Brightness temperature - obtaining physical properties of non-equipartition plasma

Abstract

The limit on the intrinsic brightness temperature, attributed to `Compton catastrophe', has been established being $10^{12}$~K. Somewhat lower limit of the order of $10^{11.5}$~K is implied if we assume that the radiating plasma is in equipartition with the magnetic field --- the idea that explained why the observed cores of active galactic nuclei sustained the limit lower than the `Compton catastrophe'. Recent observations with unprecedented high resolution by the RADIOASTRON revealed systematic exceed in the observed brightness temperature. We propose means of estimating the degree of non-equipartition regime in AGN cores. Coupled with the core-shift measurements the method allows us to estimate independently the magnetic field strength and the particles number density at the core. We show that the ratio of magnetic energy to radiating plasma energy is of the order of $10^{-5}$, which means the flow in the core is dominated by the particle energy. We show that the magnetic field obtained by the brightness temperature measurements may be underestimated. We propose for the relativistic jets with small viewing angles the non-uniform MHD model, and obtain the expression for the magnetic field amplitude about two orders higher than that for the uniform model. These magnetic field amplitudes are consistent with the limiting magnetic field suggested by the `magnetically arrested disk' model.