Diffusion in the Bohm limit and non-thermal electrons in the Cassiopeia A supernova remnant
Abstract
Acceleration of cosmic rays in our Galaxy by means of diffusive shock acceleration (DSA) is believed to occur primarily in supernova remnants. Despite a strong theoretical foundation, the precise details are still unknown in part because of the difficulty in directly observing nucleons that are accelerated to TeV energies in, and modify the structure of, SNR shocks. X-ray telescopes give us the opportunity to trace the keV synchrotron radiation emitted by accelerated electrons in the shocks, and the non-thermal bremsstrahlung radiation emitted by accelerated electrons which escape the shocks and interact with interior reverse-shocked ejecta. In our spatially-resolved spectroscopic analysis of Chandra X-ray observations of the Galactic SNR Cassiopeia A in the 0.5-10 keV band, we constrained the the diffusion coefficient of electrons in the forward shock. We demonstrated that in some locations they are accelerated nearly as fast as possible - in the Bohm limit as predicted by efficient DSA (Stage et al. 2006). We mapped the cutoff frequencies and diffusion coefficient of electrons accelerated in the forward shock on arcsecond scale by fitting the spectrum at thousands of locations along the shock filaments with a synchrotron model. Further, we used these results to understand the 10-32 keV emission observed by the Rossi X-ray Timing Explorer: a high-energy power-law tail containing contributions from non-thermal bremsstrahlung. Estimating the total synchrotron flux based on our Chandra analysis, and employing a non-thermal bremsstrahlung model we have developed, we have constrained the particle dynamics by fitting the RXTE spectrum. We show that the fraction of the non-thermal X-ray emission in the RXTE band produced by non-thermal bremsstrahlung is about two-thirds, that about 5% of the electrons have been accelerated to non-thermal energies, and that these electrons carry about 30% of the total energy in the electron distribution.
- Publication:
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37th COSPAR Scientific Assembly
- Pub Date:
- 2008
- Bibcode:
- 2008cosp...37.3015S