The effect of cooling on the accretion of circumprimary discs in merging supermassive black hole binaries

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 on to the more massive object. We included a cooling prescription controlled by the parameter beta = Omega t(cool), 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 (beta >= 30), the disc becomes thick and renders the system invisible, while for beta <= 10 the strong cooling blocks any leakage on to the secondary's orbit, allowing an enhancement in the accretion rate of two orders of magnitude stronger than the average through the rest of the simulation.