Quasi-thermal synchrotron self-Comptonization: constraints on magnetic field strength in accreting black holes.

Grzegorz Wardzi\'nski$^1$ and Andrzej A. Zdziarski$^1$

$^1$ N. Copernicus Astronomical Center, Warsaw, Poland


We investigate how a weak non-thermal tail beyond a Maxwellian electron distribution 
affects the synchrotron emission and its Comptonization in plasmas with 
parameters typical for accretion flows onto black holes. We find that the tail 
can significantly increase the net synchrotron emission and thus increase the 
luminosity produced by Comptonization of synchrotron photons. The 
enhancement of the Comptonized synchrotron luminosity can be as large as by 
$\sim 10^3$ and $\sim 10^5$ for stellar and supermassive black holes, 
respectively, when the energy content in the non-thermal tail is only 1 per 
cent. 

The presence of the tail leads to formation of a gamma-ray high-energy tail in 
the Comptonization spectrum. Since observations of these tails can constrain the 
quasi-thermal electron distribution and thus the Comptonized synchrotron 
luminosity, they provide upper limits on the strength of magnetic fields in 
accretion flows. In particular, observations of a MeV tail in the hard state of 
Cyg X-1 imply the magnetic field in this source to be at most an order of 
magnitude below equipartition.