Ultra wide 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 ultra - wide binaries in the field. Flyby gravitational interactions with other neighbors in the galaxy in these ultra - wide 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 $\sim 10$~Gpc$^{-3}$~yr$^{-1}$ ($\sim 5$~Gpc$^{-3}$~yr$^{-1}$) for elliptical (spiral) galaxies. This channel predicts higher merger rate with higher velocity dispersion of the host - galaxy, delay time distribution which shallower than uniform but steeper that $1/t$, higher merger rate for lower BH to NS mass ratio.