Detectable environmental effects in GW190521-like black-hole binaries with LISA

TL;DR

LISA will detect and localize GW190521-like black-hole binaries in gas-rich environments months to years before LIGO/Virgo, enabling early electromagnetic follow-up and studying environmental effects on gravitational wave signals.

Contribution

This paper demonstrates LISA's capability to detect and analyze environmental effects in massive black-hole binaries before ground-based detectors.

Findings

LISA can detect up to ten such binaries before LIGO/Virgo.

LISA can localize sources within approximately 1 square degree.

LISA can measure deviations caused by gas and orbital effects in waveforms.

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.