On the Absence of Wind Signatures in GRB Afterglow Spectra: Constraints on the Wolf‐Rayet Winds of GRB Progenitors

We investigate available constraints on the circumstellar medium (CSM) aroundlong-duration gamma-ray burst (GRB) progenitors from afterglow spectra. Wefirst establish a statistical sample of five GRB afterglow spectra that havebeen collected and analyzed with no prior knowledge of the line-of-sightproperties. This sample is then adopted for a uniform search of Wolf-Rayet windsignatures, as represented by CIV 1548, 1550 absorption doublets at dv=-1000 to-5000 km/s from the GRBs (hereafter CIV_{15}). We report the detection of asingle CIV_{15} absorber at dv ~ -1500 km/s from GRB 050730 and none in therest. Our search yields an estimate of 20% for the incidence of CIV_{15}absorbers with rest-frame absorption equivalent width EW(CIV 1548)>0.2 Ang nearGRB host galaxies, consistent with the incidence of intergalactic CIV_{15} nearclassical damped Lya absorbers toward quasar sightlines. Including the twoCIV_{15} absorbers previously known toward GRB 021004, we further demonstratethat the presence of H^0, C^+, and Si^+ together with the absence of excitedC^+ or Si^+ argue against a CSM origin. The null result is consistent with theexpectation that the circumburst medium is fully ionized by the afterglowradiation field. We examine possible scenarios for the survival of the C^{3+}ions, including a clumpy wind model. We find that a clumpy wind is unable toeffectively shield the ionizing radiation and allow C^{3+} to survive at r<10pc from the afterglow. We conclude that the lack of CSM-originated CIV_{15}absorbers is consistent with Wolf-Rayet winds terminating at < 30 pc from theirprogenitor stars.