General-relativistic radiative cooling in neutron star magnetospheres

TL;DR

This paper investigates how general relativity and electromagnetic field geometries influence plasma cooling and particle distributions in neutron star magnetospheres, impacting coherent radiation emission.

Contribution

It provides the first systematic analysis of radiative cooling effects in realistic neutron star environments considering general relativity and complex field geometries.

Findings

Drift velocities promote spiral-shaped momentum distributions with inverted populations.

Curved spacetime enhances the gradient of the distribution function, aiding kinetic instabilities.

Inverted momentum structures persist longer in curved spacetime, favoring coherent emission.

Abstract

Radiation reaction cooling plays an important role in describing the extreme plasma conditions found in the magnetospheres of astrophysical compact objects. Strong electromagnetic fields, characteristic of these environments, can trigger the development of anisotropic ring-shaped plasma distributions with inverted Landau populations in momentum space. In this work, we present the first systematic investigation of this mechanism in realistic astrophysical configurations, by accounting for how non-uniform electromagnetic field geometries and general-relativistic effects modify the phase-space dynamics of radiatively cooled plasmas. We demonstrate analytically that drift velocities favour the formation of spiral-shaped momentum distributions that still display inverted Landau populations, and estimate the minimum and maximum plasma injection distances required for inverted momentum distributions to be able to power the emission of coherent radiation through kinetic instabilities. From numerical simulations, we conclude that curved spacetime increases the gradient of the distribution function responsible for the development of kinetic instabilities, and prolongs the persistence of the inverted momentum structure relative to flat spacetime, confirming that realistic astrophysical conditions preserve and enhance the conditions necessary for synchrotron-powered emission of coherent radiation to occur.