Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(9)

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

PAIRITELupperlimits,aswellaslimitsreportedintheliterature,areshowninFigure1.

Welaterimagedthe eldofGRB050509bwiththeKeckI10mtelescopeandtheLRIS-Binstrument(Okeetal.1995)usingthedichroicD560(50%transmissionpointat5696 A)withGandR lters.Startingat11.25May2005,beginningnearastronomicaltwilight,wetook5ditheredimagesineachbandforatotalof1660sand1620sexposuresinGandR,respectively.Thedatawerereducedintheusualmannerandcombined,weightedbyexposuretimes.

On17May2005,08:05.5,8.17daftertheGRB,weobtaineddeepR(Ellis)(whichissimilartoHarrisR;Baconetal.2003)imagingontheEchelleteSpectrographandImager(ESI;Sheinisetal.2002)ontheKeckII10mtelescope.InthepresenceofbrightglarefromtheMoon,wecombinedseveralreducedimagesforane ectiveexposuretimeof960s.SincethereisanegligiblysmallcolorterminconvertingHarrisRtoRc,3wefoundazero-pointrelativetotheLRISRimage.TherearenonewsourcestoRc≈25.0mag(5σ),norsigni cantvariationsofthefaintsourcesintheXRTerrorcircle.

3.TheX-rayEmission

TheSwiftXRT(Burrowsetal.2000)beganobservationsofGRB050509bon2005May9at04:00:56,approximately61saftertheBATtrigger.Theobservationsconsistedofelevenblocks,eachabout2.5ksinduration(exceptthe rstobservationof1.6ksandthelastobservationof1.8ks),spreadoveraperiodof～21hr.TheXRToperatedinanumberofdi erentmodesthroughouttheobservations.Themostcommon(32.3ksofexposure)andmostusefulmodeforthisobjectwasthe“PhotonCounting”mode,whichretainsthefullimagingandspectroscopicresolutionoftheinstrument.Theimagesare480×480pixels,withascaleof2′′.36perpixel.TheXRTpoint-spreadfunctionisenergydependent,withahalf-powerdiameterof′′18at1.5keV.Theenergyresolutionisalsoafunctionofenergy,varyingfromabout50eVat0.1keVtoabout190eVat10keV.

The rstPhotonCountingobservationbeganat04:01:20(Kenneaetal.2005)andlasted1640s.AsnotedinKenneaetal.(2005)andRoletal.(2005),afaintX-raysourceisdetectedinthis rstoftheelevenobservations,butitfadedquicklybelowthebackground.WehaveobtainedtheXRTdatafromtheSwiftarchive,andhaveanalyzedthemtodeterminethepositionofthisX-rayafterglowcandidateaswellastoexamineitsvariability.Webrie yreviewthedatareduction,andthenwediscussthelocalizationoftheafterglowcandidateandattempttoquantifythedecay.

3.1.SwiftDataReduction

UsingtheLevel1datafromtheSwiftarchive,weranthexrtpipelinescriptpackagedwiththeHEAsoft6.0softwaresuppliedbytheNASAHighEnergyAstrophysicsScienceArchiveResearchCenter4.Weusedthedefaultgradeselection(grades0to12)andscreeningparameterstoproduceaLevel2event lere-calibratedaccordingtothemostcurrent(asof2005May15)calibration lesintheSwiftdatabase5.Toproduceimagesforsourcedetection,weusedthexselectsoftware(alsopartofHEAsoft6.0),witha lterto