Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(5)

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

mightstillmeettheconstraintsofshortGRBs(Woosley2001).

ThetheoreticalpredictionsfortheafterglowsofshortGRBshavebeenconsideredbyPanaitescuetal.(2001).Sincethepeak uxofthepromptemissioniscomparableforshortandlongGRBs,iftheirdistancescalesaresimilartheisotropicequivalentenergyoutputingammarays(Eγ,iso)wouldbeproportionaltothedurationoftheGRB,whichis～10 100timeslargerforlongGRBs.Ifthee ciencyforproducingthegammaraysiscomparable,thentheisotropicequivalentkineticenergyintheafterglowshock(Ek,iso)wouldhaveasimilarratiobetweenlongandshortGRBs.ThiswouldimplytheafterglowofshortGRBstobeonaverage～10 40timesdimmerin uxthanthatoflongGRBs.TheafterglowsofshortGRBswouldbeevenmuchdimmerthanthisiftheyencounteramuchsmallerexternaldensitycomparedtolongGRBs;thisistheexpectationfromshort-burstsfrombinarymergersoutsideofthehostgalaxy.Panaitescuetal.(2001)arguedthatalowexternaldensitywouldnota ecttheX-rayband,asthelatterwasassumedtolieabovethecooling-breakfrequency,νc.We ndthatforaverylowexternaldensitytheelectroncoolingbecomesveryslowsothatνccanlieabovetheX-raybandforthe rstfewdays,thusreducingtheX-ray uxcomparedtothatforahigherexternaldensitytypicaloftheinterstellarmedium(ISM)foundnearstar-formingregionsoflong-durationGRBs.