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Being arguably the most massive binary black hole merger event observed todate, GW190521 deserves special attention. The exceptionally loud ringdown ofthis merger makes it an ideal candidate to search for gravitational waveechoes, a proposed smoking gun for the quantum structure of black holehorizons. We perform an unprecedented multi-pronged search for echoes via twowell-established and independent pipelines: a template-based search forstimulated emission of Hawking radiation, or Boltzmann echoes, and themodel-agnostic coherent WaveBurst (cWB) search. Stimulated Hawking radiationfrom the merger is expected to lead to post-merger echoes at horizon modefrequency of $\sim50$ Hz (for quadrupolar gravitational radiation), repeatingat intervals of $\sim1$ second, due to partial reflection off Planckian quantumstructure of the horizon. A careful analysis using dynamic nested samplingyields a Bayesian evidence of $8^{+4}_{-2}$ (90\% confidence level) for thissignal following GW190521, carrying an excess of $6^{+10}_{-5}\%$ ingravitational wave energy, relative to the main event (consistent with thepredicted amplitude of Boltzmann echoes). The "look-elsewhere" effect isestimated by using General Relativity (plus Boltzmann echoes) injections inreal data, around the event, giving a false (true) positive detectionprobability for higher Bayes factors of $1.5^{+1.2}_{-0.9}\%$ ($35\pm7\%$).Similarly, the reconstructed waveform of the first echo in cWB carries anenergy excess of $13^{+16}_{-7}\%$. While the current evidence for stimulatedHawking radiation does not reach the gold standard of $5\sigma$ (or p-value$<3\times10^{-7}$), our findings are in line with expectations for stimulatedHawking radiation at current detector sensitivities. The next generation ofgravitational wave observatories can thus draw a definitive conclusion on thequantum nature of black hole horizons.

GW190521: Search for Echoes due to Stimulated Hawking Radiation from Black Holes