Simulations on high-z long gamma-ray burst rate

Abstract

Since the launch of <it>Swift</it> satellite, the detections of high-<it>z</it> (<it>z</it> > 4) long gamma-ray bursts (LGRBs) have been rapidly growing, even approaching the very early Universe (the record holder currently is <it>z</it>= 8.3). The observed high-<it>z</it> LGRB rate shows significant excess over that estimated from the star formation history. We investigate what may be responsible for this high productivity of GRBs at high-<it>z</it> through Monte Carlo simulations, with effective <it>Swift</it>/Burst Alert Telescope (BAT) trigger and redshift detection probabilities based on current <it>Swift</it>/BAT sample and <it>Compton Gamma-ray Observatory</it>/Burst and Transient Source Experiment LGRB sample. We compare our simulations to the <it>Swift</it> observations via log <it>N</it>– log <it>P</it>, peak luminosity (<it>L</it>) and redshift distributions. In the case that LGRB rate is purely proportional to the star formation rate, our simulations poorly reproduce the LGRB rate at <it>z</it> > 4, although the simulated log <it>N</it>– log <it>P</it> distribution is in good agreement with the observed one. Assuming that the excess of high-<it>z</it> GRB rate is due to the cosmic metallicity evolution or unknown LGRB rate increase parametrized as (1 +<it>z</it>)δ, we find that although the two scenarios alone can improve the consistency between our simulations and observations, incorporation of them gives much better consistency. We get 0.2 < ϵ < 0.6 and δ < 0.6, where ϵ is the metallicity threshold for the production of LGRBs. The best consistency is obtained with a parameter set (ϵ, δ) = (∼ 0.4, ∼ 0.4), and BAT might trigger a few LGRBs at <it>z</it>≃ 14. With increasing detections of GRBs at <it>z</it> > 4 (∼15 per cent of GRBs in current <it>Swift</it> LGRB sample based on our simulations), a window for very early Universe is opening by <it>Swift</it> and up-coming space-based multiband astronomical variable object monitor missions.