  On May 21, 2019 at 03:02:29 UTC Advanced LIGO and Advanced Virgo observed a
short duration gravitational-wave signal, GW190521, with a three-detector
network signal-to-noise ratio of 14.7, and an estimated false-alarm rate of 1
in 4900 yr using a search sensitive to generic transients. If GW190521 is from
a quasicircular binary inspiral, then the detected signal is consistent with
the merger of two black holes with masses of $85^{+21}_{-14} M_{\odot}$ and
$66^{+17}_{-18} M_{\odot}$ (90 % credible intervals). We infer that the primary
black hole mass lies within the gap produced by (pulsational) pair-instability
supernova processes, and has only a 0.32 % probability of being below $65
M_{\odot}$. We calculate the mass of the remnant to be $142^{+28}_{-16}
M_{\odot}$, which can be considered an intermediate mass black hole (IMBH). The
luminosity distance of the source is $5.3^{+2.4}_{-2.6}$ Gpc, corresponding to
a redshift of $0.82^{+0.28}_{-0.34}$. The inferred rate of mergers similar to
GW190521 is $0.13^{+0.30}_{-0.11}\,\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$.
