Item

Abstract

GW190521 is the compact binary with the largest masses observed to date, with
at least one in the pair-instability gap. This event has also been claimed to
be associated with an optical flare observed by the Zwicky Transient Facility
in an Active Galactic Nucleus (AGN), possibly due to the post-merger motion of
the merger remnant in the AGN gaseous disk. We show that the Laser
Interferometer Space Antenna (LISA) will detect up to ten of such gas-rich
black hole binaries months to years before their detection by LIGO/Virgo-like
interferometers, localizing them in the sky within $\approx1$ deg$^2$. LISA
will also measure directly deviations from purely vacuum and stationary
waveforms, arising from gas accretion, dynamical friction, and orbital motion
around the AGN's massive black hole (acceleration, strong lensing, and Doppler
modulation). LISA will therefore be crucial to alert and point electromagnetic
telescopes ahead of time on this novel class of gas-rich sources, to gain
direct insight on their physics, and to disentangle environmental effects from
corrections to General Relativity that may also appear in the waveforms at low
frequencies.