Observational black hole spectroscopy: A time-domain multimode analysis of GW150914

Abstract

The detection of the least damped quasi-normal mode from the remnant of the gravitational wave event GW150914 realised the long sought possibility to observationally study the properties of quasi-stationary black hole spacetimes through gravitational waves. Past literature has extensively explored this possibility and the emerging field has been named "black hole spectroscopy". In this study, we present results regarding the ringdown spectrum of GW150914, obtained by application of Bayesian inference to identify and characterise the ringdown modes. We employ a pure time-domain analysis method which infers from the data the time of transition between the non-linear and quasi-linear regime of the post-merger emission in concert with all other parameters characterising the source. We find that the data provides no evidence for the presence of more than one quasi-normal mode. However, from the central frequency and damping time posteriors alone, no unambiguous identification of a single mode is possible. More in-depth analysis adopting a ringdown model based on results in perturbation theory over the Kerr metric, confirms that the data do not provide enough evidence to discriminate among an l = 2 and the l = 3 subset of modes. Our work provides the first comprehensive agnostic framework to observationally investigate astrophysical black holes' ringdown spectra.

Testing the No-Hair Theorem with GW150914

“…Although LIGO [13] and Virgo [14] have already confidently detected gravitational waves from multiple binaryblack-hole coalescences [15][16][17][18][19][20][21], black hole spectroscopy has remained elusive [22][23][24][25][26][27][28]. This is because past analyses looked for the ringdown in data at late times after the signal peak, where the quasinormal modes are too weak to confidently characterize with current instruments.…”

“…This is because past analyses looked for the ringdown in data at late times after the signal peak, where the quasinormal modes are too weak to confidently characterize with current instruments. The choice to focus on the late, weak-signal regime stemmed from concerns about nonlinearities surrounding the black hole merger, which were traditionally expected to contaminate the ringdown measurement at earlier times [8,24,25,[27][28][29][30].…”

Scalar fields and parametrized spherically symmetric black holes: Can one hear the shape of space-time?

“…Of course, in principle the higher order parameters might be well constrained once the whole spectrum is being provided with pristine precision, but such a scenario is far away from actual observations. In the loudest event GW150914, only rough constraints on the fundamental quasi-normal mode are possible [10,11,55]. Since we did not consider rotating black holes in this work, our work has to be extended in the future.…”

Extracting the Gravitational Recoil from Black Hole Merger Signals

“…[63][64][65][66][67]) for the remnant mass and spin to obtain posterior distributions for these quantities. Finally, considering only the ringdown signal and varying the quasi-normalmode frequencies [68][69][70][71], the remnant mass and spin are independently measured [16,19,21,22,73,74].…”

“…The end state of the remnant is entirely characterized by its mass (m f ), spin (χ f ) and kick velocity (v f ); all additional complexities ("hair") [14,15] are dissipated away in GWs during the ringdown stage that follows the merger. The remnant mass and spin have already been measured from GW signals and used to test general relativity [16][17][18][19][20][21][22][23]. However, a measurement of the kick has remained elusive.…”