Dust Extinction of Gamma-Ray Burst Host Galaxies: Identification of Two Classes?

Dust in the host galaxies of gamma-ray bursts (GRBs) dims and reddens theirafterglow spectra. Knowledge of the nature of this dust is crucial forcorrecting for extinction, providing clues to the nature of GRB progenitors,and probing the interstellar medium of high-redshift galaxies as well as thenature of cosmic dust when the universe was much younger and galaxies were muchless evolved. The dust and extinction properties of GRB host galaxies are stillpoorly known. Unlike previous work, we derive in this Letter the extinctioncurves for 10 GRB host galaxies without a priori assumption of any specificextinction types (such as that of the Milky Way, or the Small/Large MagellanicClouds). It is found that there appears to exist two different types ofextinction curves: one is relatively flat and gray, the other displays asteeper dependence on inverse wavelength, closely resembling that of the MilkyWay but with the 2175$\Angstrom$ feature removed.Comment: 15 pages, 3 figures. Substantially revised with the same arguments; accepted for publication in The Astrophysical Journal Letter