The extinction curve of long gamma - ray burst host galaxies : Are they two different types ?

We present the extinction curves of long gamma ray burst (GRB) host galaxies (here after GRB hosts) utilizing the extinction feature of the optical afterglow of gamma ray bursts. The extinction curve of a galaxy is essential to understanding its dust and gas properties which in return will help us to reveal the origin of GRBs. We find out that the shape of the extinction curves may be divided into two clusters and are different from that of Milky Way (MW), Large and Small Magellanic Clouds (LMC and SMC). Fitting the extinction curve with the silicate-graphite model yields that in some GRB hosts (referred as Type-I) the dust size is small and their distribution is smooth and graphite content is extremely low; in some other GRB hosts (referred as Type-II) the dust distribution index and graphite content is similar to LMC but characteristic size is larger than that of LMC. In GRB hosts the dust size parameters cover a rather large range and the graphite content are low. The results may imply that GRB hosts are of young galaxies. Due to the different dust properties, it is possible that the progenitors or the explosion mechanism may be also different for this two groups of GRBs. Subject headings: dust, extinction — gamma rays bursts: afterglow, host galaxy