Multi-messenger and Multi-band Studies of Massive Black Holes: the Synergies Between LISA and SKAO

Abstract

Depending on its mass, the same gas-embedded massive black hole (BH) binary can emit gravitational waves (GWs) in one or more bands (nHz and mHz) concurrently, along with electromagnetic (EM) waves over the entire spectrum. The Square Kilometre Array Observatory (SKAO), thanks to its unparalleled sensitivity, will be pivotal in achieving coincident GW-EM (mHz-radio) and GW-GW (nHz-mHz) detections. We review the state-of-the-art predictions - achieved by means of numerical simulations and mock catalogues - of the numbers of detectable coincident GW-EM signals when employing the Laser Interferometer Space Antenna (LISA) and SKA-Mid. By exploiting the same underlying BH binary populations, to allow for a fairer comparison, we then assess the importance of a variety of EM models for the radio flares and jets, finding that the number of radio counterparts of LISA sources is relatively insensitive to the jet/flare model employed and to whether SKA-Mid AA* or AA4 is assumed. Additionally, we describe how SKAO - as part of a pulsar timing array (PTA) - and LISA will provide the opportunity to detect the first low-frequency, multi-band (nHz-mHz) GW detection of the same object. Supermassive BH binaries embedded in gas discs are subjected to hydrodynamical torques, causing perturbations that produce additional small-amplitude, higher-frequency GWs. The main carrier GWs may thus be identifiable as a deterministic signal by SKAO-era PTAs, while the higher-frequency harmonics would shine in LISA as stochastic signals. Correlating these multi-band GWs would provide unprecedented constraints on the environment of the most massive BHs.