# Radiative magnetic reconnection near accreting black holes

## TL;DR

This paper proposes a radiative mechanism for magnetic flares near black holes based on first-principle simulations, explaining hard X-ray emissions and hybrid Comptonization in accreting black hole systems.

## Contribution

It introduces a self-regulated chain of magnetic reconnection plasmoids with radiative cooling, providing a first-principles explanation for observed X-ray spectra.

## Key findings

- Reconnection occurs in a compact region near the black hole.
- The chain radiates in hard X-rays with a cutoff near 100 keV.
- Particles at X-points produce additional high-energy emission.

## Abstract

A radiative mechanism is proposed for magnetic flares near luminous accreting black holes. It is based on recent first-principle simulations of magnetic reconnection, which show a hierarchical chain of fast-moving plasmoids. The reconnection occurs in a compact region (comparable to the black hole radius), and the chain experiences fast Compton cooling accompanied by electron-positron pair creation. The distribution of plasmoid speeds is shaped by radiative losses, and the self-regulated chain radiates its energy in hard X-rays. The mechanism is illustrated by Monte-Carlo simulations of the transfer of seed soft photons through the reconnection layer. The emerging radiation spectrum has a cutoff near 100 keV similar to the hard-state spectra of X-ray binaries and AGN. We discuss how the chain cooling differs from previous phenomenological emission models, and suggest that it can explain the hard X-ray activity of accreting black holes from first principles. Particles accelerated at the X-points of the chain produce an additional high-energy component, explaining "hybrid Comptonization" observed in Cyg X-1.