Electron-Positron pairs creation close to a black hole horizon. Red-shifted annihilation line in the emergent X-ray spectra of a black hole, part I

TL;DR

This paper investigates the formation of a red-shifted 511 keV annihilation line near black hole horizons caused by photon-photon interactions leading to electron-positron pair creation, which could produce observable spectral features.

Contribution

It introduces a model for the generation of a red-shifted annihilation line close to black hole horizons due to photon-photon interactions in the accretion environment.

Findings

Annihilation line forms very close to the black hole horizon within 10^3-10^4 meters.

The line appears as a blackbody bump at energies around 511/20 keV, red-shifted by gravitational effects.

Observational evidence for this feature is discussed in an accompanying paper.

Abstract

We consider a Compton cloud (CC) surrounding a black hole (BH) in an accreting black hole system, where electrons propagate with thermal and bulk velocities.In that cloud, soft (disk) photons may be upscattered off these energetic electrons and attain several MeV energies. They could then create pairs due to these photon-photon interactions. In this paper, we study the formation of the 511 keV annihilation line due to this photon-photon interaction, which results in the creation of electron-positron pairs, followed by the annihilation of the created positrons with the CC electrons. The appropriate conditions of annihilation line generation take place very close to a BH horizon within (10^3-10^4)m cm from it, where m is a BH hole mass in solar units. As a result, the created annihilation line should be seen by the Earth observer as a blackbody bump, or so called {reflection} bump at energies around (511/20) (20/z) keV where z~20 is a typical gravitational red-shift experienced by the created annihilation line photons when they emerge. This transient feature should occur in any accreting black hole systems, Galactic or extragalactic. Observational evidences for this feature in several galactic black hole systems is detailed in an accompanying paper (II). An extended hard tail of the spectrum up to 1 MeV may be also formed due to X-ray photon up-scattering off created pairs.