Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(24)

WithessentiallynoindicationofrecentstarformationinG1,massiveprogenitorstarsleadingtocol-lapsarscannotbepresentinG1.S1andS2,thebrightestandthird-brightestsourceswithintheXRTerrorcircle,havenoindicationofrecentstarformationiftheirredshiftsare 1.3(SFR<0.05M⊙year 1forz<0.3andSFR<1M⊙year 1forz<1.2).Thefainter(andblue)objectsdiscussedin§5.1arelikelytobebackgroundgalaxies.IftheoriginofGRB050509bisfromacollapsar,itislikelythatitsredshiftexceeds

1.3.

IfGRB050509bisabackgroundobjectatz 2,someprogenitorscenariosaredi culttoreconcile.Withanobserveddurationof～30msec,therest-framedurationwouldbeonlyabout10msec.ThisisimplausiblyshortforanNS–NSmerger,andmarginallypossibleforaBH–NSmergerifthecoalescenceisthroughunstablemasstransfer(Lee&Klu´zniak1999;Rosswog2005;Miller2005).Itishardtosimulta-neouslyaccommodatetheshortintrinsictimescaleandthehigherenergybudgetoftheburstwithinanycompactmergermodel,ifitisathighredshift.

IfshortGRBstracestarformationwithatimedelaythroughdoublecompactmergerswithcoalescencetimescalesof107–1010yr(asopposedtoprompttracersofstarformationaswiththecollapsarscenarioforlongGRBs;Bagotetal.1998andBloometal.1999),thenweexpectsomefraction(10–30%)ofshortGRBstobeseeninassociationwithearly-typegalaxiesingeneralandclustersspeci cally(seeNutzmanetal.2004forratedensityinthelocaluniverse).Thisissomewhatmodeldependent,sincethedistributionofcompactmergertimescalesispoorlyconstrainedbydata,butbroadlyconsistentwithbothobservedandmodeldistributions.

Acore-collapsesupernova(SN)producesnoelectromagneticradiationuntilitsenvelopeiscompletelyconsumedbytheexplosion(althoughseeKhokhlovetal.1999).Thisphaseends,however,withabrilliant ashofX-rayorextremeultravioletphotonsastheshockreachesthestellarsurface.The“breakout” ashisdelayedintime,andvastlyreducedinenergy,relativetotheneutrinotransientproducedbycorecollapse.