,withtheburstratedeterminedbyprocessesonscalessigni cantlysmallerthanclusterlengths.Tothisend,weshouldconsiderthechanceprobabilityoftheGRBeventoccurringatcloseimpactparametertoagalaxysimilartoG1.AsreportedbyEisensteinetal.(2005),thegalaxyG1hasaPetrosianr′magnitudeof17.18±0.02magbasedonimagingbytheSDSS.TheskydensityofgalaxieswithcomparableapparentmagnitudebrighterthanG1is～40persquaredegree(Blantonetal.2003b).Therefore,theprobabilityofaneventrandomlyoccurringwithin20′′(abouttwicetheobservedo set)ofthisbrightgalaxyis～5×10 3.Weconsiderthisaconservativeestimatebecausethisprobabilitymakesnoreferencetothegalaxyredshift,type,size,orage(whichareconsistentwithaprioridiscussionsofshort-bursthostgalaxies;e.g.,Bagotetal.1998andBloometal.1999).

IfonearguesthatGRB050509bisindeedphysicallyassociatedwiththisbright,low-redshiftellipticalgalaxy,onemustconsiderwhytheseveralotherwell-localizedshortburstshavenotshownsimilarassoci-ations.The rstpossibility,thattheshortburstsarisefromamorelocalpopulation(assuggestedbythemagnetar arefrom27December2004;Hurleyetal.2005a)andGRB050509bmustthereforearisefromadi erentpopulation,wasdiscountedforfourofthebest-localizedshortbursts(Nakaretal.2005).AnotherpossibilityisthatGRB050509bwassigni cantlycloserthantheotherwell-localizedshortbursts.Astrongtestofthishypothesisistodetermineifothershortburstsareassociatedwithmoredistantclustersorin-trinsicallybright,massivegalaxies(e.g.,throughadeepimagingcampaign).Thethirdpossibilityissimplythatshort-burstprogenitorsneednotalwaysariseinsuchgalaxies.Infact,fortheNS–NShypothesiswewouldexpectmergersingalaxiesspanningawiderangeofHubbletypes.AdelayedBH–NSmergerisalsopossible,butlesslikelyifGRB050509bisassociatedwithG1:statistically,thedistributionoftimescalesforBH–NScoalescenceisasbroadasthatforNS–NScoalescence,albeitquitemodeldependent(Belczynskietal.2002;Sipior&Sigurdsson2002),butthesystemickickvelocityisexpectedtobesystematicallylowerbyafactorofafewinmosttheoreticalmodelsofformationofBH–NSbinaries(kickvelocityisroughlyinverselyproportionaltomass,somoremassivebinariesreceivelesskick).Moreover,largervelocitykicksgenerallyleadtoshortermergertimes.Thusthe～40kpco settendstofavorNS–NSoverBH–NSmergers.