Polarization Analysis of Ringdown Signals

Merging binary black holes exhibit a ringdown phase in their gravitational wave signals composed of modes excited perturbatively on the remnant. We study ringdowns to test general relativity and explore the astrophysics of mergers. When the excitations of Kerr ringdown modes have equatorial reflection symmetry, as is the case for (anti-)aligned spin black hole mergers, the measured polarization amplitudes and phases in each mode are determined by the viewing (or inclination) angle to the equatorial plane. We use such a constrained model to fit the ringdown signals of both non-precessing and precessing systems such as GW150914 and GW190521, respectively. We show that the ellipticity of ringdown signals like those of GW150914 is measurable even for events detected by only the two Hanford-Livingston LIGO detectors, and we can translate it into a measurement of the system's inclination assuming the reflection symmetry above. For precessing systems that break the spin alignment symmetry, we explore the severity of this effect in ringdown analyses using GW190521-like injections. Despite the model's apparent limitations in accurately recovering injected mode amplitudes, we find it gives an adequate measurement of the remnant mass and spin of a precessing system.