Multimode Quasinormal Spectrum from a Perturbed Black Hole.

Area of Science:

• Astrophysics
• Gravitational Wave Astronomy
• General Relativity

Background:

• Black hole mergers produce gravitational waves, with the late stage (ringdown) characterized by damped sinusoids.
• The black hole no-hair theorem posits that ringdown spectra depend solely on the final black hole's mass and spin.
• Observing multiple ringdown modes allows for testing this fundamental theorem.

Purpose of the Study:

• To search for and confirm evidence of a multimode ringdown spectrum from a black hole merger.
• To test the black hole no-hair theorem using gravitational wave observations.
• To constrain the properties of the final black hole formed during the merger.

Main Methods:

• Analysis of gravitational wave data from the event GW190521.
• Statistical model comparison using Bayes factors to distinguish between single-mode and two-mode models.
• Estimation of black hole parameters (mass, spin) and mode amplitudes.

Main Results:

• Strong evidence (Bayes factor 56±1) for a two-mode ringdown spectrum over a single mode.
• Identification of the dominant ℓ=m=2 and subdominant ℓ=m=3 harmonics.
• Estimation of the final black hole's mass (330$_{-40}^{+30}$M$_{\odot}$) and spin (0.86$_{-0.11}^{+0.06}$), consistent with the no-hair theorem.

Conclusions:

• The observed multimode ringdown spectrum from GW190521 provides significant observational support for the black hole no-hair theorem.
• The final black hole's properties are consistent with general relativity's predictions.
• Constraints were placed on deviations from general relativity for the subdominant mode's frequency.