In the past few years, we have detected several black hole (BH) mergers through the measurement of gravitational waves (GWs) with Earth-based interferometers. All these detections corresponded to stellar-mass BHs, which are not expected to be active (accreting matter) unless there is a close star feeding them. For this reason, none of these GW detections was accompanied by a clear electromagnetic signal. In the next decade, with the advent of the Laser Interferometer Space Antenna (LISA), we will be able to detect GWs from supermassive black hole mergers. These events are predicted to be a natural outcome of the fusion of galaxies. Contrary to stellar-mass BH binaries, supermassive black hole binary systems (SMBHBs) are located in gas-rich environments, namely at the nuclei of merged galaxies. As a consequence of this, they may present similar phenomenology to single AGNs, in particular, accretion disks and jets. Of course, due to the presence of two black holes instead of a single one, there will be important differences as well. When the two black holes are close to each other, the system would be formed by a circumbinary disk that feeds a central cavity with two (mini-) accretion disks around the BHs. The electromagnetic emission from SMBHBs should present specific signatures that would differentiate them from single AGNs. With our group, we are trying to identify these signatures via detailed modeling of the physics of these systems. In particular, we are about to publish papers on the two following approaches:
Detailed predictions via ray tracing of GRMHD simulations
In this line of research, we take data from detailed GRMHD simulations of equal-mass supermassive black holes at close separations (Bowen et al. 2019, Combi et al. 2021), and we post-process the data with the ray-tracing code Bothros, in a continuation of the work done by d'Ascoli et al. (2018). The black holes are fed by two minidisks, which in turn are fed by a circumbinary disk via two streams. In the movie on the right, you can see a far-UV ray-traced movie of the system, next to its Spectral Energy Distribution (SED). The various components of the binary: circumbinary disk, minidisks, streams, and cavity, are shown in different colors and linestyles.
For more details, you can check my talk at the LISA Astrophysics Working Group meeting.