Greybody factors imprinted on black hole ringdowns. II. Merging binary black holes

The spectral amplitude of the merger-ringdown gravitational wave (GW) emitted by a comparable mass-ratio black hole merger is modeled by the greybody factor of the remnant black hole. We also include the post-Newtonian correction to the greybody factor model. Our model includes only a few fitting parameters, which could evade the overfitting issue. We perform the mass-spin inference from the simulating extreme spacetimes (SXS) data without tuning the data range of each SXS waveform. Also, we find that the exponential damping in the ringdown spectral amplitude can be modeled well with the exponential damping in the greybody factor at high frequencies. Our findings could be consistent with a conjecture that the light ring of the remnant black hole, which sources the ringdown, forms as early as during the merger stage. We discuss the formation of the light ring in the static binary solution as a first step towards the understanding of how the separation of merging black holes may affect the formation of the light ring.