Ultrawide Black Hole—Neutron Star Binaries as a Possible Source for Gravitational Waves and Short Gamma-Ray Bursts

Abstract

The third observing run of the LIGO/Virgo/KARGA collaboration reported a few neutron star–black hole (NSBH) merger events. While NSBH mergers have yet to receive extensive theoretical attention, they may have a promising electromagnetic signature in the form of short gamma-ray bursts. Here we show that NSBH dynamical mergers can naturally form from ultrawide binaries in the field. Flyby gravitational interactions with other neighbors in the galaxy in these ultrawide systems may result in high eccentricity that drives the binary into a merger. We show that this process can result in a merger rate at the order of ∼10 Gpc−3 yr−1 (∼1 Gpc−3 yr−1) for elliptical (spiral) galaxies. This channel predicts a higher merger rate with a higher velocity dispersion of the host galaxy, a delay time distribution that is steeper than uniform but shallower than 1/t, and a higher merger rate for a lower black hole to neutron star mass ratio.