The Origin of Diffuse X-Ray Emission from the Galactic Ridge. II. Nonequilibrium Emission Due to In Situ Accelerated Electrons
Abstract
The origin of the Galactic ridge X-ray emission has been investigated from various points of view, such as the iron K line, the hard part of the continuum, and energetics. We propose a ridge plasma model based on stochastic particle acceleration in the interstellar medium to explain the properties of the soft and hard X-rays consistently. In situ accelerated electrons form a spectrum that consists of three components: bulk thermal (Maxwellian), quasi-thermal, and nonthermal (power law) through diffusion in momentum space. For the bulk temperature of a few hundred eV, the quasi-thermal component extends up to a few tens of keV. While nonthermal electrons are collisionless, so as to stay in the acceleration regime, quasi-thermal electrons interact with the bulk electrons through Coulomb collisions. Thus, the interaction with the bulk plasma significantly alters the X-ray emission, and the resultant spectrum can explain the observed features that resemble the emission from a multitemperature or nonequilibrium plasma of order of keV. From a comparison of the model predictions with the observed spectrum, we found that the Galactic ridge X-ray emission is explained by electron acceleration in interstellar gas with temperature 0.3-0.6 keV and density (6.5-3.4)×10-3 cm-3, which can be bound by Galactic gravity. This model can also solve the energetics problem: since a substantial part of the X-ray flux is accounted for by nonequilibrium emission due to quasi-thermal electrons of a small fraction of the medium, we need neither hot plasmas of the order of keV nor higher rates of supernova explosion in the Galaxy to explain the ridge X-ray emission.
- Publication:
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The Astrophysical Journal
- Pub Date:
- December 2002
- DOI:
- 10.1086/344247
- Bibcode:
- 2002ApJ...581.1071M
- Keywords:
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- Acceleration of Particles;
- ISM: Cosmic Rays;
- Galaxy: Disk;
- ISM: Structure;
- Stars: Supernovae: General;
- X-Rays: ISM