Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(12)

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

ForeachXRTsourceotherthantheafterglowcandidate,Figure3plotstheo setbetweentheXRTpositionandthepositionoftheclosestopticalsource.TwoXRTsourceshadtwoopticalsourceswithin5′′;forthese,theclosestopticalsourceisrepresentedbydashedlinesandthenextclosestbydottedlines.Thereisanobviouslocusarounda4′′.5di erenceinR.A.,suggestingthattheseXRTsourcesareassociatedwiththecorrespondingnearestopticalsources.Atadetectionsensitivityaround10 14ergcm 2s 1,itisnotsurprisingto ndsomanyopticalcounterpartsinthemoderatelydeepBokimage.InaChandra/SubarustudyoftheR.A.=13hrXMM/ROSAT eld,McHardyetal.(2003) ndunambiguousopticalcounterparts¯=20.7,for61ofthe66X-raysourcesabove10 14ergcm 2s 1.ThemeanRmagnitudeofthesesourcesisRandthefaintestcounterpartisatR=24.4mag.

Usingthe14sourcesintheabovelocus(excludingthetwosourceswithmultiplepossiblecounterparts),wederiveano setbetweentheXRTframetotheopticalframeof4′′.49±0′′.72WinR.A.and0′′.42±0′′.30SinDecl.OurbestestimateforthelocationoftheX-rayafterglowisthereforeR.A.=12h36m13s59,Decl.=+28 59′04′′.9(J2000);thisis4.1′′westand3.9′′northoftherevisedXRTpositionreportedinRoletal.(2005).TheuncertaintyinourpositionisacombinationofthestatisticaluncertaintyoftheXRTlocalization(3′′.6inR.A.,3′′.5inDecl.)andtheuncertaintyinshiftingtheXRTframetotheICRS(0′′.76inR.A.,0′′.40inDecl.).

TheastrometryinouroriginalreportsfromWIYNandKeckimagingwerebasedonaframeofapprox-imately10starsinthe2MASScatalog.ThereleaseoftheSDSSdataandcalibrationsofthis eldallowustoimprovetheastrometrictietotheICRS.We ttheKeck/LRISG-bandimageto91sourcesincommonwiththeSDSSobjectcatalogwithathird-orderpolynomialsolutionusingIRAF/CCMAP.TheuncertaintyintheastrometrictietoSDSS,baseduponresidualsfromthe t,isσ(R.A.)=0.134′′andσ(Decl.)=0.153′′.Assuminga75masastrometricuncertaintyintheSDSSastrometriccalibrationtotheICRS(Pieretal.2003),weestimatetheabsoluteuncertaintyintheKeck-ICRStieisσ(R.A.)=0.154′′andσ(Decl.)=0.171′′.

TheXRTlocationis11.2′′±3.6′′(or40±13kpcinprojection)fromG1aswe rstnotedinBloometal.(2005a).Spectroscopyofthissourcerevealsthatitisindeedanearly-typegalaxy(see§5)andisamemberofaclusterNSCJ123610+285901atz≈0.22(Galetal.2003;Barthelmyetal.2005a).NearthelocationoftherevisedXRTerrorcircle,we nd～11faintsources(allofwhichweorothershavereportedpreviously;seeabove).Figure2showstheKeckGandRimageswithidenti edsourcelabeled.Table2givestheastrometricpositionsandmagnitudesofthesources.

3.3.X-rayAfterglowDecay

Weexaminethe rst1.6ksblockofobservationstocharacterizethetemporalpropertiesoftheX-rayafterglow.AKolmogorov-Smirno (K-S)testonthearrivaltimesofthe14photonsgivesaprobabilityof0.06%thattheycomefromasourcewithconstantcountrate.ThenextstepinmodelcomplexityisoneinwhichtheX-raycountrateRXinthisregionhasaconstantcomponent(duetothebackgroundanddi useclusteremission)plusacomponentwithapower-lawdependenceontime(duetothefadingafterglow).OurmodelisthusRX(t)=A(t t0) α+B,whereBistheconstant(backgroundpluscluster)countrate,t0isthetimeoftheBATtrigger,andAisanormalizationchosensuchthatthemodelpreservesthedetected uxoverthe1.6ksunderconsideration.WedetermineBtobe0.00107counts 1fromthelaterobservations.

Weconsideredarangeofαfrom0to4andcomputedtheK-Sprobabilityoftheobserveddatacomingfromthemodelforeachvalueofα.TheK-Sprobabilitywashighest(97.8%)atα≈1.3.Forα 1(0.77)andα 1.7(2.1),theK-Sprobabilitydroppedbelow32%(5%).Forα=1.3,thenormalization