Electromagnetic signatures from supermassive binary black holes approaching merger

Theoretical models predict that when two galaxies merge, the supermassive black holes at their nuclei may end up forming close binary systems of sub-parsec scales. The gravitational waves emitted by these systems are the target of current Pulsar Timing Array efforts and future interferometers as LISA (Laser Interferometer Space Antenna). Unlike most stellar-mass binary black holes, supermassive binary black holes (SMBBHs) live and die in gas-rich environments (the core of a merged galaxy) and hence might accrete copious amounts of matter, become active, and emit electromagnetic radiation. Detailed knowledge of specific emission signatures from SMBBHs is crucial to differentiate them from normal AGNs, identifying potential targets for future multi-messenger observations of electromagnetic radiation and gravitational waves. In this talk, I will present novel relativistic predictions of the electromagnetic emission from the surrounding gas of an SMBBH system approaching merger. We ray-traced data from General Relativistic Magnetohydrodynamic (GRMHD) simulations of supermassive binary black holes approaching merger (at a separation ~20M), and generated images, time-dependent spectra, and light curves. We analyze how the spin of the black holes influences the observed emission. We found that prograde black hole spin makes mini-disks brighter because the smaller ISCO angular momentum demands more dissipation before matter plunges to the horizon. However, compared to mini-disks in larger separation binaries with spinning black holes, our minidisks are less luminous: unlike those systems, their mass accretion rate is lower than in the circumbinary disk, and they radiate with lower efficiency because their inflow times are shorter. Finally, we identify specific signatures that might differentiate SMBBHs from normal AGNs.