Positron Annihilation Spectroscopy of Active Galactic Nuclei
Authors: Doikov Dmytry N. , Yushchenko Alexander V. , Jeong Yeuncheol 
Published: May 2023

This paper focuses on the interpretation of radiation fluxes from active galactic
nuclei. The advantage of positron annihilation spectroscopy over other methods of
spectral diagnostics of active galactic nuclei (therefore AGN) is demonstrated. A
relationship between regular and random components in both bolometric and spectral
composition of fluxes of quanta and particles generated in AGN is found. We consider
their diffuse component separately and also detect radiative feedback after the passage
of high-velocity cosmic rays and hard quanta through gas-and-dust aggregates surrounding
massive black holes in AGN. The motion of relativistic positrons and electrons in such
complex systems produces secondary radiation throughout the whole investigated region of
active galactic nuclei in form of cylinder with radius R= 400-1000 pc and height
H=200-400 pc, thus causing their visible luminescence across all spectral bands. We
obtain radiation and electron energy distribution functions depending on the spatial
distribution of the investigated bulk of matter in AGN. Radiation luminescence of the
noncentral part of AGN is a response to the effects of particles and quanta falling from
its center created by atoms, molecules and dust of its diffuse component. The
cross-sections for the single-photon annihilation of positrons of different energies
with atoms in these active galactic nuclei are determined. For the first time we use the
data on the change in chemical composition due to spallation reactions induced by
high-energy particles. We establish or define more accurately how the energies of the
incident positron, emitted γ-quantum and recoiling nucleus correlate with the atomic
number and weight of the target nucleus. For light elements, we provide detailed tables
of all indicated parameters. A new criterion is proposed, based on the use of the ratio
of the fluxes of γ-quanta formed in one- and two-photon annihilation of positrons in a
diffuse medium. It is concluded that, as is the case in young supernova remnants, the
two-photon annihilation tends to occur in solid-state grains as a result of active loss
of kinetic energy of positrons due to ionisation down to thermal energy of free
electrons. The singlephoton annihilation of positrons manifests itself in the gas
component of active galactic nuclei. Such annihilation occurs as interaction between
positrons and K-shell electrons; hence, it is suitable for identification of the
chemical state of substances comprising the gas component of the investigated media.
Specific physical media producing high fluxes of positrons are discussed; it allowed a
significant reduction in the number of reaction channels generating positrons. We
estimate the brightness distribution in the γ-ray spectra of the gas-and-dust media
through which positron fluxes travel with the energy range similar to that recorded by
the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA)
research module. Based on the results of our calculations, we analyse the reasons for
such a high power of positrons to penetrate through gas-and-dust aggregates. The energy
loss of positrons by ionisation is compared to the production of secondary positrons by
high-energy cosmic rays in order to determine the depth of their penetration into
gas-and-dust aggregations clustered in active galactic nuclei. The relationship between
the energy of γ-quanta emitted upon the single-photon annihilation and the energy of
incident electrons is established. The obtained cross sections for positron interactions
with bound electrons of the diffuse component of the non-central, peripheral AGN regions
allowed us to obtain new spectroscopic characteristics of the atoms involved in
singlephoton annihilation.