X-ray Spectra from Plasmas with High-Energy Electrons: kappa-distributions and e-e Bremsstrahlung

TL;DR

This paper develops a model to calculate X-ray spectra from plasmas with kappa electron distributions, incorporating electron-electron bremsstrahlung, and compares results with existing models, revealing significant high-energy contributions.

Contribution

It introduces a general model for plasma emission with kappa distributions using the AtomDB database and adds e-e bremsstrahlung to improve spectral predictions.

Findings

Kappa distributions significantly affect X-ray spectra.

Electron-electron bremsstrahlung contributes over 10% at high energies.

Total spectra differ by more than 10% above 60 keV.

Abstract

Shocks, turbulence and winds all influence the electron velocity distribution in hot plasmas, exciting lower-energy electrons and generating a high-energy (typically power-law) tail. This effect, typically described as a kappa distribution can affect both the line and continuum X-ray spectrum emitted by the plasma. Hahn & Savin (2015) proposed a "Maxwellian decomposition" to generate the rate coefficients of kappa distributions. Using their method and the AtomDB atomic database, we have developed a general model to calculate the emission from a plasma with a kappa distribution. We compare our kappa results for the charge state distribution and spectra of oxygen to those from KAPPA package with the ion data available within the CHIANTI atomic database. Sufficiently energetic electrons, created either in a kappa distribution or merely a very hot Maxwellian plasma, can also emit via electron-electron (e-e) bremsstrahlung, a process not previously included in AtomDB. We have added this process to AtomDB and apply it to calculate the temperature gradients, as well as the total spectra from the post-shock regions of an accreting magnetic cataclysmic variable (CV). We find the contribution of e-e bremsstrahlung to the total spectra exceeds 10% at KT\sim 100 keV, with the total emissivity in the post-shock accretion stream differing by more than 10% at energies above 60 keV.