Enhanced gamma radiation toward the rotation axis from the immediate vicinity of extremely rotating black holes

TL;DR

This paper models gamma-ray emission from extremely rotating black holes, showing that gamma-ray flux is significantly enhanced along the rotation axis and could be detectable by current telescopes during high-energy flares.

Contribution

It applies pulsar outer-gap theory to black hole magnetospheres, revealing a new mechanism for gamma-ray production near the event horizon influenced by black hole spin and accretion rate.

Findings

Gamma-ray luminosity increases as accretion rate decreases.

Flux is enhanced along the rotation axis for high black hole spin.

Detectability of flares depends on viewing angle and flare duration.

Abstract

We investigate the acceleration of electrons and positrons by magnetic-field-aligned electric fields in the polar funnel of an accreting black hole (BH). Applying the pulsar outer-gap theory to BH magnetospheres, we find that such a lepton accelerator arises in the immediate vicinity of the event horizon due to frame-dragging, and that their gamma-ray luminosity increases with decreasing accretion rate. Furthermore, we demonstrate that the gamma-ray flux is enhanced along the rotation axis by more than an order of magnitude if the BH spin increases from $a=0.90M$ to $a=0.9999M$. As a result, if a ten-solar-mass, almost-maximally rotating BH is located within 3 kpc, when its accretion rate is between 0.005% and 0.01% of the Eddington rate, its high-energy flare becomes detectable with the Fermi/Large Area Telescope, provided that the flare lasts longer than 1.2 months and that we view the source nearly along the rotation axis. In addition, its very-high-energy flux is marginally detectable with the Cherenkov Telescope Array, provided that the flare lasts longer than a night and that our viewing angle is about 45 degrees with respect to the rotation axis.