Signs of higher multipoles and orbital precession in GW151226

We present a reanalysis of GW151226, the second binary black hole merger discovered by the LIGO-Virgo Collaboration. Previous analysis showed that the best-fit waveform for this event corresponded to the merger of a ∼14 M⊙ black hole with a ∼7.5 M⊙ companion, and the posterior distribution in mass ratio (q≤1) is rather flat. In this work, we perform parameter estimation using a waveform model that includes the effects of orbital precession and higher-order radiative multipole modes, and we find that the source parameters of GW151226 shift toward the low q and high effective spin (χ_eff) region and that q is better measured. The new solution has a log likelihood roughly two points higher than when either higher multipoles or orbital precession is neglected and can alter the astrophysical interpretation of GW151226. Additionally, we find it useful to use a flat-in-χ_eff prior, which does not penalize the large |χ_eff| region, in order to uncover the higher likelihood region for GW151226. Our solution has several interesting properties: (a) the secondary black hole mass is close to the upper limit of the hypothesized lower mass gap of astrophysical black hole population; and (b) orbital precession is driven by the primary black hole spin, which has a dimensionless magnitude as large as ∼0.85 and is tilted away from the orbital angular momentum at an angle of ∼57°. Since GW151226 is a relatively weak signal, an unambiguous claim of the detection of these effects in the signal cannot be made.