Mass ratio of binary black holes determined from LIGO/Virgo data restricted to small false alarm rate

Abstract

ABSTRACT We focus on gravitational wave events of binary black hole mergers up to the third observing run with the minimum false alarm rate smaller than $10^{-5}\, {\rm yr}^{-1}$. These events tell us that the mass ratio of two black holes follows $m_2/m_1=0.723$ with the chance probability of 0.00301 per cent for the chirp mass $M_{\rm chirp}\,\gt 18\, {\rm M}_{\odot }$. We show that the relation of $m_2/m_1=0.723$ is consistent with the binaries originated from population III stars which are the first stars in the Universe. On the other hand, it is found for $M_{\rm chirp}\,\lt\, 18 \,{\rm M}_{\odot }$ that the mass ratio follows $m_2/m_1=0.601$ with the chance probability of 0.117 per cent if we ignore GW190412 with $m_2/m_1\sim 0.32$. This suggests a different origin from that for $M_{\rm chirp}\ \gt\ 18 \,{\rm M}_{\odot }$.