On the absence of Wind Signatures in GRB Afterglow Spectra Constraints on the Wolf-Rayet Wi.pdfVIP

a r X i v : a s t r o - p h / 0 6 1 1 0 7 9 v 3 1 0 A p r 2 0 0 7 Accepted for Publication in the Astrophysical Journal Preprint typeset using LATEX style emulateapj v. 20/04/00 ON THE ABSENCE OF WIND SIGNATURES IN GRB AFTERGLOW SPECTRA: CONSTRAINTS ON THE WOLF-RAYET WINDS OF GRB PROGENITORS HSIAO-WEN CHEN1, JASON X. PROCHASKA2, ENRICO RAMIREZ-RUIZ2,3, JOSHUA S. BLOOM4,5, MIROSLAVA DESSAUGES-ZAVADSKY6, AND RYAN J. FOLEY4 Accepted for Publication in the Astrophysical Journal ABSTRACT We investigate available constraints on the circumstellar medium (CSM) around long-duration γ- ray burst (GRB) progenitors from afterglow spectra. We first establish a statistical sample of five GRB afterglow spectra that have been collected and analyzed with no prior knowledge of the line-of- sight properties. This sample is then adopted for a uniform search of Wolf-Rayet wind signatures, as represented by C IV λλ 1548, 1550 absorption doublets at ? v = ?1000 to ?5000 km s?1 from the GRBs (hereafter C IV15). We report the detection of a single C IV15 absorber at ? v ≈ ?1500 km s ?1 from GRB050730 and none in the rest. Our search yields an estimate of 20% for the incidence of C IV15 absorbers with rest-frame absorption equivalent width EW(C IV 1548) 0.2 A? near GRB host galaxies, consistent with the incidence of intergalactic C IV15 near classical damped Lyα absorbers toward quasar sightlines. Including the two C IV15 absorbers previously known toward GRB021004, we further demonstrate that the presence of H0, C+, and Si+ together with the absence of excited C+ or Si+ argue against a CSM origin. The null result is consistent with the expectation that the circumburst medium is fully ionized by the afterglow radiation field. We examine possible scenarios for the survival of the C3+ ions, including a clumpy wind model. We find that a clumpy wind is unable to effectively shield the ionizing radiation and allow C3+ to survive at r 10 pc from the afterglow. We conclude that the lack of C