Searching for a Ringdown Overtone in GW150914

Abstract

We reanalyze the GW150914 post-merger data searching for quasinormal modes beyond the fundamental, quadrupolar mode. There is currently an ongoing disagreement in the literature about whether, and to what extent, the data contains evidence for a quasinormal mode overtone. We use a frequency-domain approach to ringdown data analysis that was recently proposed by the authors. Our analysis has several advantages compared to other analyses performed mainly in the time domain; in particular, the source sky position and the ringdown start time are marginalized over (as opposed to simply being fixed) as part of a Bayesian ringdown analysis. We find tentative evidence for an overtone in GW150914, but at a lower significance than reported elsewhere. Our preferred analysis, marginalizing over the uncertainty in the time of peak strain amplitude, gives a posterior on the overtone amplitude peaked away from zero at ∼ 1.8σ.

Collective filters: a new approach to analyze the gravitational-wave ringdown of binary black-hole mergers

“…This led to a test of the no-hair theorem at the ∼ 20% level. Recently, Cotesta et al [53] raised an opposing viewpoint that the search for the first overtone in the ringdown of GW150914 might be impacted by noises, therefore the conclusion still remains controversial [54,55]. On the other hand, Capano et al [56] studied the QNM spectrum of GW190521 [57] and found the l = m = 3 harmonic.…”

Generation and propagation of nonlinear quasinormal modes of a Schwarzschild black hole

“…For instance, [19] concludes that for precessing binary systems, linear QNMs do not always fit well GW signals from NR simulations starting from the merger time. Nevertheless, these results have motivated the use of the entire post-merger signal of current GW events, such as GW150914 [20], to detect the fundamental QNM (n = 0) as well as the first overtone (n = 1), and to perform tests of gravity [8,[21][22][23], although different conclusions have been obtained [24][25][26][27].…”

Nonlinearities in Black Hole Ringdowns

“…To date, the linear QNM spectrum has been used to analyze current GW detections [17][18][19], forecast the future detectability of ringdown [20][21][22], and perform tests of gravity in the strong field regime [23,24].…”