Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(23)

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

2001),implyingthatthetotalenergyoutput(Eγ,iso≈Lp(γ)/η×duration,withηastheconversione ciencytogammarays)isatleastanorderofmagnitudesmallerforshortbursts.AsarguedbyPanaitescuetal.(2001),sinceafterglowbrightnessscaleswithEγ,iso(1 η),short-burstafterglowswouldbesystematicallyfaint.

NowthatthereisadetectedX-rayafterglowweareinapositiontodirectlytestthefaintnessclaim,byinferringthegamma-rayenergyreleaseandX-rayafterglowluminosity(aproxyforthekineticenergyintheblastwave).FromFigure9itisclearthatthisratioforGRB050509bissimilartothatfoundinlong-durationGRBs.Thisisastrikingobservationalbridgetolong-durationburstsandsuggestsacommonphysicalmechanismsforpromptanddelayed(afterglow)emissionforbothlong-durationandshort-durationGRBs,eventhoughtheirprogenitorsareprobablydi erent.

Atentativedetectionofanafterglowsignalbyaddinguptheemissionof76shortBATSEburstswasreportedbyLazzatietal.(2001)(seealsoConnaughton2002).Thesignalpeakedatt≈30safterthebursttriggerwitharelatively atνFν≈5×10 10ergcm 2s 1.ThiswouldcorrespondtoanX-ray uxinthe0.2–10keVrange,ofFX≈2×10 9ergcm 2s 1.TheX-ray uxoftheafterglowofGRB050509bisbestconstrainedaroundt≈200s,andisfoundtobeFX≈6.5×10 13ergcm 2s 1.Extrapolatingthis uxtot≈30swithapower-lawindexintherangeinferredfromthedata,1.0 α 1.7,givesa uxthatislowerthantheonefoundbyLazzatietal.byafactorof～(1 5)×102.ThismightsuggestthateitherthepossibledetectionbyLazzatietal.(2001)wasnotstatisticallysigni cant,ortheX-rayafterglowofGRB050509bisunderluminouscomparedtotheaveragevalueforshortGRBsbyatleasttwoordersofmagnitude.