Near-Infrared Colors of Gamma-Ray Burst Afterglows and Cosmic Reionization History

We discuss a way to study the cosmic reionization history using near-infrared (NIR) observations of the afterglows of high-redshift (5 ≲ z ≲ 25) gamma-ray bursts (GRBs) that will be detected by the Swift satellite. In principle, details of the cosmic reionization history should be imprinted in the NIR spectra of GRB afterglows. However, spectroscopy with a space telescope is required to obtain such information for very high redshifts (z ≳ 15) unless the neutral fraction of the high-z universe is less than 10-6. The broadband photometry has higher sensitivity than the spectroscopy, so that NIR photometric follow-up of GRB afterglows is very promising for examining cosmic reionization history. A few minutes exposure with a 8 m class ground-based telescope of the afterglows of the high-z GRBs will reveal how many times reionization occurred in the universe.