Shock-driven periodic variability in a low-mass-ratio supermassive black hole binary

Abstract

ABSTRACT We investigate the time-varying electromagnetic emission of a low-mass-ratio supermassive black hole binary (SMBHB) embedded in a circumprimary disc, with a particular interest in variability of shocks driven by the binary. We perform a 2D, locally isothermal hydrodynamics simulation of an SMBHB with mass ratio q = 0.01 and separation a = 100 Rg, using a physically self-consistent steady disc model. We estimate the electromagnetic variability from the system by monitoring accretion on to the secondary and using an artificial viscosity scheme to capture shocks and monitor the energy dissipated. The SMBHB produces a wide, eccentric gap in the disc, previously only observed for larger mass ratios, which we attribute to our disc model being much thinner (H/R ≈ 0.01 near the secondary) than is typical of previous works. The eccentric gap drives periodic accretion on to the secondary SMBH on a time-scale matching the orbital period of the binary, $t_{\rm {bin}}\approx 0.1\,\,\rm {yr}$, implying that the variable accretion regime of the SMBHB parameter space extends to lower mass ratios than previously established. Shocks driven by the binary are periodic, with a period matching the orbital period, and the shocks are correlated with the accretion rate, with peaks in the shock luminosity lagging peaks in the accretion rate by 0.43 tbin. We propose that the correlation of these quantities represents a useful identifier of SMBHB candidates, via observations of correlated variability in X-ray and ultraviolet monitoring of active galactic nuclei, rather than single-waveband periodicity alone.