Tracking black hole kicks from gravitational wave observations

TL;DR

This paper demonstrates that gravitational wave observations can be used to infer the recoil velocity of black holes, providing the first potential observational signature of linear momentum transport by gravitational waves.

Contribution

It introduces a method to extract the line-of-sight component of black hole recoil velocities from gravitational wave data using higher-order modes.

Findings

For certain binary black hole systems, radial kicks of 120 km/s can be statistically distinguished from zero at SNR 15.

The method applies to non-spinning binaries with mass ratio ≥ 2 and total mass around 100 solar masses.

This approach enables the first observational evidence of linear momentum transfer via gravitational waves.

Abstract

Coalescing binary black holes emit anisotropic gravitational radiation. This causes a net emission of linear momentum that produces a gradual acceleration of the source. As a result, the final remnant black hole acquires a characteristic velocity known as recoil velocity or gravitational kick. The symmetries of gravitational wave emission are reflected in the interactions of the gravitational wave modes emitted by the binary. In this work we make use of the rich information encoded in the higher-order modes of the gravitational wave emission to infer the component of the kick along the line-of-sight (or \textit{radial kick}). We do this by performing parameter inference on simulated signals given by numerical relativity waveforms for non-spinning binaries using numerical relativity templates of aligned-spin (non-precessing) binary black holes. We find that for suitable sources, namely those with mass ratio $q\geq 2$ and total mass $M \sim 100M_\odot$, and for modest radial kicks of $120km/s$, the $90\%$ credible intervals of our posterior probability distributions can exclude a zero kick at a signal-to-noise ratio of $15$; using a single Advanced LIGO detector working at its early sensitivity. The measurement of a non-zero radial kick component would provide the first observational signature of net transport of linear momentum by gravitational waves away from their source.