# Black hole spectroscopy with coherent mode stacking

## TL;DR

This paper introduces a coherent mode stacking technique to enhance the detection of specific black hole ringdown modes in gravitational wave data, enabling more precise tests of General Relativity and alternative gravity theories.

## Contribution

The authors propose a novel coherent stacking method that combines multiple gravitational wave signals to improve the detection of targeted black hole ringdown modes.

## Key findings

- Method significantly boosts signal-to-noise ratio for target modes.
- Likely detection of collective ringdown modes with one year of data at advanced detector sensitivity.
- Simulation results demonstrate the effectiveness of the stacking technique.

## Abstract

The measurement of multiple ringdown modes in gravitational waves from binary black hole mergers will allow for testing fundamental properties of black holes in General Relativity, and to constrain modified theories of gravity. To enhance the ability of Advanced LIGO/Virgo to perform such tasks, we propose a coherent mode stacking method to search for a chosen target mode within a collection of multiple merger events. We first rescale each signal so that the target mode in each of them has the same frequency, and then sum the waveforms constructively. A crucial element to realize this coherent superposition is to make use of a priori information extracted from the inspiral-merger phase of each event. To illustrate the method, we perform a study with simulated events targeting the l=m=3 ringdown mode of the remnant black holes. We show that this method can significantly boost the signal-to-noise ratio of the collective target mode compared to that of the single loudest event. Using current estimates of merger rates we show that it is likely that advanced-era detectors can measure this collective ringdown mode with one year of coincident data gathered at design sensitivity.