Gamma-ray bursts (GRB) are among the most energetic events observed in the universe at all epochs. In particular, long-duration GRBs are believed to originate in the catastrophic death of massive stars. Many GRBs are followed by optical afterglows that can peak as high as V ~ 5 magnitude, M(r) ~ -38 magnitude in the restframe (see the right panel), soon after the GRB and fade away quickly with time following a power-law model. Similar to high-redshift quasars (QSOs), GRB afterglows can serve as a sensitive probe of intervening gas that is either local or external to the burst progenitor environments. Since GRB afterglows rapidly decline, however, an effective exploitation of these lighthouses requires rapid follow-up spectroscopy (t < 2 day) even with 8 m-class telescopes.

The advantages of applying GRB afterglows as a background source to study intervening gas are two fold. First, the afterglows are viable competition for QSOs as a probe of the IGM at all redshifts including the reionization epoch at z > 7, due to their extreme luminosity. The left panel shows the redshift distribution of 219 GRBs with known redshift measurements, in comparison to those of QSOs found in the Sloan Digital Sky Survey with i' < 20. While no more than 3% of all known i' < 20 mag QSOs are at z > 3.5, the afterglows offer a more uniform probe of primordial gas in the early epoch if we observe these sources shortly after the initial bursts.

Second, the transient nature of GRB afterglows make these ideal lines of sight for studying correlations between stellar content and gas properties derived from absorption line analysis. Specifically, traditional QSO absorption spectroscopy has allowed us to study the chemical enrichment history in neutral gas over a large cosmic time interval, but searches for the absorbing galaxies have often been hampered by the glare of the background QSOs. The afterglows will not outshine the absorbing galaxies after they fade, substantially increasing the likelihood of finding faint galaxies close to the sightlines. In addition, afterglow spectroscopy will also allow us to study the ISM of the GRB host galaxies, which are presumably starburst environment at high redshifts.