Despite the re-birth of multi-messenger astronomy, no unambiguous electromagnetic (EM) counterpart to stellar-mass binary black hole (BBH) pre-/post-merger has been reported. In 2035+, LISA will be launched and will detect the gravitational waves from supermassive BBHs, expected to be gas-rich and therefore EM-loud — and even multi-messenger — systems. Detecting the EM pre-merger counterpart (e.g. with NewAthena in X-rays, V. Rubin Observatory in optical) would allow for optimal follow-up, while an EM-GW detection would be precious for several fundamental questions of astrophysics or cosmology. However, uncertainties remain: on the one hand, no supermassive BBH detection has been confirmed yet, despite a growing list of candidates. On the other hand, the accretion properties onto supermassive BBHs and their EM signatures are not firmly identified because few numerical codes are able to model accretion and emission around BBHs in General Relativity (GR).
After an introductory part, I will present e-NOVAs (“extended Numerical Observatory for Violent Accreting systems”), the first European GR magneto-hydrodynamical+GR ray-tracing code incorporating an analytical BBH spacetime. I will show the latest results obtained with e-NOVAs (and comparable tools worldwide) to understand the BBH circumbinary flow evolution and the multi-wavelength observables that derive from it. Those are the signatures we are looking for to detect, for the first time, and from now on, supermassive BBHs.
