Systematic study for gas-to-dust ratio of short gamma-ray burst afterglows

Extragalactic X-ray absorption and optical extinction are often found in gamma-ray burst (GRB) afterglows, and they could be tracers of both circumburst and host galaxy environments. By performing spectral analyses of the spectral energy distribution of nine short GRB (SGRB) afterglows with a known redshift, we investigated the ratio of the equivalent hydrogen column density to the dust extinction, N_H^rest/A_V^rest, in the rest frame of each SGRB. We found that the distribution of N_H^rest/A_V^rest is systematically smaller than for long GRBs, and is roughly consistent with the gas-to-dust ratio in the Milky Way. This result means that the measured gas-to-dust ratio of SGRBs would originate from the interstellar medium in each host galaxy. This scenario supports the prediction that SGRBs occur in non-star-forming regions in the host galaxies.