Gyro-orbit size, brightness temperature limit and implausibility of coherent emission by bunching in synchrotron radio sources

TL;DR

The paper derives an upper brightness temperature limit for incoherent synchrotron sources based on gyro-orbit sizes, challenging the plausibility of coherent bunching as an explanation for extremely high brightness temperatures observed in variable radio sources.

Contribution

It introduces a new constraint on brightness temperatures from gyro-orbit sizes and argues against coherent bunching as a mechanism for high brightness temperatures in extragalactic radio sources.

Findings

Brightness temperature limit derived from gyro-orbit size.

Coherent bunching cannot explain observed high brightness temperatures.

Bunches disperse faster than the orbital period, making coherence impossible.

Abstract

We show that an upper limit on the maximum brightness temperature for a self-absorbed incoherent synchrotron radio source is obtained from the size of its gyro orbits, which in turn must lie well within the confines of the total source extent. These temperature limits are obtained without recourse to inverse Compton effects or the condition of equipartition of energy between magnetic fields and relativistic particles. For radio variables, the intra-day variability (IDV) implies brightness temperatures $\sim 10^{19}$ K in the co-moving rest frame of the source. This, if interpreted purely due to an incoherent synchrotron emission, would imply gyro radii $>10^{28}$ cm, the size of the universe, while from the causality arguments the inferred maximum size of the source in such a case is $\stackrel{<}{_{\sim}} 10^{15}$ cm. Such high brightness temperatures are sometimes modeled in the literature as some coherent emission process where bunches of non-thermal particles are somehow formed that radiate in phase. We show that, unlike in case of curvature radiation models proposed in pulsars, in the synchrotron radiation mechanism the oppositely charged particles would contribute together to the coherent phenomenon without the need to form separate bunches of the opposite charges. At the same time we show that bunches would disperse over dimensions larger than a wavelength in time shorter than the gyro orbital period ($\stackrel{<}{_{\sim}}0.1$ sec). Therefore a coherent emission by bunches cannot be a plausible explanation of the high brightness temperatures inferred in extragalactic radio sources showing variability over a few hours or longer.