Author: Daniel Perley
Abstract: While luminous long-duration GRBs can be detected well beyond z>7, actually identifying high-redshift bursts and studying them in detail is challenging, both in principle and in practice. Only a small fraction of observed GRBs originate from the highest redshifts: recent uniformly-selected afterglow and host-galaxy samples limit the rate to approximately 1-3 percent of all Swift GRBs, or 1-3 events per year on average. While even this number is surprisingly high given the rapid fall-off of the star-formation rate density beyond z>4 suggested by deep-field galaxy studies, it nevertheless presents a major challenge for observers of high-redshift bursts: separating genuine high-z events from the much larger number of false positives during the narrow time window when the afterglow is bright enough for definitive observations (infrared spectroscopy) to be possible is quite challenging. In spite of this, in recent years observers have become increasingly efficient at isolating high-z events, with two GRBs confidently detected beyond z>7 in 2009 and several other likely candidates found in more recent years. These results bolster the case for a higher intrinsic GRB rate at z>7, suggesting either a dominant role for ultra-faint galaxies below the threshold of the HST-UDF survey in cosmic star formation or continued evolution in the GRB-to-SFR ratio (due to metallicity, IMF variations, or similar effects) even at z>∼3. Host galaxy observations, both at z∼3 and z>6, will help to distinguish these hypotheses.