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Abstract

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 $\delta f_{330} =
-0.010^{+0.073}_{-0.121}$