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          Institute: MPI für extraterrestrische Physik     Collection: High Energy Astrophysics at MPE     Display Documents

ID: 5696.0, MPI für extraterrestrische Physik / High Energy Astrophysics at MPE
The origin of diffuse X-ray emission from the Galactic ridge. II. Nonequilibrium emission due to in situ accelerated electrons
Authors:Masai, Kuniaki; Dogiel, Vladimir A.; Inoue, Hajime; Schonfelder, Volker; Strong, Andrew W.
Date of Publication (YYYY-MM-DD):2002-12-20
Title of Journal:The Astrophysical Journal
Journal Abbrev.:Astrophys. J.
Issue / Number:2
Start Page:1071
End Page:1079
Review Status:Peer-review
Audience:Experts Only
Abstract / Description: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) x 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.
Free Keywords:acceleration of particles; cosmic rays; Galaxy : disk; ISM : structure; supernovae : general; X-rays : ISM
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für extraterrestrische Physik
External Affiliations:Tokyo Metropolitan Univ, Dept Phys, Tokyo 1920397, Japan; Tokyo Metropolitan Univ, Dept Phys, Tokyo 1920397, Japan; Inst Space & Astronaut Sci, Kanagawa 2298510, Japan; PN Lebedev Phys Inst, IE Tamm Dept Theoret Phys, Moscow 119991, Russia; Max Planck Inst Extraterr Phys, D-85741 Garching, Germany
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