Large black hole recoils can help identify spin precession
Remnant kicks in signals from binary black holes improve measurements of spin precession in data from current gravitational-wave detectors
When the spins of black hole binaries are not aligned with the orbital angular momentum of the system, the orbital plane precesses. This, in turn, leaves an imprint in the emitted gravitational-wave signal. Identifying this spin precession in the data from current gravitational-wave detectors is very challenging. So far, there is only one gravitational-wave event (GW200129_065458) with strong evidence for spin precession. Interestingly, its remnant also shows a large recoil (“kick”) velocity. Motivated by this fact, a team of researchers from the independent Max Planck Research Group “Binary Merger Observations and Numerical Relativity” investigated whether the presence of a kick helps to extract more information about the spins. They simulated signals from black hole binaries with and without kicks, but with the same amount of spin precession. They found that significant kicks correlate to more signal structure and lead to more informative measurements of the binary's properties. As a result, spin precession could be detectable even for signals that lie in challenging regions of the parameter space, where it was previously thought to be almost impossible.
Paper abstract
Remnants of binary black-hole mergers can gain significant recoil or kick velocities when the binaries are asymmetric. The kick is the consequence of the anisotropic emission of gravitational waves, which may leave a characteristic imprint in the observed signal. So far, only one gravitational-wave event supports a nonzero kick velocity: GW200129_065458. This signal is also the first to show evidence for spin precession. For most other gravitational-wave observations, spin orientations are poorly constrained as this would require large signal-to-noise ratios, unequal mass ratios, or inclined systems. Here we investigate whether the imprint of the kick can help to extract more information about the spins. We perform an injection and recovery study comparing binary black-hole signals with significantly different kick magnitudes, but the same spin magnitudes and spin tilts. To exclude the impact of higher signal harmonics in parameter estimation, we focus on equal-mass binaries that are oriented face-on. This is also motivated by the fact that equal-mass binaries produce the largest kicks and many observed gravitational-wave events are expected to be close to this configuration. We generate signals with IMRPhenomXO4a, which includes mode asymmetries. These asymmetries are the main cause for the kick in precessing binaries. For comparison with an equivalent model without asymmetries, we repeat the same injections with IMRPhenomXPHM. We find that signals with large kicks necessarily include large asymmetries, and these give more structure to the signal, leading to more informative measurements of the spins and mass ratio. Our results also complement previous findings that argued precession in equal-mass, face-on, or face-away binaries is nearly impossible to identify. In contrast, we find that in the presence of a remnant kick, even those signals become more informative and allow determining precession with signal-to-noise ratios observable already by current gravitational-wave detectors.
