  The first direct observation of a binary neutron star (BNS) merger was a
watershed moment in multi-messenger astronomy. However, gravitational waves
from GW170817 have only been observed prior to the BNS merger, but
electromagnetic observations all follow the merger event. While post-merger
gravitational wave signal in general relativity is too faint (given current
detector sensitivities), here we present the first tentative detection of
post-merger gravitational wave "echoes" from a highly spinning "black hole"
remnant. The echoes may be expected in different models of quantum black holes
that replace event horizons by exotic Planck-scale structure and tentative
evidence for them has been found in binary black hole merger events. The fact
that the echo frequency is suppressed by $\log M$ (in Planck units) puts it
squarely in the LIGO sensitivity window, allowing us to build an optimal
model-agnostic search strategy via cross-correlating the two detectors in
frequency/time. We find a tentative detection of echoes at $f_{\rm echo} \simeq
72$ Hz, around 1.0 sec after the BNS merger, consistent with a 2.6-2.7
$M_\odot$ "black hole" remnant with dimensionless spin $0.84-0.87$. Accounting
for all the "look-elsewhere" effects, we find a significance of $4.2 \sigma$,
or a false alarm probability of $1.6\times 10^{-5}$, i.e. a similar
cross-correlation within the expected frequency/time window after the merger
cannot be found more than 4 times in 3 days. If confirmed, this finding will
have significant consequences for both physics of quantum black holes and
astrophysics of binary neutron star mergers [Note added: This result is
independently confirmed by arXiv:1901.04138, who use the electromagnetic
observations to infer $t_{\rm coll}=0.98^{+0.31}_{-0.26}$ sec for black hole
formation].
