Accretion into the Central Cavity of a Circumbinary Disk
  A near-equal mass binary black hole can clear a central cavity in a
circumbinary accretion disk; however, previous works have revealed accretion
streams entering this cavity. Here we use 2D hydrodynamical simulations to
study the accretion streams and their periodic behavior. In particular, we
perform a suite of simulations, covering different binary mass ratios
$q=M_2/M_1$ in the range $0.01 \leq q \leq 1$. 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) while the binary is
efficient in maintaining a low-density cavity, the time-averaged mass accretion
rate into the cavity, through narrow coherent accretion streams, is suppressed
by at most a factor of $\sim 5$, compared to a disk with a single BH with the
same mass; (ii) the largest suppression occurs for $q\approx 0.05$; binaries
whose mass ratios are either lower or higher both suppress accretion less
significantly; (iii) for $q \gtrsim 0.05$, 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 (v) for $q \lesssim 0.05$, the accretion rate becomes steady, with
no time-variations. Most binaries produced in galactic mergers are expected to
have $q\gtrsim 0.05$. 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 disk properties.
