Importance of mirror modes in binary black hole ringdown waveform

Abstract: The post-merger signal in binary black hole merger is described by linear, black-hole perturbation theory. Historically, this has been modeled using the dominant positive-frequency (corotating) fundamental mode. Recently, there has been a renewed effort in modeling the post-merger waveform using higher, positive-frequency overtones in an attempt to achieve greater accuracy in describing the waveform at earlier times using linear perturbation theory. It has been shown that the inclusion of higher overtones can shift the linear regime to the peak of $(l,m)=(2,2)$ spherical harmonic mode. In this work, we show that the inclusion of negative-frequency (counterrotating) modes, called 'mirror' modes, extends the validity of linear perturbation theory to even earlier times, with far lower systematic uncertainties in the model in recovering the remnant parameters at these early times. A good description of the signal at early times also enables for a greater signal-to-noise ratio to be accumulated in the ringdown phase, thereby, allowing for a more accurate measurement of remnant parameters and tests of general relativity.