Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(13)

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

Ais22counts 1.Wecantranslatethistoanenergy- uxnormalizationbydeterminingtheconversionfromcountstoergcm 2.Weconsideronlythe rst300sofdataforthisdeterminationinordertoreducecontaminationfromthebackground.Foreachofthe9counts,weknowitsenergyaswellasthee ectiveareaoftheXRTatthatenergy.Theaverageis3.14×10 11ergcm 2count 1.OurmodelfortheX-ray ux(0.3–10keV)oftheafterglowonlyisthenFX(t)=A′(t t0) α.Forα=1.3,thenormalizationisA′=6.9×10 10ergcm 2s 1.Forexample,theX-ray uxoftheafterglowatt=200saftertheBATtriggerforα=1,1.3,and1.7isFX(200)=5.8,7.0,and6.5×10 13ergcm 2s 1,respectively.

Figure4shows,onacommonscale,X-raylightcurvesforanumberofGRBsandcore-collapsesuper-novae.ThelocationoftheX-raytransientassociatedwithGRB050509b,whenplacedatz=0.2248,isstriking.WhiletheslopeofthetransientagreeswellwiththoseoftypicalGRBs,its uxfallswellbelowthetypicallong-GRBrange.Infact,foranyreasonableredshift(i.e.,z 3–5),GRB050509bwouldstillbesigni cantlyunderluminousinitsX-rayafterglowwhencomparedtothoseoflong-durationGRBs(seeFigure4).Fortheassumedredshiftofthetentativehostgalaxy,theextrapolatedX-rayluminosityatafewdays,whichisalsoconsistentwiththeChandraupperlimit(Pateletal.2005),isclosetothoseseenintypicalcore-collapsesupernovae.