Multimode ringdown modeling with qnmfits and KerrRingdown

Gravitational-wave astronomy is dominated by the study of the inspiral and ringdown phases of the signal. In the last decade, the ringdown community has made large strides in understanding the aftermath of binary black hole mergers through the study of numerical simulations. In this note, we introduce two flavors of fitting algorithms, that have been verified against each other, for the extraction of quasinormal mode amplitudes from ringdown waveforms - qnmfits in Python and KerrRingdown in Mathematica. In addition, these codes are able to simultaneously fit for the remnant mass and spin of the system. Different from other methods, we include a greedy algorithm that automatically chooses which modes to model based on their power content. These tools can be applied to any generic waveform, including those extracted using Cauchy-characteristic Evolution.