A second decoupling between merging binary black holes and the inner disc - impact on the electromagnetic counterpart

The coalescence of two supermassive black holes (SMBHs) produces powerful gravitational wave 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 an 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(10²) d 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.