An observational limit on the earliest gamma-ray bursts

ABSTRACT We predict the redshift of the first observable (i.e. in our past light cone) gamma-ray burst (GRB) and calculate the GRB rate redshift distribution of the Population III stars at very early times (z = 20-60). Using the last two year of data from Swift, we place an upper limit on the efficiency (η_GRB) of GRB production per solar mass from the first generation of stars. We find that the first observable GRB is most likely to have formed at redshift 60. The observed rate of extremely high redshift GRBs (XRGs) is a subset of a group of 15 long GRBs per year, with no associated redshift and no optical afterglow counterparts, detected by Swift. Taking this maximal rate, we get that η_GRB <1.1 × 10^-3 GRBs per solar mass in stars. A more realistic evaluation, for example, taking a subgroup of 5 per cent of the total sample of Swift gives an upper limit of η_GRB <3.2 × 10^-4 GRBs per solar mass.