Accretion into the central cavity of a circumbinary disc

A near-equal-mass binary black hole (BH) can clear a central cavity in a circumbinary accretion disc; however, previous works have revealed accretion streams entering this cavity. Here we use 2D hydrodynamical simulations to study the accretion streams and their periodic behaviour. In particular, we perform a suite of simulations, covering different binary mass ratios q = M2/M1 in the range 0.003 ≤ q ≤ 1. In each case, we follow the system for several thousand binary orbits, until it relaxes to a stable accretion pattern. We find the following results: (i) the binary is efficient in maintaining a low-density cavity. However, the time-averaged mass accretion rate into the cavity, through narrow coherent accretion streams, is suppressed by at most a factor of a few compared to a disc with a single BH with the same mass; (ii) for q ≳ 0.05, the accretion rate is strongly modulated by the binary, and depending on the precise value of q, the power spectrum of the accretion rate shows either one, two or three distinct periods; and (iii) for q ≲ 0.05, the accretion rate becomes steady, with no time variations. Most binaries produced in galactic mergers are expected to have q ≳ 0.05. If the luminosity of these binaries tracks their accretion rate, then a periodogram of their light curve could help in their identification, and to constrain their mass ratio and disc properties.