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 their afterglow spectra. Knowledge of the nature of this dust is crucial for correcting for extinction, providing clues to the nature of GRB progenitors, and probing the interstellar medium of high-redshift galaxies as well as the nature of cosmic dust when the universe was much younger and galaxies were much less evolved. The dust and extinction properties of GRB host galaxies are still poorly known. Unlike previous work, we derive in this Letter the extinction curves for 10 GRB host galaxies without a priori assumption of any specific extinction types (such as that of the Milky Way, or the Small/Large Magellanic Clouds). It is found that there appears to exist two different types of extinction curves: one is relatively flat and gray, the other displays a steeper dependence on inverse wavelength, closely resembling that of the Milky Way but with the 2175$\Angstrom$ feature removed. 
Shilun Chen, Aigen Li, Daming Wei