Black hole mergers: the first light

TL;DR

This paper investigates the energy dissipation and observable signatures resulting from recoiling supermassive black holes surrounded by circumbinary discs, revealing significant potential energy release and potential electromagnetic signals following black hole mergers.

Contribution

It provides new analytical and simulation-based insights into the energy dissipation mechanisms and light curves associated with recoiling black holes, differing from previous studies.

Findings

Potential energy release dominates energy dissipation.

Recoil angle significantly affects the light curve.

Observable electromagnetic signals are possible for high-velocity recoils.

Abstract

The coalescence of supermassive black hole binaries occurs via the emission of gravitational waves, that can impart a substantial recoil to the merged black hole. We consider the energy dissipation, that results if the recoiling black hole is surrounded by a thin circumbinary disc. Our results differ significantly from those of previous investigations. We show analytically that the dominant source of energy is often potential energy, released as gas in the outer disc attempts to circularize at smaller radii. Thus, dimensional estimates, that include only the kinetic energy gained by the disc gas, underestimate the real energy loss. This underestimate can exceed an order of magnitude, if the recoil is directed close to the disc plane. We use three dimensional Smooth Particle Hydrodynamics (SPH) simulations and two dimensional finite difference simulations to verify our analytic estimates. We also compute the bolometric light curve, which is found to vary strongly depending upon the kick angle. A prompt emission signature due to this mechanism may be observable for low mass (10^6 Solar mass) black holes whose recoil velocities exceed about 1000 km/s. Emission at earlier times can mainly result from the response of the disc to the loss of mass, as the black holes merge. We derive analytically the condition for this to happen.