Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(10)

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

includeonlycountsinPIchannels30–1000(correspondingtophotonenergiesof0.3–10keV).ThePIchanneltophotonenergyconversionwasaccomplishedwiththeredistribution leswxpc0to12

2seriesstartingat4pixels(4,5.657,8,11.314,16),andthesigni cancethresholdwassetat4×10 6,correspondingtoa～1falsepositivedetectionofapointsourceintheimage.Wedetect22compactsourcesintheentire32.3ksdataset.

Tostudythepropertiesoftheafterglowcandidate,weextractedthealltheeventswithinanareaofradius10pixelsaroundthenominalwavdetectposition.Inthe rstobservationof1.6ks,thereare14countsinthisregion.Whenexaminingaplotofthecumulativedistributionversustime,wenoticedthatthemajorityofthecountsfromthisregionoccurredinthe rst300s.Wethereforefurtherinvestigatedthisbriefinterval.

Inthe rst300softhe rstPhotonCountingobservation,theXRTdetected92countsontheentirechip,with73ofthemoutsideofthe22sourceregions.Withinany10-pixelradiussourceregion,wethereforeexpectanaverageof0.1backgroundcounts.Wedetect9countsinthisregionoftheX-rayafterglow,ingthemeanlocationofjustthese9counts,wecanobtainarelativelyuncontaminatedestimateofthesourceposition.Wecalculatethe68%√con denceintervalineachdirectionasTσj/

isdistributedbytheNationalOpticalAstronomyObservatory,whichisoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Inc.,undercooperativeagreementwiththeNationalScienceFoundation.6IRAF