Brightness temperature constraints on coherent processes in magnetospheres of neutron stars

TL;DR

This paper examines the extreme brightness temperatures observed in pulsars and FRBs, showing that coherent curvature emission by bunches is energetically implausible, but Free Electron Laser mechanisms are more feasible.

Contribution

It provides quantitative constraints on coherent emission models in neutron star magnetospheres, highlighting the energetic limitations of bunch-based curvature emission.

Findings

Bunch electrostatic energy exceeds total bulk energy for observed brightness temperatures.

Coherent curvature emission by bunches is energetically prohibitive.

Free Electron Laser mechanism constraints are more easily satisfied.

Abstract

We discuss constraints that the observed brightness temperatures impose on coherent processes in pulsars and Fast Radio Bursts (FRBs), and in particular on the hypothesis of coherent curvature emission by bunches. We estimate the peak brightness temperature that a bunch of charge $Ze$ can produce via synchrotron and/or curvature emission as $k_B T \sim (Z e)^2/\lambda$, where $\lambda$ is the typical emitted wavelength. We demonstrate that the bunch's electrostatic energy required to produce observed brightness temperature is prohibitively high, of the order of the total {\it bulk } energy. We compare corresponding requirements for the Free Electron Laser mechanism (Lyutikov 2021) and find that in that case the constraints are much easier satisfied.