Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(4)

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

wasundertaken.IncludingGRB050509b,thiscorrespondstoaratioof1:18forshort-hardtolong-softburstdetectionswithSwift,muchsmallerthantheBATSEresult.2

Aswithlong-durationbursts,thedistributionofshortburstsappearsverynearlyisotropic(Kouveliotouetal.1993;Briggsetal.1996),andtheirbrightnessdistribution(<V/Vmax>≈0.35)isconsistentwithbeingacosmologicalpopulation.Still,thereisnostrongevidencetosupporttheideathatshortburstsarepreferentiallyseenfromz<～0.37richAbellclusters(Hurleyetal.1997),noraretheyclearlyconnectedwithstarformationwithin～100Mpc(Nakaretal.2005).

Withoutpreciseandrapidlocalizations,thepopulationstatisticsdonotprovideastrongconstraintontheshort-burstprogenitors.Still,ithasbeenlargelyreckonedthattheleadingcandidatesforshortburstsarethemergerofaneutronstarbinary(NS–NS;Blinnikovetal.1984;Paczy´nski1986,1991;Narayanetal.1992;Katz&Canel1996;Ru ert&Janka1999;Rosswog&Ramirez-Ruiz2002;Rosswogetal.2003)orablackhole–neutronstarbinary(BH–NS;Lattimer&Schramm1976;Eichleretal.1989;Mochkovitchetal.1993;Kluzniak&Lee1998;Bethe&Brown1999;Pophametal.1999;Fryeretal.1999).Thesesystemsholdseveralparticularattractions.First,althoughuncertain,theestimatedrateofmergers(between1.5–20per106yrpergalaxy;Belczynskietal.2002;Sipior&Sigurdsson2002;Rosswogetal.2003)iscomparabletotheshort-burstrate(Schmidt2001).Second,thedynamicaltimescaleofsuchmergersisseveralmillisecondsandthesound-crossingtimesareofordertenmilliseconds,comparabletotheshortestobservedbursts(Miller2005).Third,compactmergersystemsarelikelytocontainenoughmass-energyinatransienttorustopowershort-burst uencesaswouldbeobservedifatcosmologicaldistances(Rosswogetal.2003;Leeetal.2004;Rosswog2005).Thetypicaldynamicaltimescaleinsuchbinariesimmediatelypriortocoalescence(ms)ismuchshorterthantheobservedburstduration,andsoitrequiresthecentralenginetoevolveintoacon gurationthatisstable,whileretainingasu cientamountofenergytopowertheburst(Leeetal.2004).

Mergersofsuchcompactremnantsarebynomeanstheonlypossiblechanneltoproduceshortbursts.Evaporatingprimordialblackholesmayproduceshort(<100ms)GRBs(Clineetal.1999),thoughbasicenergeticsargumentssuggestthatitwouldbedi culttoseesuchsourcesfromdistanceswellbeyondtheGalaxy.Therecentdiscoveryofamega arefromSGR1806 20(Mereghettietal.2005;Hurleyetal.2005a;Palmeretal.2005;Terasawaetal.2005)ledtoplausiblesuggestionsthatasubstantialfraction(≈40%)ofshortburstscouldbeproducedbyextragalacticmagnetars(Hurleyetal.2005a).However,positional(Palmeretal.2005;Nakaretal.2005)andspectral(Lazzatietal.2005)argumentshaveledotherworkerstosuggestthatatmostafewpercentoftheBATSEcatalogcouldconsistofshort-burstmagnetars.Notethatnotallcompactmergerscreatefertileconditions(atransienttorusaroundaBH)formakingashortburst(e.g.,Janka&Ru ert2001;Rosswogetal.2004).Thedurationoftheburstinacompactbinarymergerisdeterminedbytheviscoustimescaleoftheaccretinggas,whichissigni cantlylongerthanthedynamicaltimescale,thusaccountingnaturallyforthelargedi erencebetweenthedurationsofburstsandtheirfastvariability(Leeetal.2004).Inthecollapsarscenarioforlong-durationbursts,ontheotherhand,theburstdurationisgivenbythefall-backtimeofthegas(Woosley1993;MacFadyen&Woosley1999),whichistypicallygreaterthanafewseconds.However,amodi edcollapsarscenarioinwhichtheburstdurationisdeterminednotbyfall-backbutratherbythedynamicaltimescalesassociatedwiththeexpandingout ow

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