Signatures of Mass Ratio Reversal in Gravitational Waves from Merging Binary Black Holes

The spins of merging binary black holes offer insights into their formation history. Recently it has been argued that in the isolated binary evolution of two massive stars the firstborn black hole is slowly rotating, while 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 nonnegligible 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 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 nonnegligible spin (χ > 0.05) in up to 25% of binary black holes, with among those the more (less) massive black hole is spinning in 0%-80% (20%-100%) of the 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.