On Dust Extinction of Gamma-Ray Burst Host Galaxies

doi: 10.1086/591049Although it is well recognized that gamma-ray burst (GRB) afterglows are obscured and reddened by dust in their host galaxies, the wavelength dependence and quantity of dust extinction are still poorly known. Current studies on this mostly rely on fitting the afterglow spectral energy distributions (SEDs) with template extinction models. The inferred extinction (both quantity and wavelength dependence) and dust-to-gas ratios are often in disagreement with that obtained from dust depletion and X-ray spectroscopy studies. We argue that this discrepancy could result from the prior assumption of a template extinction law. We propose an analytical formula to approximate the GRB host extinction law. With the template extinction laws self-contained, and the capability of revealing extinction laws differing from the conventional ones, it is shown that this is a powerful approach in modeling the afterglow SEDs to derive GRB host extinction.A. L. and S. L. are supported in part by aNASA/ Swift Theory Program, a NASA/Chandra Theory Program, and the NSFC Outstanding Oversea Young Scholarship. D. M. W. is supported by the NSFC grants 10621303 and 10673034, and the National Basic Research Program of China (973 Program 2007CB815404). D.A.K. and S. K. acknowledge financial support by DFG grant Kl 766/13-2