Maximum brightness temperature for an incoherent synchrotron radio source

TL;DR

This paper examines the theoretical maximum brightness temperature of incoherent synchrotron radio sources, comparing the inverse Compton limit with a tighter equipartition-based limit, and discusses their implications.

Contribution

It introduces a tighter brightness temperature limit based on energy equipartition, independent of inverse Compton effects, and analyzes both limits' advantages and disadvantages.

Findings

Maximum brightness temperature ~10^{12} K from inverse Compton limit.

Tighter limit ~10^{11.5} K from equipartition condition.

Discussion of the pros and cons of both limits.

Abstract

We discuss here a limit on the maximum brightness temperature achievable for an incoherent synchrotron radio source. This limit, commonly referred to in the literature as an inverse Compton limit, prescribes that the brightness temperature for an incoherent synchrotron radio source may not exceed ~10^{12} K, a fact known from observations. However one gets a somewhat tighter limit on the brightness temperatures, T_{b}~10^{11.5} K, independent of the inverse Compton effects, if one employs the condition of equipartition of energy in magnetic fields and relativistic particles in a synchrotron radio source. Pros and cons of the two brightness temperature limits are discussed.