We propose a method for examining the cosmic reionization history by using near-infrared (NIR) colors of afterglows of high redshift (5 < z < 25) gamma-ray bursts (GRBs) that will be detected by the Swift satellite. The broad-band photometry has a much higher sensitivity than that of the spectroscopy. A prompt NIR photometric follow-up of the high-z GRB afterglows will reveal how many times the reionization occurred in the universe.
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