Closing in on a Short-Hard Burst Progenitor Constraints from Early-Time Optical Imaging and(18)

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

Toestimatethevelocitydispersionofthegalaxy,wehavecomparedthespectrumagainstatemplatespectrumofHD72324(e.g.,Kelsonetal.2000)smoothedbyawiderangeofσ.ThebestmatchtotheabsorptionlinesofG1withλrest=4000–5300 Aisσ=275±40kms 1.Accountingfortheinstrumentalresolution,wederivealight-weightedvelocitydispersionforthisgalaxyof260±40kms 1.

ThespectralfeaturesevidentinFigure6aretypicalofearly-typegalaxies.Thespectraltypeandvelocitydispersionindicateamassiveellipticalgalaxywithnoapparentongoingstarformation.Aquantitativelimittothecurrentstar-formationrate(SFR)canbeinferredfromtheupperlimittotheHαluminosityofthisgalaxy.Theemission-line uxina10 Awindow( v≈300kms 1)centeredattheexpectedwavelengthofHαhasa3σupperlimitof1.2×10 16ergs 1cm 2.Adoptingthecurrentconcordancecosmology,wederiveanHαluminosityLHα<1.2×1040ergs ingtheempiricalrelationbetweenSFRandLHα(Kennicut1998),the3σupperlimittothecurrentSFRis0.1M⊙yr 1.