Effects of Spin on High-Energy Radiation from Accreting Black Holes

TL;DR

This study investigates how black hole spin influences high-energy radiation in accreting black holes, revealing that observed X-ray and gamma-ray emissions depend on spin and viewing angle, with implications for understanding jet power and black hole properties.

Contribution

The paper introduces detailed general-relativistic radiative transport calculations on magnetically arrested disks, highlighting the impact of spin and inclination on high-energy emissions and their relation to jet power.

Findings

X-ray and gamma-ray emissions depend strongly on spin and inclination.

High-energy power is affected by redshift effects, not directly by jet power.

X-ray to near-infrared power ratio indicates black hole spin.

Abstract

Observations of jets in X-ray binaries show a correlation between radio power and black hole spin. This correlation, if confirmed, points towards the idea that relativistic jets may be powered by the rotational energy of black holes. In order to examine this further, we perform general-relativistic radiative transport calculations on magnetically arrested accretion flows, which are known to produce powerful jets via the Blandford-Znajek (BZ) mechanism. We find that the X-ray and gamma-ray emission strongly depend on spin and inclination angle. Surprisingly, the high-energy power does not show the same dependence on spin as the BZ jet power, but instead can be understood as a redshift effect. In particular, photons observed perpendicular to the spin axis suffer little net redshift until originating from close to the horizon. Such observers see deeper into the hot, dense, highly-magnetized inner disk region. This effect is largest for rapidly rotating black holes due to a combination of frame dragging and decreasing horizon radius. While the X-ray emission is dominated by the near horizon region, the near-infrared radiation originates at larger radii. Therefore, the ratio of X-ray to near-infrared power is an observational signature of black hole spin.