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

13 Apr 2023

By Gaia Lops & David Izquierdo-Villalba

The Laser Interferometer Space Antenna (LISA) will search for gravitational waves (GWs) at 0.1 − 100 mHz where loud signals from inspiralling and coalescing massive black holes binaries (MBHBs) of 10⁴−10⁷ M_Sun at z<20 are expected. The detection of these MBHBs will open an important avenue for multimessenger studies since the simultaneous detection of MBHBs in the GW and electromagnetic band will constrain the luminosity distance-redshift relation. While the GW signal gives the source luminosity distance, the location of the electromagnetic emission associated with the MBHB provides the redshift information. However, this latter search is completely dependent on the sky-localization capabilities of LISA, which can pin down regions varying from hundreds to fractions of deg² and, thus, on the number of galaxies lying within these regions.

In this paper - led by Gaia Lops and David Izquierdo-Villalba (B-Massive group at Milano-Biocca University) - they explored the possibility of unequivocally identifying the host galaxies of the inspiralling and merging MBHBs. The authors used a novel approach of creating a simulated mock survey for LISA that accounts for the cosmological evolution of galaxies, AGNs and MBHBs, and assigning to each merging MBHB of 10⁴-10⁷ M_Sun realistic estimations of the LISA sky regions. This paper shows that, regardless of redshift and time remaining to the merge, the large uncertainties in the LISA sky-localization imply that as many as 10⁵ galaxies will be identified as candidates hosting the detected LISA MBHB. To overcome this limitation, the authors explored the possibility of seeking only X-ray AGN sources, assuming that GW sources would be uniquely associated with Eddington-limited AGNs. By using the future Athena X-ray observatory as the chosen detector, they proved that the X-ray selection reduces the number of host candidates by up to 2 orders of magnitude. However, the poor sky-location of LISA together with the small field of view and large exposure times of Athena limits the X-ray detection produced by MBHBs within 10 hours prior to the merger. Despite that, at merger time, when no time constraint is applied, Athena will be able to detect the X-ray emission associated with the coalescing MBHB. Under these circumstances, less than 10 potential X-ray AGN candidates would be found by Athena inside the LISA sky region. Thus, the synergy between Athena and LISA will be fundamental for detecting MBHBs and opening a bright future for multimessenger studies.

Access to the manuscript in ADS.