Signatures of mass ratio reversal in gravitational waves from merging binary black holes

Abstract

The spins of merging binary black holes offer insights into their formation history. Recently it has been argued that in isolated binary evolution of two massive stars the firstborn black hole is slowly rotating, whilst the progenitor of the second-born black hole can be tidally spun up if the binary is tight enough. Naively, one might therefore expect that only the less massive black hole in merging binaries exhibits non-negligible spin. However, if the mass ratio of the binary is "reversed" (typically during the first mass transfer episode), it is possible for the tidally spun up second-born to become the more massive black hole. We study the properties of such mass-ratio reversed (MRR) binary black hole mergers using a large set of 560 population synthesis models. We find that the more massive black hole is formed second in 70% of binary black holes observable by LIGO, Virgo, and KAGRA for most model variations we consider, with typical total masses 20 M and mass ratios q = m 2 /m 1 ∼ 0.7 (where m 1 > m 2 ). The formation history of these systems typically involves only stable mass transfer episodes. The second-born black hole has non-negligible spin (χ > 0.05) in up to 25% of binary black holes, with among those the more (less) massive black hole spinning in 0%-80% (20%-100%) of cases, varying greatly in our models. We discuss our models in the context of several observed gravitational-wave events and the observed mass ratio -effective spin correlation.

No peaks without valleys: The stable mass transfer channel for gravitational-wave sources in light of the neutron star-black hole mass gap

“…For β acc = 0.0, M primary is always M BH,a (cf. Broekgaarden et al 2022;Zevin & Bavera 2022). For β acc = 1, the distribution in M primary drops off steeply below about 8 M , while for β acc = 0 there no real gap left in the mass distribution.…”

Signatures of spin precession and nutation in isolated black-hole binaries

“…Mass-ratio reversal (MRR) occurs when the primary star evolves into the less massive BH, e.g. 's, [31,32]. In Pathways A1 and A2, it depends crucially on the fraction f a of the donor's envelope that is accreted during stable mass transfer, while in Pathway B2 it also depends on Kerr-limit mass loss in BH formation of the primary star (i.e., isotropic mass loss causes MRR, and negligible mass loss does not).…”

“…We hope that this study can help to motivate further investigation into the possibility of observing precession and nutation in GW data of BBHs, which may uncover the likely formation channels of the BBH population. An evolutionary scenario where both binary components lose their envelopes through stable mass transfer [32] could result in a nutating BBH if efficient accretion yields two high spins. Also, the chemically homogeneous evolution scenario may provide nutating BBHs if the tidally or rotationally induced high spins are retained [54,55].…”

Four eccentric mergers increase the evidence that LIGO--Virgo--KAGRA's binary black holes form dynamically

“…Merger products should develop high spins as they accumulate angular momentum through repeated mergers, so high spin magnitude can indicate dynamical formation Kimball et al 2020;Tagawa et al 2021a), since the spins of black holes that form via stellar collapse are typically expected to be small. However, chemically homogeneous evolution (Marchant et al 2016;Qin et al 2019), mass transfer and/or tidal locking in tight binaries (Valsecchi et al 2010;Qin et al 2019;Neijssel et al 2021;Izzard et al 2003;Belczynski et al 2020;Bavera et al 2020;Zevin & Bavera 2022;Broekgaarden et al 2022), and differential rotation between the stellar core and envelope (Hirschi et al 2005) can also produce rapidly-spinning black holes.…”