text: 104,202,1392285789652926467,130881465,Alex Nitz,"In work led by @CollinCapano we find support for a sought after sub-dominant mode in the 'ringdown' of GW190521. This enables a test of the nature of the final black hole and suggests limits on the mass ratio of the initial binary. <LINK> <LINK> General relativity predicts that for a Kerr black hole (spinning, no charge) if it is perturbed, such as when resulting from the merger of two progenitor black holes, it will emit gravitational radiation during this 'ringdown' with a predictable discrete spectrum of frequencies. A key principle is that spectrum of these modes only depends on the total mass and spin of the black hole. (the relative amplitudes may however depend on the initial system). By comparing the observed modes and the prediction from General relativity, we can test the nature of the final black hole and look for signs of new physics. We find the observed signal is very consistent with what you'd expect from a perturbed Kerr black hole. <LINK> The presence of one particular mode also indicates that the original system was unlikely to have been made of equal mass black holes (NR simulations suggest that unequal masses are required to excite this mode). This allows one to predict what the mass ratio may have been. <LINK> GW190521 has been somewhat of mystery for the gravitational-wave community. It is the highest mass merger observed to date, there is a possible EM flare observed in coincidence, and several studies have suggested the merger may be eccentric. Likely mysteries remain!",https://arxiv.org/abs/2105.05238,"When two black holes merge, the late stage of gravitational wave emission is a superposition of exponentially damped sinusoids. According to the black hole no-hair theorem, this ringdown spectrum depends only on the mass and angular momentum of the final black hole. An observation of more than one ringdown mode can test this fundamental prediction of general relativity. Here we provide strong observational evidence for a multimode black hole ringdown spectrum using the gravitational wave event GW190521, with a Bayes factor of $\sim 40$ preferring two fundamental modes over one. The dominant mode is the $\ell=m=2$ harmonic, and the sub-dominant mode corresponds to the $\ell=m=3$ harmonic. We estimate the redshifted mass and dimensionless spin of the final black hole as $330^{+30}_{-40}\,\mathrm{M}_\odot$ and $0.87^{+0.05}_{-0.10}$, respectively. The detection of the two modes disfavors a binary progenitor with equal masses; the mass ratio is constrained to $0.4^{+0.2}_{-0.3}$. We find that the final black hole is consistent with the no hair theorem and constrain the fractional deviation from general relativity of the sub-dominant mode's frequency to be $-0.01^{+0.07}_{-0.11}$. ","Observation of a multimode quasi-normal spectrum from a perturbed black