Inverse Compton X-rays from relativistic flare electrons and positrons

TL;DR

This paper evaluates the potential of inverse Compton scattering of photospheric photons by relativistic electrons and positrons in solar flares to produce detectable hard X-ray emissions, offering a new diagnostic tool for flare particle populations.

Contribution

It provides a detailed calculation of ICS X-ray fluxes considering anisotropic photon fields, improving previous isotropic assumptions, and explores the diagnostic potential of ICS for flare particles.

Findings

ICS can produce hard X-ray spectra in solar flares.

Limb flares show stronger ICS signals due to anisotropy.

Some coronal X-ray sources may be explained by ICS from relativistic particles.

Abstract

In solar flares, inverse Compton scattering (ICS) of photospheric photons might give rise to detectable hard X-ray photon fluxes from the corona where ambient densities are too low for significant bremsstrahlung or recombination. Gamma-ray lines and continuum in some large flares imply the presence of the necessary ~100 MeV electrons and positrons, the latter as by-products of GeV energy ions. Recent observations of coronal hard X-ray sources in particular prompt us to reconsider here the possible contribution of ICS. We aim to evaluate the ICS X-ray fluxes to be expected from prescribed populations of relativistic electrons and positrons in the solar corona. The ultimate aim is to determine if ICS coronal X-ray sources might offer a new diagnostic window on relativistic electrons and ions in flares. We use the complete formalism of ICS to calculate X-ray fluxes from possible populations of flare primary electrons and secondary positrons, paying attention to the incident photon angular distribution near the solar surface and thus improving on the assumption of isotropy made in previous solar discussions. Both primary electrons and secondary positrons produce very hard ICS X-ray spectra. The anisotropic primary radiation field results in pronounced centre-to-limb variation in predicted fluxes and spectra, with the most intense spectra, extending to the highest photon energies, expected from limb flares. Acceptable numbers of electrons or positrons could account for RHESSI coronal X/gamma-ray sources. Some coronal X-ray sources at least might be interpreted in terms of ICS by relativistic electrons or positrons, particularly when sources appear at such low ambient densities that bremsstrahlung appears implausible.