Galaxy fields of LISA massive black hole mergers in a simulated universe

Laser Interferometer Space Antenna (LISA) will extend the search for gravitational waves (GWs) at 0.1 - 100 mHz where loud signals from coalescing binary black holes of 10(4) - 10(7) M-circle dot are expected. Depending on their mass and luminosity distance, the uncertainty in the LISA sky-localization decreases from hundreds of deg(2) during the inspiral phase to fractions of a deg(2) after the merger. By using the semi-analytical model L-Galaxies applied to the Millennium-I merger trees, we generate a simulated universe to identify the hosts of z <= 3 coalescing binaries with total mass of 3 x 10(5), 3 x 10(6), and 3 x 10(7) M-circle dot, and varying mass ratio. We find that, even at the time of merger, the number of galaxies around the LISA sources is too large (greater than or similar to 10(2)) to allow direct host identification. However, if an X-ray counterpart is associated to the GW sources at z < 1, all LISA fields at merger are populated by less than or similar to 10 active galactic nuclei (AGNs) emitting above similar to 10(-17) erg cm(-2) s(-1). For sources at higher redshifts, the poorer sky-localization causes this number to increase up to similar to 10(3). Archival data from eRosita will allow discarding similar to 10 per cent of these AGNs, being too shallow to detect the dim X-ray luminosity of the GW sources. Inspiralling binaries in an active phase with masses less than or similar to 10(6) M-circle dot at z <= 0.3 can be detected, as early as 10 h before the merger, by future X-ray observatories in less than a few minutes. For these systems, less than or similar to 10 AGNs are within the LISA sky-localization area. Finally, the LISA-Taiji network would guarantee the identification of an X-ray counterpart 10 h before merger for all binaries at z less than or similar to 1.

Automated summary

The Laser Interferometer Space Antenna (LISA) aims to detect gravitational waves in the frequency range of 0.1 - 100 mHz, particularly those generated by binary black holes with masses of 10^4 - 10^7 solar masses. Through a simulated universe, it is found that the identification of hosts around LISA sources is challenging due to a large number of galaxies. However, if an X-ray counterpart is associated, a small number of active galactic nuclei (AGNs) can be observed. Additionally, future X-ray observatories can detect inspiralling binaries with masses less than or equal to 10^6 solar masses before the merger.