Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(27)

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

Brighteningemissionfrommosttypesofsupernovaewouldhavebeenseeninourimaging,sothelackofsuchemissionappearsinconsistentwiththenotionthatshortburstsareduetocollapsarsorvariantsthereof.Ourafterglowmodelingisalsoconsistentwith,thoughdoesnotrequire,acircumburstmediumhavinglowerdensitythanthatinferredinlong-durationGRBs;iftrue,thiswouldsuggestthattheprogenitorproducesaGRBinanenvironmentthatisbaryonpoorcomparedtothatexpectedforcollapsars.Moreover,wehaveseennoevidenceforongoingstarformationintheputativehost,sotherearelikelynoremainingmassivestars.Giventheshortactivelifeofaneutronstarhavingahighmagnetic eld,thisalsodisfavorsthemagnetarhypothesis.

Thenon-detectionofbrighteningemissionmayplacelimitsonthepresenceofathermal“mini-supernova”fromnon-relativisticejectaofacompactmergersystem(Li&Paczy´nski1998;Rosswog&Ramirez-Ruiz2002).Inthisscenario,thesmalldensemass(mej)ingthescalingsofLi&Paczy´nski(1998)andcrudelyassumingthat10%ofthebolometriclightatpeakisradiatedintheRband,theR-bandbrightnessshouldpeakatobservertimet≈1.2(mej/0.01M⊙)1/2daftertheburst,withabsolutemagnitudeMR≈ 18.5 1.25log(mej/0.01M⊙)mag.AssumingthattheGRBdidindeedoriginatefromtheredshiftz=0.2248,uponinspectionofFigure1,withnon-detectionsatMR≈ 16magatt≈1d,wecanveryroughlyexcludemej>few×10 3M⊙.ThoughtheLi&Paczy´nski(1998)modelwasintendedasasimplisticsketchofthephenomenon,thislimitonmejissomewhatsurprisinggiventheamountofescapingnon-relativisticmaterialexpectedincompactmergers(Rosswog&Ramirez-Ruiz2002).Indeed,weconsiderthislackofa“mini-supernova”asweakevi-denceagainstaz=0.22originfromacompactmergersystem.Still,theselimitsaresubjecttoconsiderableuncertaintyinanumberofuncertainparametersofejecta.Forinstance,ifthevelocityoftheejectaweretobe～0.01cinsteadof0.3c(asassumedbyLi&Paczy´nski1998)thenthepeakofthethermalemissionwouldoccurafterabout1month,andwouldnothavebeendetectedwiththecurrentlimits.

WeconcludebyemphasizingthatintheNS–NSorBH–NSprogenitorhypothesisforshort-hardbursts,thehostsgalaxiesmaybearangeofHubbletypes(e.g.,Livioetal.1998).CompactmergersystemscoalesceinappreciableratesfromMyrtoGyrafterastarburst(e.g.,Fryeretal.1999;Bloometal.1999).Obviously,thelongerthetimesincethestarburst,thelargerthedistanceabinarysystemwilltravelbeforecoalescence.Aclearpredictionfromthismodelisthatasmoreshortburstsarelocalized,thoseassociatedwithlater-typegalaxiesofagivenmassshouldbepreferentiallyclosertothestar-formationcentersofthehost;thatis,weexpectamoreconcentrateddistributionaroundaspiralgalaxywiththesamemassasanearly-type.Ontheotherhand,dwarfstar-forminghostshaveshallowenoughpotentialsthatmergersystemsfromthesegalaxiescouldcoalesceatappreciabledistances(>～100kpc)evenshortlyafterstarburst.AsSwiftlocalizesmoreshort-hardbursts,weexpectthattheo setdistributionaroundgalaxieswillfurtherelucidatetheprogenitorquestion.

Wethanktheanonymousrefereefortheirinsightfulanddetailedcommentsthatleadtotheimprovementofthispaper.MuchofthisworkcouldnothavebeenundertakenwithouttheworkofS.BarthelmyrunningtheGCNCircularsandtheSwiftteamhavingdeployedsuchamarvelousexperiment.WethanktheGeminista fortheirexcellentworkinobservingGRB050509b,especiallyK.Roth.WethankR.Duncan,A.Loeb,P.Kumar,andE.Quataertforinformativeexchanges.WethankB.Simmonsforherassistancewithusinggal t.WealsothankM.Skrutskie,D.Starr,W.Peters,B.HutchinsfortheircontributionstoandassistancewithPAIRITEL.WeareindebtedtoG.WirthandR.ChornockfortheircontributionsinacquiringsomeoftheKeckdata.J.S.B.,J.X.P.,andH.-W.C.arepartiallysupportedbyNASA/SwiftgrantNNG05GF55G.TheworkofA.V.F.issupportedbyNSFgrantAST-0307894andNASA/Swiftgrant

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