An Eccentric Circumbinary Accretion Disk and the Detection of Binary Massive Black Holes

We present a two-dimensional grid-based hydrodynamic simulation of a thin,viscous, locally-isothermal corotating disk orbiting an equal-mass Newtonianbinary point mass on a fixed circular orbit. We study the structure of the diskafter multiple viscous times. The binary maintains a central hole in theviscously-relaxed disk with radius equal to about twice the binary semimajoraxis. Disk surface density within the hole is reduced by orders of magnituderelative to the density in the disk bulk. The inner truncation of the diskresembles the clearing of a gap in a protoplanetary disk. An initially circulardisk becomes elliptical and then eccentric. Disturbances in the disk contain acomponent that is stationary in the rotating frame in which the binary is atrest; this component is a two-armed spiral density wave. We measure thedistribution of the binary torque in the disk and find that the strongestpositive torque is exerted inside the central low-density hole. We makeconnection with the linear theory of disk forcing at outer Lindblad resonances(OLRs) and find that the measured torque density distribution is consistentwith forcing at the 3:2 (m=2) OLR, well within the central hole. We alsomeasure the time dependence of the rate at which gas accretes across the holeand find quasi-periodic structure. We discuss implications for variability anddetection of active galactic nuclei containing a binary massive black hole.