The possible existence of Pop III NS-BH binary and its detectability

TL;DR

This paper explores the potential existence and detectability of Population III neutron star-black hole binaries, suggesting that their merger rate could be detectable by current gravitational wave observatories due to natal kicks.

Contribution

It introduces population synthesis simulations including neutron star kicks to estimate the merger rate of Pop III NS-BH binaries, highlighting their possible observability.

Findings

Estimated merger rate of ~1 Gpc^{-3} yr^{-1} for Pop III NS-BH binaries.

Pop III NS-BH mergers could be detected by Advanced LIGO and Virgo in upcoming observation runs.

Neutron star kicks significantly increase the likelihood of Pop III NS-BH mergers within observable timescales.

Abstract

In the population synthesis simulations of Pop III stars, many BH (Black Hole)-BH binaries with merger time less than the age of the Universe $(\tau_{\rm H})$ are formed, while NS (Neutron Star)-BH binaries are not. The reason is that Pop III stars have no metal so that no mass loss is expected. Then, in the final supernova explosion to NS, much mass is lost so that the semi major axis becomes too large for Pop III NS-BH binaries to merge within $\tau_{\rm H}$. However it is almost established that the kick velocity of the order of $200-500{\rm~ km~s^{-1}}$ exists for NS from the observation of the proper motion of the pulsar. Therefore, the semi major axis of the half of NS-BH binaries can be smaller than that of the previous argument for Pop III NS-BH binaries to decrease the merging time. We perform population synthesis Monte Carlo simulations of Pop III NS-BH binaries including the kick of NS and find that the event rate of Pop III NS-BH merger rate is $\sim 1 {\rm Gpc^{-3} yr^{-1}}$. This suggests that there is a good chance of the detection of Pop III NS-BH mergers in O2 of Advanced LIGO and Advanced Virgo from this autumn.