Microphysics of Shock Acceleration from Observations of X-Ray Synchrotron Emission from Supernova Remnants

Abstract

Recent observations of non-thermal X-rays from supernova remnants have been attributed to synchrotron radiation from the loss-steepened tail of a non-thermal distribution of electrons accelerated at the remnant blast wave. In diffusive shock acceleration, the maximum particle energy depends on the energy-dependence of the diffusion coefficient kappa. Most work on shock acceleration has made the plausible assumption that kappa is proportional to the particle gyroradius, implying a wavelength-independent spectrum of MHD turbulence, where Kolmogorov or Kraichnan spectra might be more physically plausible. I derive the wave-spectrum-dependence of the maximum electron energy resulting from limitations due to radiative (synchrotron and inverse-Compton) losses and to finite remnant age (or size). I then exhibit calculations of synchrotron X-ray spectra, and model images, for supernova remnants as a function of spectrum, and compare to earlier results. Spectra can be considerably altered, and images are dramatically different, for Kolmogorov or Kraichnan spectra of turbulence. The predicted images are quite unlike observed remnants, suggesting that the turbulence near SNRs is generated by the high-energy particles themselves.