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

Abstract

Using the near-infrared (NIR) observations of the afterglows of the high redshift ($5 \la z \la 25$) gamma-ray bursts (GRBs) which will be detected by the {\it Swift} satellite, we discuss a way to study the cosmic reionization history. In principle the details of the cosmic reionization history are well imprinted in the NIR spectra of the GRB afterglows. However the spectroscopy with a space telescope is required to obtain such an information for a very high redshift ($z\ga 15$) unless the neutral fraction of the high-$z$ universe is less than $10^{-6}$. The broad-band photometry has the higher sensitivity than that of the spectroscopy, so that the NIR photometric follow-up of the GRB afterglows is very promising to examine the cosmic reionization history. A few minutes exposure with a 8-m class ground-based telescope of the afterglow of the high-$z$ GRBs will reveal how many times the reionization occurred in the universe.