On the role and origin of nonthermal electrons in hot accretion flows

TL;DR

This study models X-ray spectra of hot accretion flows around black holes, showing that including nonthermal electrons from pion decay and acceleration improves agreement with observed AGN data.

Contribution

It introduces a comprehensive model incorporating nonthermal synchrotron seed photons from pion decay and direct acceleration, enhancing the explanation of AGN X-ray spectra.

Findings

Nonthermal electrons significantly contribute to seed photon flux.

Pion-decay electrons dominate in low-delta models.

Nonthermal acceleration fraction ~0.1 aligns with observations.

Abstract

We study the X-ray spectra of tenuous, two-temperature accretion flows using a model involving an exact, Monte Carlo computation of the global Comptonization effect as well as general relativistic description of both the flow structure and radiative processes. In our previous work we found that in flows surrounding supermassive black holes, thermal synchrotron radiation is not capable of providing a sufficient seed photons flux to explain the X-ray spectral indices as well as the cut-off energies measured in several best-studied AGNs. In this work we complete the model by including seed photons provided by nonthermal synchrotron radiation and we find that it allows to reconcile the hot flow model with the AGN data. We take into account two possible sources of nonthermal electrons. First, we consider e+- produced by charged-pions decay, which should be always present in the innermost part of a two-temperature flow due to proton-proton interactions. We find that for a weak heating of thermal electrons (small delta) the synchrotron emission of pion-decay e+- is much stronger than the thermal synchrotron emission in the considered range of bolometric luminosities, L Ledd. The small-delta model including hadronic effects in general agrees with the AGN data, except for the case of a slowly rotating black hole and a thermal distribution of protons. For large-delta, the pion-decay e+- have a negligible effect and then in this model we consider nonthermal electrons produced by direct acceleration. We find an approximate agreement with the AGN data for the fraction of the heating power of electrons which is used for the nonthermal acceleration eta~0.1.