A Review of X-ray Observations of Supernova Remnants

Although radio and optical observations have played an important role in identifying and characterizing supernova remnants (SNRs), most of the shock heated material is primarily visible in X-rays, having temperatures of typically 10 − 10 K. The emission mechanisms are bremsstrahlung and line radiation, but a number of remnants are now known to emit Xray synchrotron radiation from ultra-relativistic electrons, as well. This was already known for pulsar wind nebula dominated, or Crab-like remnants, which fall outside the scope of this review, but also shelltype remnants like SN 1006 emit synchrotron radiation from electrons accelerated by the blast wave [1]. The X-ray line emission of SNRs is in particular interesting, as it allows the study of the most abundant metals, the so called α-elements, which prominently feature in the X-ray spectra of young SNRs like Cas A, Tycho (SN 1576) and Kepler (SN 1064). This line emission, and mass estimates based on the X-ray emission, give important information on the nucleosynthesis yields of the explosions, which can then be compared to theoretical supernova (SN) models [2,3]. The line and bremsstrahlung emission give us a window on the past of the SNR: its progenitor and the SN explosion. X-ray synchrotron radiation, on the other hand, is interesting as it provides a window on cosmic ray shock acceleration. SNRs are thought to be the primary source of cosmic rays up to, and possibly beyond, energies of 10 eV, at which energy the spectrum has a break, usually referred to as “the knee”, However, direct evidence for a SNR origin for cosmic ray particles other than electrons is lack-