Observation of a multimode quasi-normal spectrum from a perturbed black hole

We provide strong observational evidence for a multimode black hole ringdown spectrum, using the gravitational wave event GW190521. We show strong evidence for the presence of at least two ringdown modes, with a Bayes factor of $43.4^{+8.1}_{-6.8}$ preferring two modes over one. The dominant mode is the fundamental $\ell=m=2$ harmonic, and the sub-dominant mode corresponds to the fundamental $\ell=m=3$ harmonic. We estimate the redshifted mass and dimensionless spin of the final black hole as $332^{+31}_{-35}\,M_\odot$ and $0.871^{+0.052}_{-0.096}$ respectively. The detection of the two modes disfavors a binary progenitor with equal masses, and the mass ratio is constrained to $0.45^{+0.22}_{-0.29}$. General relativity predicts that the frequency and damping time of each mode in the spectrum depends only on two parameters, the black hole mass and angular momentum. Consistency between the different modes thus provides a test of general relativity. As a test of the black hole no-hair theorem, we constrain the fractional deviation of the sub-dominant mode frequency from the Kerr prediction to $\delta f_{330} = -0.010^{+0.073}_{-0.121}