Merger history of primordial black-hole binaries

Abstract

Primordial black holes (PBHs), as dark matter candidates, have attracted more and more attention as they could be possible progenitors of the heavy binary black holes (BBHs) observed by LIGO/Virgo. Accurately estimating the merger rate of PBH binaries will be crucial to reconstruct the mass distribution of PBHs. It was pointed out the merger history of PBHs may shift the merger rate distribution depending on the mass function of PBHs. In this paper, we use 10 BBH events from LIGO/Virgo O1 and O2 observing runs to constrain the merger rate distribution of PBHs by taking into account the effect of merger history. We perform the analysis by explicitly considering two commonly used PBH mass functions: the power-law and the log-normal ones, respectively. It is found that the second merger process makes a subdominant contribution to the total merger rate, and hence the merger history effect can be safely neglected. We also confirm that the main components of dark matter should not be made of stellar-mass PBHs.