The coalescence of two supermassive black holes (SMBHs) produces powerful gravitationalwave (GW) radiation and, if gas is present in the vicinity, also an electromagnetic (EM) counterpart. In the standard picture, an EM outburst will be produced when the binary "decouples" from the circum-binary disc and starts "squeezing" the disc inside the secondary orbit, resulting in its quick accretion on to the primary black hole. Here we use analytical arguments and numerical simulations to show that the disc within about 20 R S of a SMBH survives the merger without being depleted. The reason is a "second decoupling": the inner disc thickens due to tidal heating and inefficient cooling, effectively decoupling from the interaction of the binary. We show that this second decoupling quenches the heating sources in the disc O(10 2 ) days before coalescence. This will render the peak UV/X-ray luminosity significantly weaker than previously thought. After the merger, the residual disc cools down and expands, merging with the outer disc rather than being completely accreted. This results in continuous EM emission, hindering the detection of the cut-off and re-brightening proposed in earlier studies.

The gas dissipation from a protoplanetary disk is one of the key processes affecting planet formation, and it is widely accepted that it happens on timescales of a few million years for disks around single stars. In recent years, several protoplanetary disks have been discovered in multiple-star systems, and despite the complex environment in which they find themselves, some of them seem to be quite old, a situation that may favor planet formation. A clear example of this is the disk around HD 98800 B, a binary in a hierarchical quadruple stellar system, which at an ∼10 Myr age seems to still be holding significant amounts of gas. Here we present a 1D+1D model to compute the vertical structure and gas evolution of circumbinary disks in hierarchical triple-star systems considering different stellar and disk parameters. We show that tidal torques due to the inner binary, together with the truncation of the disk due to the external companion, strongly reduce the viscous accretion and expansion of the disk. Even allowing viscous accretion by tidal streams, disks in these kind of environments can survive for more than 10 Myr, depending on their properties, with photoevaporation being the main gas dissipation mechanism. We particularly apply our model to the circumbinary disk around HD 98800 B and confirm that its longevity, along with the current nonexistence of a disk around the companion binary HD 98800 A, can be explained with our model and by this mechanism.

Supermassive black holes (SMBHs) pair and form bound binaries after their host galaxies merge. In a gas-rich merger, accretion discs are expected to form around the binary and its components. These discs control the binary orbital evolution until the system is compact enough for gravitational waves to drive the SMBHs to coalescence. In this work, we implemented a time-dependent 1D model to follow the long-term evolution of the coupled binary + discs system, from a separation of 10 5 down to 20 Schwarzschild radii. We run different models changing the system parameters, including the binary mass ratio q 0.3 and a factor γ that controls the inflow across the gap created by the secondary. We find that our implementation yields higher residual masses and longer binary residence times than previous studies. Our main conclusion is the non-steady-state nature of the evolution of the system: the properties the disc had when the binary was still at large separations influence its whole evolution. To recover steady state, the binary residence time would have to be much longer than the inflow time-scale of the disc throughout their entire history, which in general is not satisfied.

At the final stages of a supermassive black hole coalescence, the emission of gravitational waves will efficiently remove energy and angular momentum from the binary orbit, allowing the separation between the compact objects to shrink. In the scenario where a circumprimary disc is present, a squeezing phase will develop, in which the tidal interaction between the disc and the secondary black hole could push the gas inwards, enhancing the accretion rate on to the primary and producing what is known as an electromagnetic precursor. In this context, using 3D hydrodynamic simulations, we study how an adiabatic circumprimary accretion disc responds to the varying gravitational potential as the secondary falls onto the more massive object. We included a cooling prescription controlled by the parameter β = Ωtcool, which will determine how strong the final accretion rate is: a hotter disc is thicker, and the tidal interaction is suppressed for the gas outside the binary plane. Our main results are that for scenarios where the gas cannot cool fast enough (β ≥ 30) the disc becomes thick and renders the system invisible, while for β ≤ 10 the strong cooling blocks any leakage on to the secondary’s orbit, allowing an enhancement in the accretion rate two orders of magnitude stronger than the average through the rest of the simulation.