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 C IV lambda lambda 1548, 1550 absorption doublets at Delta v approximate to -1000 to -5000 km s(-1) from the GRBs (hereafter C IV15). We report the detection of a single C IV15 absorber at Delta v approximate to -1500 km s(-1) from GRB 050730 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 angstrom near GRB host galaxies, consistent with the incidence of intergalactic C IV15 near classical damped Ly alpha absorbers toward quasar sight lines. Including the two C IV15 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 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 CSM-originated C IV15 absorbers is consistent with Wolf-Rayet winds terminating at < 30 pc from their progenitor stars.