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.