Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(22)

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

lessenergeticthantypicallongGRBs(seeFigure9).

AsalsoarguedbyLeeetal.(2005),thefactthattheX-rayafterglowluminosityofGRB050509bismuchsmallerthanthatoflongGRBsisprobablybecausetheeventwassub-energetic,ratherthanduetodi erencesonthevaluesoftheexternaldensityorthemicrophysicalparameters.ThisisillustratedinFigure8bya ttothecurrentlyavailableafterglowdatausingparametervaluesthataretypicalforlongGRBs,exceptfortheisotropicequivalentenergyintheafterglowshock,Ek,iso,whichisheretakentobeequaltoEγ,isoassumingz=0.2248.Otherparametervaluescouldalsogiveareasonabledescriptionoftherathersparsedata.InTable3wedemonstrateafewdi erentsetsofparametersthat ttheafterglowdata.Again,wereferthereadertoLeeetal.(2005)foramoredetaileddescription.Regardlessoftheredshift,itwillbeverydi culttodetecttheafterglowintheradio,sincethemaximal uxdensity(giventheobservationalconstraints)isunlikelytoexceed～15µJy.

IfshortGRBsoccursigni cantlyoutsideoftheirhostgalaxies,asmaybecommonforbinarymergers(Tutukov&Yungelson1994;Bloometal.1999;Fryeretal.1999;Buliketal.1999;Belczy´nskietal.2000),thenonemightexpecttheexternaldensityencounteredbytheafterglowshockofsomeGRBstobeverylow,typicaloftheintergalacticmedium(IGM),nIGM≈10 6.5(1+z)3cm 3.Thismayhelpexplainwhysomeshortburstscouldhaveveryfaintafterglows.SinceGRB050509bhappenedtooccurnearthecenterofagalaxyclusterwheretheexternaldensityisrelativelyhigh,itsX-rayafterglowwasrelativelybrighter.IfindeedGRB050509bisassociatedwiththegalaxyclusteratz≈0.22,thenonemightexpecttheexternaldensitytobeintermediatebetweentheIGMandISM:theGRBis～76′′fromthecenteroftheclusterasdeterminedbytheX-rayposition(§3.4),correspondingto～270kpcinprojectionandwellwithinthedi useemissionfromthehotintraclustermediumgas(whichextendstoaradiusof～1Mpc).ThissuggestsanambientdensitynearthepositionoftheGRBofn≈10 3–10 2cm 3,thoughthisestimateisuncertainbecausethespacepositionoftheburstrelativetotheclustercenterandtheintraclustermedium(ICM)densitypro learenotknownprecisely.

7.Discussion

ThelackofastrongafterglowsignaturesetsGRB050509bapartfrommostotherGRBs.8Asacom-parison,thelow-redshiftlong-softburst(GRB030329,z=0.1685;Greineretal.2003),ifplacedattheredshiftofG1,wouldhavebeenR≈14magatt=8000s;thisisapproximate10magbrighterthanthedetectionlimitsfoundherein.Evenatz=3,theopticalafterglowofaGRB030329-likeburstshouldhavebeendetectedatearlytimes(neglectingthee ectsofdustextinction).Ournon-detections(R>～24mag)ofvariabilityat1.3hrinthewhatwouldbetherestframeatz=1ismorethan3.5magnitudesdeeperthanthefaintestopticaltranisentfoundforalongGRB(021211;z=1.0;see g.2ofFoxetal.2003).

Thelackofdetectableoptical/infraredafterglowisnotsurprisingongroundsrelatedtotheprogenitorsandtoGRBafterglowtheory.First,sincetheluminosityoflong-wavelengthafterglowsscaleswiththesquarerootoftheambientdensity(Begelmanetal.1993;M´esz´arosetal.1998),eventsthatoccurintheISMorIGMshouldbeintrinsicallyfainter(atoptical/infraredwavelengths)thanthoseoccurringinthecircumburstenvironmentsofcollapsars(seePanaitescuetal.2001).Second,basedon<V/Vmax>studies,theisotropic-equivalentpeakluminosity(Lp(γ))ofshortburstsissimilartothatoflongbursts(Schmidt

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