Shock-Driven Periodic Variability in a Low-Mass-Ratio Supermassive Black Hole Binary

TL;DR

This study uses hydrodynamic simulations to explore how low-mass-ratio supermassive black hole binaries cause periodic shock-driven variability in electromagnetic emissions, revealing potential observational signatures for identifying such systems.

Contribution

It demonstrates that low-mass-ratio SMBHBs can produce eccentric gaps and periodic shocks, extending the known parameter space for variability detection to lower mass ratios.

Findings

Eccentric gaps form at low mass ratios in thin disks.

Periodic accretion occurs on the binary orbital timescale.

Shock luminosity peaks lag behind accretion rate peaks by 0.43 orbital periods.

Abstract

We investigate the time-varying electromagnetic emission of a low-mass-ratio supermassive black hole binary (SMBHB) embedded in a circumprimary disk, with a particular interest in variability of shocks driven by the binary. We perform a 2D, locally isothermal hydrodynamics simulation of a SMBHB with mass ratio $q=0.01$ and separation $a=100\;R_g$, using a physically self-consistent steady disk model. We estimate the electromagnetic variability from the system by monitoring accretion onto 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 disk, previously only observed for larger mass ratios, which we attribute to our disk model being much thinner ($H/R\approx0.01$ near the secondary) than is typical of previous works. The eccentric gap drives periodic accretion onto the secondary SMBH on a timescale matching the orbital period of the binary, $t_{\rm{bin}}\approx0.1\;\rm{yr}$, 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 $0.43\;t_{\rm{bin}}$. We propose that the correlation of these quantities represents a useful identifier of SMBHB candidates, via observations of correlated variability in X-ray and UV monitoring of AGN, rather than single-waveband periodicity alone.