Chasing Super-Massive Black Hole merging events with Athena and LISA by L. Piro et al. on Monday 28 November

The European Space Agency is studying two large-class missions bound to

operate in the $2030$s, and aiming at investigating the most energetic

phenomena in the Universe. $Athena$ is poised to study the physical conditions

of baryons in large-scale structures, as well as to yield a census of accreting

super-massive black holes down to the epoch of reionization; the Laser

Interferometer Space Antenna (LISA) will extend the hunt for Gravitational Wave

(GW) events to the mHz regime. While the science cases of the two missions are

independently outstanding, we discuss in this paper the $additional$ science

that their concurrent operation could yield. We focus on the multi-messenger

study of Super-Massive (M$\lesssim 10^7\rm M_{\odot}$) Black Hole Mergers

(SMBHMs), accessible to $Athena$ up to $z\sim2$. The simultaneous measurement

of their electro-magnetic (EM) and GW signals may enable unique experiments in

the domains of astrophysics, fundamental physics, and cosmography. Key to

achieve these 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 ($\simeq0.4$deg$^2$). LISA will achieve such an

accuracy several hours prior to merging for the highest signal-to-noise events.

While theoretical predictions of the EM emission are still uncertain, this

opens in principle the possibility of truly concurrent EM and GW studies of the

merger phase. LISA localization improves significantly at merging, and is

likely to reach the arcminute-level for a sizeable fraction of events at

$z\lesssim 0.5$ and masses $\lesssim10^6\rm M_{\odot}$, well within the

detection capability of $Athena$. We also briefly discuss the prospective of

$Athena$ studies for other classes of GW-emitting black hole binaries, for

which theoretical predictions are admittedly extremely uncertain. [abridged]

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.13759v1