GW190521: Search for echoes due to stimulated Hawking radiation from black holes

Being arguably the most massive binary black hole merger event observed to date, GW190521 deserves special attention. The exceptionally loud ringdown of this merger makes it an ideal candidate to search for gravitational wave echoes, a proposed smoking gun for the quantum structure of black hole horizons. We perform a multipronged search for echoes via two well-established and independent pipelines; a templatebased search for stimulated emission of Hawking radiation, or Boltzmann echoes, and the model-agnostic coherent WaveBurst (cWB) search. Stimulated Hawking radiation from the merger is proposed to lead to postmerger echoes at horizon mode frequency of similar to 50 Hz (for quadrupolar gravitational radiation), repeating at intervals of similar to 1 second, due to partial reflection off Planckian quantum structure of the horizon. An analysis using dynamic nested sampling yields a Bayesian evidence of 8(-2)(+4) (90% confidence level) for this signal following GW190521, carrying an excess of 6(-5)(+10)% in gravitational wave energy, relative to the main event (consistent with the predicted amplitude of Boltzmann echoes). The "look-elsewhere" effect is estimated by using general relativity (plus Boltzmann echoes) injections in real data, before and after the event, giving a false (true) positive detection probability for higher Bayes factors of 1.5(-0.9)(+1.2)% (35 +/- 7%). Similarly, the reconstructed waveform of the first echo in cWB carries an energy excess of 13(-7)(+16)%. While the current evidence for stimulated Hawking radiation does not reach the gold standard of 5 sigma (or p-value < 3 x 10(-7)), our findings are in line with predictions for stimulated Hawking radiation at current detector sensitivities. The next generation of gravitational wave observatories can thus draw a definitive conclusion on the quantum nature of black hole horizons.