INCIDENCE RATE OF GRB-HOST DLAs AT HIGH REDSHIFT

We study the incidence rate of damped Ly alpha systems associated with the host galaxies of gamma-ray bursts (GRB-host DLAs) as functions of neutral hydrogen column density (N-H (I)) and projected star formation rate (SFR) using cosmological SPH simulations. Assuming that the occurrence of GRBs is correlated with the local SFR, we find that the median N-H I of GRB-host DLAs progressively shifts to lower N-H I values with increasing redshift, N-H I and the incidence rate of GRB- host DLAs with log N-H I > 21.0 decreases rapidly at z >= 6. Our results suggest that the likelihood of observing the signature of IGM attenuation in GRB afterglows increases toward higher redshift, because it will not be blocked by the red damping wing of DLAs in the GRB host galaxies. This enhances the prospects of using high-redshift GRBs to probe the reionization history of the universe. The overall incidence rate of GRB-host DLAs decreases monotonically with increasing redshift, whereas that of QSO DLAs increases up to z = 6. A measurement of the difference between the two incidence rates would enable an estimation z p 6 of the value of eta(GRB) which is the mass fraction of stars that become GRBs for a given amount of star formation. Our predictions can be tested by upcoming high-z GRB missions, including JANUS (Joint Astrophysics Nascent Universe Scout) and SVOM (Space multiband Variable Object Monitor).