On the spatial distribution of electron energy loss due to gyro-cooling in hot-star magnetospheres

TL;DR

This paper investigates how energetic electrons in hot-star magnetospheres cool via gyro-synchrotron emission, affecting observed radio signals, and finds relativistic electrons are less impacted by collisional cooling, shaping future spectral models.

Contribution

It derives the spatial distribution of gyro-cooling energy loss for electrons in dipole magnetospheres, highlighting the dominance of relativistic electrons in observed radio emission.

Findings

Cooling distribution is independent of initial energy for sub-relativistic electrons.

Coulomb collisions can quench emission from sub-relativistic electrons.

Relativistic electrons are less affected by collisional cooling, explaining observed radio emissions.

Abstract

Hot magnetic stars often exhibit incoherent circularly polarized radio emission thought to arise from gyro-synchrotron emission by energetic electrons trapped in the circumstellar magnetosphere. Theoretical scalings for electron acceleration by magnetic reconnection driven by centrifugal breakout match well the empirical scalings for observed radio luminosity with both the magnetic field strength and the stellar rotation rate. This paper now examines how energetic electrons introduced near the top of closed magnetic loops are subsequently cooled by the energy loss associated with their gyro-synchrotron radio emission. For sample assumed distributions for energetic electron deposition about the loop apex, we derive the spatial distribution of the radiated energy from such "gyro-cooling". For sub-relativistic electrons, we show explicitly that this is independent of the input energy, but also find that even extensions to the relativistic regime still yield a quite similar spatial distribution. However, cooling by coulomb collisions with even a modest ambient density of thermal electrons can effectively quench the emission from sub-relativistic electrons, indicating that the observed radio emission likely stems from relativistic electrons that are less affected by such collisional cooling. The overall results form an initial basis for computing radio emission spectra in future models that account for such cooling and multimode excitation about the fundamental gyro-frequency. Though motivated in the context of hot-stars, the basic results here could also be applied to gyro-emission in any dipole magnetospheres, including those of ultra-cool dwarfs and even (exo)-planets.