Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(7)

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

2.ObservationsandReduction

Initially,severalgroupsreported(Ryko etal.2005;Ugarteetal.2005;Bloometal.2005a;Torii2005)nonewoptical/infraredsourcethatwasconsistentwiththeXRTpositionofGRB050509b(Kenneaetal.2005).At07:21:27wehighlightedtheproximityoftheXRTtoabrightredgalaxy(hereafterG1=2MASXJ12361286+2858580)andsuggestedaplausiblephysicalassociation(Bloometal.2005a)basedonitspresumedmembershipinaz≈0.22cluster(Barthelmyetal.2005a).WelaterreportedthedeterminationoftheredshiftinProchaskaetal.(2005)andProchaskaetal.(2005).At08:44:13wenotedthepresenceofafaint,compactsource(hereafterS1;see2)intheoutskirtsofG1,whichwedeemedaplausiblecandidatecounterpart(Bloometal.2005b).Averysimilarsuggestionwasmadeat09:36:49byCenkoetal.(2005c);inaddition,theynotedapparentvariabilityofthecandidate(laterretractingthevariabilityclaiminCenkoetal.2005b)anddetectionofthreeotherfaintsources(S2–S4)consistentwiththeXRTposition(seealsoCenkoetal.2005a).Twoadditionalsources(S5andS6)intheXRTlocationweresubsequentlynotedfromVeryLargeTelescope(VLT)imagingbyHjorthetal.(2005),followedbyanother5sources(J1–J5)reportedbyBloometal.(2005).Noradioemission(Parkinson2005b;vanderHorstetal.2005)orGeV/TeVemission(Parkinson2005a)isconsistentwiththeXRTerrorlocalization.Belowwediscusstheobservations,andfurtherinterpretation,leadingtothesereports.

2.1.OpticalandInfraredImaging

Weobservedthe eldofGRB050509bonMay9withtheWIYN3.5mtelescopeandtheOPTICCCDimagerwitha9.6′×9.6′ eldofviewandaplatescaleof0.14′′/pixel.Underpoor(～2′′)seeingconditions,twoexposurestotaling360swereobtainedinthei′bandbeginningat04.344hr.Inaddition,weobtained2400sofintegrationinther′bandunderimprovedseeingconditions(～1′′)beginningat06.088hr.

Thedatawerereducedintheusualmannerusing at- eldsfromboththeilluminateddomeandthetwilightsky.TheastrometricsolutionstotheindividualimageswerecalculatedbycomparisontotheUSNOB-1.0catalogwitharoot-mean-square(rms)residualof0.1′′.Thephotometriczero-pointsoftheimageswerecalculatedbycomparisontomorethan50starsintheSloanDigitalSkySurvey(SDSS)pho-tometryprovidedbyEisensteinetal.(2005).Thezero-pointsoftheWIYNimagesareuncertainataboutthe3%level.Limitingmagnitudeswereestimatedfromthehistogramof uxesin104seeing-matchedaperturesplacedrandomlywithinthe eld.Thedispersion(σ)ofaGaussian ttedtothisdistributionwasusedtoestimatethe5σlimiting uxineachimage,whichwasconvertedtoamagnitudeusingtheknownzero-point.

ThebrightgalaxyG1tothewestoftheXRTpositioncontaminatesasigni cantportionofthe8′′radiusXRTerrorcircle.Weusedgal t(Pengetal.2002)to tasmoothS´ersicpro letothisgalaxyinordertoremovemostofthecontaminantlightpriortoexaminingtheXRTerrorcircle.Aseriesof1000seeing-matchedaperturesplacedrandomlywithintheXRTerrorcircleidenti ednonewsources.ThefaintgalaxyS1wasdetectedatthe>5σlevelinourdeeperr′images.

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