  The gravitational wave event, GW190521 is the most massive binary black hole
merger observed by ground-based gravitational wave observatories LIGO/Virgo to
date. While the observed gravitational-wave signal is mainly in the merger and
ringdown phases, the inspiral gravitational-wave signal of GW190521-like binary
will be more visible by space-based detectors in the low-frequency band. In
addition, the ringdown gravitational-wave signal will be more loud with the
next generation (3G) of ground-based detectors in the high-frequency band,
displaying a great potential of the multiband gravitational wave observations.
In this paper, we explore the scientific potential of multiband observations of
GW190521-like binaries with milli-Hz gravitational wave observatory: LISA,
deci-Hz observatory: B-DECIGO, and (next generation of) hecto-Hz observatories:
aLIGO and ET. In the case of quasicircular evolution, the triple-band
observation by LISA, B-DECIGO and ET will provide parameter estimation errors
of the masses and spin amplitudes of component black holes at the level of
order 1% -- 10%. This would allow consistency tests of general relativity in
the strong-field at an unparalleled precision, particularly with the "B-DECIGO
+ ET" observation. In the case of eccentric evolution, the multiband
signal-to-noise ratio by "B-DECIGO + ET" observation would be larger than 100
for a five year observation prior to coalescence, even with high final
eccentricities.
