High-energy signatures of binary systems of supermassive black holes

TL;DR

This paper proposes a method to identify supermassive black hole binaries by analyzing high-energy emissions from their jets, focusing on distinctive spectral features caused by disk perturbations and gamma-ray absorption.

Contribution

It introduces a new criterion based on broadband spectral analysis of jet emissions, considering disk perturbations and gamma-ray opacity, to detect supermassive black hole binaries.

Findings

Enhanced external Compton emission in X-rays due to disk photon excess

Deep gamma-ray absorption features at tens of TeV energies

Potential for Cherenkov telescopes to identify black hole binaries

Abstract

Context. Binary systems of supermassive black holes are expected to be strong sources of long gravitational waves prior to merging. These systems are good candidates to be observed with forthcoming space-borne detectors. Only a few of these systems, however, have been firmly identified to date. Aims. We aim at providing a criterion for the identification of some supermassive black hole binaries based on the characteristics of the high-energy emission of a putative relativistic jet launched from the most massive of the two black holes. Methods. We study supermassive black hole binaries where the less massive black hole has carved an annular gap in the circumbinary disk, but nevertheless there is a steady mass flow across its orbit. Such a perturbed disk is hotter and more luminous than a standard thin disk in some regions. Assuming that the jet contains relativistic electrons, we calculate its broadband spectral energy distribution focusing on the inverse Compton up-scattering of the disk photons. We also compute the opacity to the gamma rays produced in the jet by photon annihilation with the disk radiation and take into account the effects of the anisotropy of the target photon field as seen from the jet. Results. We find that the excess of low-energy photons radiated by the perturbed disk causes an increment in the external Compton emission from the jet in the X-ray band, and a deep absorption feature at energies of tens of TeVs for some sets of parameters. According to our results, observations with Cherenkov telescopes might help in the identification of supermassive black hole binaries, especially those black hole binaries that host primaries from tens to hundreds of million of solar masses.