Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(17)

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

aparallel.Still,basedontheargumentsabove,weproceedbyacceptingthehypothesisthatGRB050509bisphysicallyassociatedwithG1.

5.APutativeHostGalaxyatz=0.2248

WeobtainedaspectrumofG1withDEIMOS(Faberetal.2003)ontheKeckII10mtelescopeunderphotometricconditions.Thedatawereacquiredinaseriesoftwoexposuresstartingat07.47hronthenightoftheburst.Theinstrumentalsetupincludedthe600linemm 1gratingblazedat7500 Aandcenteredat7200 A,theGG455order-blocking lter,andstandardCCDbinning.Thissetupgivesnearlycontinuouswavelengthcoverageintherange4500–9000 A.Weobservedthegalaxythougha1.1′′×20′′slitatskypositionangle90 andanairmassof1.0.ThissetupyieldsaFWHMresolutionof～5 A(i.e.,σ=100

1kms).ThedatawerereducedandcalibratedwiththeDEEPspectroscopicpipelineforDEIMOSdata(Cooperetal.2006).Wavelengthcalibrationand at- eldingwereperformedusingspectraofXe-Ne-Kr-Arandquartzlamps(respectively)obtainedthatnight.

Thesoftwareprovidesatwo-dimensional,sky-subtractedimageofthespectrumacrosstwoCCDsoftheDEIMOSmosaic.Unfortunately,theCCDthatincludesthebluestdatahasapairofblockedcolumnswhichlienearthecenterofthegalaxypro le.Therefore,weextractedtheone-dimensional(1D)spectrumonthisCCDusingoptimalextractiontechniquesassumingaGaussianpro lewithσ=9.2pixels(i.e.,1.1′′).FortheotherCCD,weextracteda1Dspectrumbyadoptinga26-pixel(3′′)boxcaraperture.Finally,weprocessedandcalibratedaspectrophotometricstandardstar(BD+28 4211)observedattheendofthisnight.Aftercomparingitsobserved ux(indigitalnumbers)againsttheSTISCALSPECcalibration7,wecalculatedasensitivityfunctionwhichcouldbeappliedtoourgalaxyspectra.

SpectroscopicobservationsofG1,S1,S2,and2unidenti edsourceswereobtainedwiththeGMOSspectrometer(Hooketal.2004)ontheGeminiNorth8mtelescopebeginningat10.27May2005underphotometricconditions.Weuseda0.75′′slit,aR400gratingblazedat7640 A,GG455order-blocking lter,andsetthecentralwavelengthto6500 A.Theairmasswaslow(1.0–1.1),sothee ectsofatmosphericdispersionwerenegligible(Filippenko1982).StandardCCDprocessingandspectrumextractionwereac-complishedwithIRAFusinga1.74′′apertureforthesesourcesofinterest(S1,S2).ThedatawereextractedusingtheoptimalalgorithmofHorne(1986).Low-orderpolynomial ingtechniquesdiscussedinWade&Horne(1988)andMathesonetal.(2000),weemployedIRAFandourownIDLroutinesto ux-calibratethedataandtoremovetelluriclinesusingthewell-exposedcontinuaofthespectrophotometricstandardEG–131(Bessell1999).

Figure6presentsthe1D ux-calibratedspectrumofG1againstavacuum,heliocentric-correctedwave-lengtharray.Thedottedlinetracesa1σerrorarraybasedonPoissoncountingstatistics.Wehavemarkedanumberofdetectedabsorption-linefeaturesandalsotheexpectedpositionforseveralstrongtransitionsfrequentlyobservedinemission-linegalaxies(e.g.,Hα,[OIII]).Wehave tadouble-Gaussianpro letoCaIIH&Kandmeasurez=0.2248±0.0002.ThisisconsistentwiththeredshiftinferredphotometricallyforthisclusterfromDPOSS(Galetal.2003).Atthisredshift,theluminositydistanceis1117.4Mpc,and1′′correspondsto3.61kpcinprojection.