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

Abstract

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 late inspiral of supermassive black hole binaries with circumbinary gas discs in the LISA band

“…(5) No "second decoupling". Fontecilla et al (2017) suggested that the inner (circum-primary) disc will become geometrically thick (h/r ∼ 1) in the late stages of the inspiral, as a result of tidal heating. They find, using a 1D analytical models and numerical simulation, that the viscous time in the inner disc decreases to t vis ∼ t GW , and it effectively decouples from the binary.…”

Non-steady-state long-term evolution of supermassive black hole binaries surrounded by accretion discs

“…During the GW emission phase the secondary will migrate inward faster than the viscous time of the inner disc, pushing the gas inward and squeezing it. The final curve shows the moment when the binary separation is a = 20 R S , where a so-called "second decoupling" should occur due to the thickening of the disc, which invalidates the thin-disc assumptions (Fontecilla et al 2017). The figures shows that the gradient of the surface density in the inner region of the circumbinary disc becomes shallower, as it is not affected by the tidal barrier.…”

“…It can be shown that there is only one real and positive solution for the central temperature. Solving the equations allows us to obtain the sound speed c s , the thickness h of the disc, and finally the viscosity itself (Fontecilla et al 2017). Having the central temperature at each radius and assuming that the discs emit as a multi-temperature blackbody (Frank et al 2002), we can obtain the spectral energy distribution (SED) and the bolometric luminosity (L bol ) of the system as a function of time.…”

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

“…For this reason, an electromagnetic (EM) counterpart is crucial to know the position of the event (Centrella et al 2010). In the case of a BH-BH system, this counterpart comes from the material in the vicinity minutes prior the final merger (Armitage & Natarajan 2002;Chang et al 2010;Tazzari & Lodato 2015;Fontecilla et al 2017). As such, this counterpart works as an alert, or precursor, for an imminent burst of GWs.…”

“…Due to the GW emission, the migration timescale of the black holes eventually becomes shorter than the viscous timescale of each accretion disc in the system: around ∼ 10 2 R S a first decoupling occurs: the circumbinary disc is left behind, and the migration becomes entirely dominated by the GW emission (Armitage & Natarajan 2002;Lodato et al 2009). At even smaller distances ( ∼ 40R S ) the inner disc strongly couples with the gravitational potential of the binary and is pushed inwards in the squeezing phase, potentially enhancing the accretion rate on to the primary (Chang et al 2010;Tazzari & Lodato 2015;Fontecilla et al 2017). For this process to be efficient and produce the precursor, the disc-binary coupling needs to be maintained: the closer the gas is to the midplane, the stronger is the gravitational interaction.…”