A Second Decoupling Between Merging Binary Black Holes and the Inner Disc--Impact on the Electromagnetic Counterpart

TL;DR

This study reveals a second decoupling in SMBH mergers that prevents the inner disc from being depleted, leading to weaker electromagnetic signals and continuous emission post-merger, challenging previous models.

Contribution

It introduces the concept of a second decoupling caused by tidal heating, altering the expected electromagnetic signatures of SMBH mergers.

Findings

Inner disc survives merger without depletion.

Second decoupling weakens pre-merger EM signals.

Post-merger EM emission remains continuous.

Abstract

The coalescence of two supermassive black holes (SMBHs) produces powerful gravitational-wave (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 ${\cal O}(10^2)$ 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.