Minidisks in Binary Black Hole Accretion

TL;DR

This paper uses advanced hydrodynamical simulations to study the structure, dynamics, and radiation of minidisks around black holes in binary systems, revealing shock-driven accretion and distinctive X-ray signatures.

Contribution

It introduces a new relativistic moving mesh simulation method and provides detailed analysis of minidisk dynamics, shock structures, and observable signatures in binary black hole systems.

Findings

Spiral shock waves drive efficient accretion in minidisks.

Simulated spectra show brighter hard X-ray emission than standard models.

Effective alpha viscosity parameter is around 0.01.

Abstract

Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole ("minidisks"), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using two-dimensional hydrodynamical simulations performed with a new general relativistic version of the moving mesh code Disco. We introduce a co-moving energy variable which enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the ISCO providing a Reynolds stress which causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov-Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling. We find that the spiral shock structure is in agreement with the relativistic dispersion relation for tightly-wound linear waves. We measure the shock induced dissipation and find outward angular momentum transport corresponding to an effective alpha parameter of order 0.01. We perform ray-tracing image calculations from the simulations to produce theoretical minidisk spectra and viewing angle dependent images for comparison with observations.