White dwarf–black hole binary progenitors of low-redshift gamma-ray bursts

ABSTRACT Although there is strong evidence that many long gamma-ray bursts (LGRBs) are associated with the collapse of a massive star, tantalizing results in recent years have upended the direct association of all LGRBs with massive stars. In particular, kilonova signals in some LGRB light curves as well as a suggested uptick in the rate density of LGRBs at low redshifts (deviating significantly from the star formation rate) suggest that compact object mergers may be a non-negligible fraction of the LGRB population. Here, we investigate the contribution of white dwarf–black hole mergers to the LGRB population. We present evidence for the deviation of the LGRB rate density from the star formation rate at low redshifts, and provide analytic and numerical arguments for why a white dwarf–black hole merger system may be a viable progenitor to explain this deviation. We show the range of parameter space in which the durations, energetics, and rates of these systems can account for a significant subpopulation of low-redshift LGRBs.