The late inspiral of supermassive black hole binaries with circumbinary gas discs in the LISA band

TL;DR

This study uses hydrodynamical simulations to explore the electromagnetic signatures of supermassive black hole binaries during late inspiral, revealing periodic luminosity variations that could aid LISA gravitational wave counterpart identification.

Contribution

It provides detailed simulation results showing gas accretion and electromagnetic variability patterns during SMBHB inspiral, advancing understanding of potential EM counterparts to gravitational wave signals.

Findings

Binary captures gas and forms minidiscs during inspiral

System exhibits strong periodic luminosity at twice the orbital frequency

X-ray emission shows distinct variability patterns in different energy bands

Abstract

We present the results of 2D, moving-mesh, viscous hydrodynamical simulations of an accretion disc around a merging supermassive black hole binary (SMBHB). The simulation is pseudo-Newtonian, with the BHs modeled as point masses with a Paczynski-Wiita potential, and includes viscous heating, shock heating, and radiative cooling. We follow the gravitational inspiral of an equal-mass binary with a component mass M_bh=10^6 M_sun from an initial separation of 60r_g (where r_g=GM_bh/c^2 is the gravitational radius) to the merger. We find that a central, low-density cavity forms around the binary, as in previous work, but that the BHs capture gas from the circumbinary disc and accrete efficiently via their own minidiscs, well after their inspiral outpaces the viscous evolution of the disc. The system remains luminous, displaying strong periodicity at twice the binary orbital frequency throughout the entire inspiral process, all the way to the merger. In the soft X-ray band, the thermal emission is dominated by the inner edge of the circumbinary disc with especially clear periodicity in the early inspiral. By comparison, harder X-ray emission is dominated by the minidiscs, and the light curve is initially more noisy but develops a clear periodicity in the late inspiral stage. This variability pattern should help identify the EM counterparts of SMBHBs detected by the space-based gravitational-wave detector LISA.