Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(6)

Todatethedeepestearly-timeobservations( t<～1hr)yieldedupperlimitsVlim≈14magfromthe0.3mROTSE-Iexperiment(Kehoeetal.2001).Hurleyetal.(2002)compileddeepernon-detectionsatopticalandradiowavelengthsattimesfromdaystoweeksafterfourshortbursts,withthefaintestnon-detectionofR≈22.3magat t=20hr(seealsoGandol etal.2000).Clearly,deepandearlyobservationsinsearchofashort-burstafterglowwouldrequirearapidlocalizationtoanuncertaintycomparabletothe eldofviewofmeter-class(andlarger)telescopes.

GRB050509b(Gehrelsetal.2005)triggeredtheBATcoded-maskimageron-boardSwifton9May200504:00:19.23(UTdatesandtimesareusedthroughoutthispaper;Hurkettetal.2005).ThepositionofGRB050509b,withanuncertaintyof4′radius,wasrelayedtothegroundwithinafewseconds.TheinitiallocalizationwaslaterrevisedtoapositionR.A.=12h36m18s,Decl.=+28 59′28′′,witha95%con denceerrorradiusof2.8′(Barthelmyetal.2005a).Barthelmyetal.(2005a)describetheburstasasingle-peakedsourcewithdurationof～30ms,peak uxof2100countss 1(15–350keV),andahardnessratioconsistentwiththatoftheshort-hardpopulation.At06:29:23,afadingX-raysourcewasreportedwitha6′′localization(Kenneaetal.2005)andlaterupdatedtoan8′′uncertaintyradiusatpositionR.A.=12h36m13.9s,Decl.=+28 59′01′′(Roletal.2005).

GRB050509bthusrepresentsthe rstshort-hardburstlocalizedinrealtimetoapositionsuitableforimmediatefollow-upobservationsfromasuiteofground-basedfacilities.Inthispaperwedescribetheresultsofourobservationsofthe eldofGRB050509bandwhatbearingthesedatahaveonthenatureofshortburstsandthephysicsofshort-burstafterglows.In§2wedescribeimagingandspectroscopyofthe eld.OuranalysisoftheX-rayafterglowofGRB050509bisgivenin§3,leadingtoalocalizationnearanellipticalgalaxy(§4).In§5wepresentaspectrumofthatgalaxy,itsredshift,andinferredproperties.Wethenargue,onstatisticalgrounds,foraplausibleassociationofthisgalaxyandtheGRB.Wedemonstratein§6howGRB050509bappearstobeasubluminousburstrelativetolong-durationGRBs,butwitharatioofblast-waveenergytogamma-rayenergythatisconsistentwiththelong-durationpopulation.Intheremainingsectionswedescribenewconstraintsonthenatureofshort-burstprogenitors.Throughout,weassumeaconcordancecosmologywithH0=70kms 1Mpc 1, Λ=0.7,and m=0.3.Alloftheresultspresentedherein,thoughgenerallyconsistentwithourpreviousresultsinGCNCirculars,supersedethem.