Radiation from Accreting Supermassive Black Hole Binaries

When two galaxies merge, the supermassive black holes at their nuclei might end up forming close binary systems of sub-parsec scales. The gravitational waves emitted by these systems are targets of current Pulsar Timing Arrays and of future interferometers such as LISA. Unlike most stellar-mass black hole mergers, supermassive black hole binaries live and die in gas-rich environments and can present similar phenomenology to single AGNs; namely, accretion disks, jets and the subsequent multi-wavelength emission.

By combining GRMHD simulations with semi-analytical modeling, I investigate how supermassive black hole binaries imprint unique electromagnetic signatures on their surrounding plasma. Recognizing these signatures will be key to distinguishing them from normal AGNs and to unlocking the full scientific return of the forthcoming multimessenger era. In (Gutiérrez et al., 2025), I wrote a comprehensive review on these phenomena for the book "New Frontiers in GRMHD Simulations (Springer)".

In (Gutiérrez et al., 2022), I combined GRMHD simulations and GR radiative transfer to produce some of the most realistic electromagnetic predictions for accreting SMBHBs near merger to date. The simulations explored both spinning and nonspinning equal-mass binary black holes. I produced images, light curves at different frequencies, and time-varying spectra, identifying the main periodic radiative features associated with the intrinsic dynamical properties of the accretion flow.
In (Gutiérrez et al., 2024), I proposed a novel and promising idea for an unique EM signature from accreting SMBHBs: repetitive nonthermal flares from magnetic reconnection at the region of interaction of two relativistic jets. I developed a semi-analytical model to give quantitative estimates for the properties of these flares, such as energetics and relevant timescales, and calculated the radiated spectrum arising from the interacting region. Such flares can overcome the luminosity of the source at both ends of the EM spectrum, namely at low (radio/IR) and high (X-rays/γ-rays) energies.