ON THE ABSENCE OF WIND SIGNATURES IN GRB AFTERGLOW SPECTRA: CONSTRAINTS ON THE WOLF-RAYET WINDS OF GRB PROGENITORS

摘要

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 λλ1548, 1550 absorption doublets at Δv = —1000 to -5000 km s~(-1) from the GRBs (hereafter C Ⅳ_(15)). We report the detection of a single C Ⅳ_(15) absorber at Δv ≈ -1500 km s~(-1) from GRB 050730 and none in the rest. Our search yields an estimate of 20% for the incidence of C Ⅳ_(15) 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 Ⅳ_(15) near classical damped Lyα absorbers toward quasar sight lines. Including the two C Ⅳ_(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 C Ⅳ_(15) absorbers is consistent with Wolf-Rayet winds terminating at < 30 pc from their progenitor stars.