  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 an unprecedented multi-pronged search for echoes via two
well-established and independent pipelines: a template-based 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 expected to lead to post-merger echoes at horizon mode
frequency of $\sim50$ Hz (for quadrupolar gravitational radiation), repeating
at intervals of $\sim1$ second, due to partial reflection off Planckian quantum
structure of the horizon. A careful analysis using dynamic nested sampling
yields a Bayesian evidence of $8^{+4}_{-2}$ (90\% confidence level) for this
signal following GW190521, carrying an excess of $6^{+10}_{-5}\%$ 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, around the event, giving a false (true) positive detection
probability for higher Bayes factors of $1.5^{+1.2}_{-0.9}\%$ ($35\pm7\%$).
Similarly, the reconstructed waveform of the first echo in cWB carries an
energy excess of $13^{+16}_{-7}\%$. While the current evidence for stimulated
Hawking radiation does not reach the gold standard of $5\sigma$ (or p-value
$<3\times10^{-7}$), our findings are in line with expectations 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.
