The cosmic merger rate of neutron stars and black holes

TL;DR

This study combines population-synthesis simulations with cosmological data to estimate the cosmic merger rates of neutron star and black hole binaries, revealing dependencies on physical assumptions and aligning with gravitational wave observations.

Contribution

It introduces a comprehensive simulation approach coupling population synthesis with cosmological models to estimate merger rates under various physical assumptions.

Findings

BHB merger rate: 150-240 Gpc$^{-3}$ yr$^{-1}$, mildly dependent on CE efficiency.

DNS merger rate: 20-600 Gpc$^{-3}$ yr$^{-1}$, strongly dependent on assumptions.

NSBH merger rate: 10-100 Gpc$^{-3}$ yr$^{-1}$, sensitive to model parameters.

Abstract

Six gravitational wave detections have been reported so far, providing crucial insights on the merger rate of double compact objects. We investigate the cosmic merger rate of double neutron stars (DNSs), neutron star-black hole binaries (NSBHs) and black hole binaries (BHBs) by means of population-synthesis simulations coupled with the Illustris cosmological simulation. We have performed six different simulations, considering different assumptions for the efficiency of common envelope (CE) ejection and exploring two distributions for the supernova (SN) kicks. The current BHB merger rate derived from our simulations spans from $\sim{}150$ to $\sim{}240$ Gpc$^{-3}$ yr$^{-1}$ and is only mildly dependent on CE efficiency. In contrast, the current merger rates of DNSs (ranging from $\sim{}20$ to $\sim{}600$ Gpc$^{-3}$ yr$^{-1}$) and NSBHs (ranging from $\sim{}10$ to $\sim{}100$ Gpc$^{-3}$ yr$^{-1}$) strongly depend on the assumptions on CE and natal kicks. The merger rate of DNSs is consistent with the one inferred from the detection of GW170817 only if a high efficiency of CE ejection and low SN kicks (drawn from a Maxwellian distribution with one dimensional root mean square $\sigma{}=15$ km s$^{-1}$) are assumed.