We study sub-relativistic and ultra-relativistic components of high-energy particles associated with supernova remnants (SNRs) using X-ray imaging and spectroscopic observations with ASCA and Chandra. Based on the extensive analysis of two shell-type SNRs, γ Cygni and RX J1713.7－3946, we have found new X-ray features in SNRs, namely (1) extremely flat-spectrum X-ray emission, and (2) unexpectedly complex structures in synchrotron X-ray images.
In the SNR γ Cygni, we found an extremely hard X-ray component from several clumps located in the northern part of the remnant, in addition to thermal plasma emission with a temperature of kTe ~ 0.8 keV. The energy spectra are described by a power law with a photon index of Γ ≅ 0.8－1.5. Also, in the vicinity of the SNR RX J1713.7－3946, we discovered a hard X-ray source, which is likely to be associated with a molecular cloud. The energy spectrum shows a flat continuum that is described by a power law with Γ =1.0±0.2. These hard X-ray sources, presumably of nonthermal origin, cannot be explained by the synchrotron or inverse-Compton mechanisms. Unusually flat spectrum obtained from these sources can be best interpreted in terms of characteristic bremsstrahlung emission from the Coulomb-loss-flattened distribution of either sub-relativistic protons or mildlyrelativistic electrons, in the dense environment. The strong shock waves of the both SNRs, which are interacting probably with the molecular cloud, as evidenced by observations of CO-lines, seem to be a natural site of acceleration of such sub- or mildly-relativistic nonthermal particles. Regardless of acceleration sites, the characteristic bremsstrahlung X-ray spectrum we discovered from these SNRs provides us a new diagnostic tool to study the largely-unknown component of low-energy cosmic rays in the Galaxy.
Synchrotron X-ray emission from shell-type SNRs enables us to study the highest energy region of shock-accelerated electrons and production sites of such high energy electrons. We study the northwest rim of the SNR RX J1713.7－3946 based on observations by Chandra. We found a complex network of nonthermal (synchrotron) X-ray filaments and hotspots, as well as a curious ’void’ type structure － a dim region of a circular shape － in the northwest rim. It is remarkable that despite distinct brightness variations, the X-ray spectra everywhere in this region can be well fitted with a power-law model with a similar photon index ranging Γ = 2.1－2.5. Spectral analysis implies that the high energy cutoff of the synchrotron X-ray spectrum extends most likely above 10 keV. The same line of analysis is performed for SN 1006, which shows also filamentary X-ray structures. We discuss implications of these results and argue that the resolved X-ray features may challenge the perceptions of standard (diffusive shock-acceleration) models concerning the production, propagation and radiation of relativistic particles in supernova remnants.