2023MNRAS.521.2577P

The European Space Agency is studying two large-class missions bound to operate in the decade of the 30s, and aiming at investigating the most energetic and violent phenomena in the Universe. Athena is poised to study the physical conditions of baryons locked in large-scale structures from the epoch of their formation, as well as to yield an accurate census of accreting supermassive black holes down to the epoch of reionization; LISA will extend the hunt for Gravitational Wave (GW) events to the hitherto unexplored mHz regime. We discuss in this paper the science that their concurrent operation could yield, and present possible Athena observational strategies. We focus on Supermassive (M$\lesssim 10^7\, \rm {M_\odot }$) Black Hole Mergers (SMBHMs), potentially accessible to Athena up to z ∼ 2. The simultaneous measurement of their electromagnetic (EM) and GW signals may enable unique experiments in the domains of astrophysics, fundamental physics, and cosmography, such as the magnetohydrodynamics of fluid flows in a rapidly variable space-time, the formation of coronae and jets in Active Galactic Nuclei, and the measurement of the speed of GW, among others. Key to achieve these breakthrough results will be the LISA capability of locating a SMBHM event with an error box comparable to, or better than the field-of-view of the Athena Wide Field Imager ($\simeq 0.4\,$ deg²) and Athena capability to slew fast to detect the source during the inspiral phase and the post-merger phase. Together, the two observatories will open in principle the exciting possibility of truly concurrent EM and GW studies of the SMBHMs