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) around long-duration gamma-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 CIV 1548, 1550 absorption doublets at dv=-1000 to -5000 km/s from the GRBs (hereafter CIV_{15}). We report the detection of a single CIV_{15} absorber at dv ~ -1500 km/s from GRB 050730 and none in the rest. 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 near GRB host galaxies, consistent with the incidence of intergalactic CIV_{15} near classical damped Lya absorbers toward quasar sightlines. Including the two CIV_{15} absorbers previously known toward GRB 021004, we further demonstrate that the presence of H^0, 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 C^{3+} ions, including a clumpy wind model. We find that a clumpy wind is unable to effectively shield the ionizing radiation and allow C^{3+} to survive at r<10 pc 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 their progenitor stars.