High-energy and very-high-energy emission from stellar-mass black holes moving in gaseous clouds

TL;DR

This paper explores how rapidly rotating stellar-mass black holes in gaseous clouds can produce detectable very-high-energy gamma-ray emissions through electron-positron pair cascades in their magnetospheres, with specific spectral and luminosity characteristics.

Contribution

It introduces a model of gamma-ray emission from black hole magnetospheres considering spacetime frame dragging effects and predicts observable signatures for nearby black holes.

Findings

Black holes can emit detectable VHE gamma-rays under certain conditions.

Gamma-ray spectra feature broad and sharp peaks at specific energies.

Luminosity peaks at low accretion rates (~0.01% Eddington).

Abstract

We investigate the electron-positron pair cascade taking place in the magnetosphere of a rapidly rotating black hole. Because of the spacetime frame dragging, the Goldreich-Julian charge density changes sign in the vicinity of the event horizon, which leads to an occurrence of a magnetic-field aligned electric field, in the same way as the pulsar outer-magnetospheric accelerator. In this lepton accelerator, electrons and positrons are accelerated in the opposite directions, to emit copious gamma-rays via the curvature and inverse-Compton processes. We examine a stationary pair cascade, and show that a stellar-mass black hole moving in a gaseous cloud can emit a detectable very-high-energy flux, provided that the black hole is extremely rotating and that the distance is less than about 1 kpc. We argue that the gamma-ray image will have a point-like morphology, and demonstrate that their gamma-ray spectra have a broad peak around 0.01-1 GeV and a sharp peak around 0.1 TeV, that the accelerators become most luminous when the mass accretion rate becomes about 0.01% of the Eddington rate, and that the predicted gamma-ray flux little changes in a wide range of magnetospheric currents. An implication of the stability of such a stationary gap is discussed.