Are gamma-ray bursts the same at high redshift and low redshift?

The majority of Swift gamma-ray bursts (GRBs) observed at z ≳ 6 have prompt durations of T90 ≲ 30 s, which, at first sight, is surprising given that cosmological time dilation means this corresponds to ≲5 s in their rest frames. We have tested whether the high-redshift GRBs are consistent with being drawn from the same population as those observed at low redshift by comparing them to an artificially redshifted sample of 114 z < 4 bursts. This is accomplished using two methods to produce realistic high-z simulations of light curves based on the observed characteristics of the low-z sample. In Method 1 we use the Swift/Burst Alert Telescope (BAT) data directly, taking the photons detected in the harder bands to predict what would be seen in the softest energy band if the burst were seen at higher z. In Method 2 we fit the light curves with a model, and use that to extrapolate the expected behaviour over the whole BAT energy range at any redshift. Based on the results of Method 2, a Kolmogorov–Smirnov test of their durations finds a ∼1 per cent probability that the high-z GRB sample is drawn from the same population as the bright low-z sample. Although apparently marginally significant, we must bear in mind that this test was partially a posteriori, since the rest-frame short durations of several high-z bursts motivated the study in the first instance.