Measuring precession in asymmetric compact binaries

Gravitational-wave observations of merging compact binaries hold the key to precision measurements of the objects' masses and spins. General relativistic precession, caused by spins misaligned with the orbital angular momentum, is considered a crucial tracer for determining the binary's formation history and environment and it also improves mass estimates; its measurement is therefore of particular interest with wide-ranging implications. Precession leaves a characteristic signature in the emitted gravitational-wave signal that is even more pronounced in binaries with highly unequal masses. The recent observations of GW190412 and GW190814 have confirmed the existence of such asymmetric compact binaries. Here we perform a systematic study to assess the confidence in measuring precession in gravitational-wave observations of high-mass-ratio binaries and our ability to measure the mass of the lighter companion in neutron-star-black-hole-type systems. Using Bayesian model selection, we show that precession can be decisively identified for low-mass binaries with mass ratios as low as 1:3 and mildly precessing spins with magnitudes 0.4, even in the presence of systematic waveform errors.