Stochastic Variability of Binary Accretion

Abstract

We measure the power spectral density (PSD) of the accretion rate time series in an unequal mass (q = 0.2) binary surrounded by a circumbinary gas disk, using very high-resolution 2D hydrodynamics simulations. Our aim is to identify new signposts of supermassive black hole (SMBH) binaries in active galactic nuclei (AGN), based on the shape of the continuum PSD, to complement well-studied line features in the PSD (periodicities). We find that the continuum PSD is a broken power-law, transitioning from flat (white noise) to a slope of -4 at a break frequency generically ~5 times the binary orbital frequency. This form is expected when (a) delivery of gas from the circumbinary disk to the individual "minidisks" is a damped random walk with correlation time equal to binary orbital period and (b) the minidisks function as low-pass filters acting at the Kepler frequency of the outer edge of the smaller black hole's minidisk; we show numerical evidence for both. The broken power-law PSD is attained in a limit where the secondary black hole is much smaller than its minidisk, realized numerically by a sufficiently small "sink" region; larger sinks lead to excess high-frequency noise seen as accretion rate spikes, and we argue these should be regarded as artificial when the black holes themselves are smaller than the sink regions. The broken power-law PSD is reminiscent of stochastic variability in ordinary AGN, inviting the conjecture that canonical AGN variability could result from widespread binarity, however pulsar timing experiments may exclude this possibility.