Evolution of an Accretion Disk in Binary Black Hole Systems

TL;DR

This paper models the evolution of accretion disks in binary black hole systems, revealing a possible early reactivation of accretion and jet formation years before merger, which could produce detectable electromagnetic signals.

Contribution

The authors improve existing dead disk models by incorporating tidal torque effects and ionization thresholds, proposing a new scenario for disk revival occurring thousands of years before merger.

Findings

Dead disk mass is lower than previous estimates.

Tidal torque can reactivate MRI and accretion long before merger.

Potential electromagnetic signals detectable within 10 Mpc for massive black holes.

Abstract

We investigate evolution of an accretion disc in binary black hole (BBH) systems and possible electromagnetic counterparts of the gravitational waves from mergers of BBHs. Perna et al. (2016) proposed a novel evolutionary scenario of an accretion disc in BBHs in which a disc eventually becomes "dead", i.e., the magnetorotational instability (MRI) becomes inactive. In their scenario, the dead disc survives until {\it a few seconds before} the merger event. We improve the dead disc model and propose another scenario, taking account of effects of the tidal torque from the companion and the critical ionization degree for MRI activation more carefully. We find that the mass of the dead disc is much lower than that in the Perna's scenario. When the binary separation sufficiently becomes small, the mass inflow induced by the tidal torque reactivates MRI, restarting mass accretion onto the black hole. We also find that this disc "revival" happens {\it more than thousands of years before} the merger. The mass accretion induced by the tidal torque increases as the separation decreases, and a relativistic jet could be launched before the merger. The emissions from these jets are too faint compared to GRBs, but detectable if the merger events happen within $\lesssim 10$ Mpc or if the masses of the black holes are as massive as $\sim 10^5 M_{\odot}$.