X-Ray Observations of Supernova Remnants

Abstract

Supernova remnants (SNRs) are low density, coronal plasmas that are shock heated by the explosion of a supernova. Spectroscopic observations of several bright SNRs have allowed us to deduce some of the physical properties of the emitting plasma. I emphasize the question of equilibration between electron temperature, ion temperature and ionization temperature. Inconsistency between several observations leaves open the question of whether or not the electrons and ions have reached equilibrium. On the other hand, both moderate and high resolution spectra show clear departures from ionization equilibrium. Ionization ages deduced using ratios of various emission lines, including the forbidden and resonance lines of He-like ions, are derived for Puppis A, Cas A and the Cygnus loop and found to be consistent with estimates from independent measurements. Even the 20,000 year old Cygnus Loop SNR is found to be underionized for its temperature. We consider possible departures from Maxwellian electron distributions, although there is neither evidence for them nor a compelling reason to expect them. Future missions, such as ASTRO-D, Spectrum X-Gamma, and particularly AXAF, will greatly enhance our ability to study the plasma properties of SNRs. As one example, AXAF will permit the use of satellite lines as diagnostics of electron temperature, ionization state and non-Maxwellian electron distribution functions on many dozen SNRs.