X-rays And Gamma Rays From Supernova Remnants (Clouds ,Nebulae Star Births And Deaths)
X-rays have been detected from several supernova remnants by the use of satellite-borne telescopes. The case of the Crab Nebula has already been mentioned; here the X-rays are almost certainly due to synchrotron radiation. Most SNRs, however, do not contain a pulsar that can maintain the supply of the ultra-relativistic electrons which are necessary for X-ray synchrotron emission.
In these other cases, the X-radiation is probably thermal emission from very hot gas that has been heated to temperatures of the order often million Kelvins. The heating results from the collision between the expanding supernova shell and the static interstellar medium which surrounds it. X-rays are seen from young super¬nova remnants such as Gas A and also from much older ones such as the CYGNUS LOOP . This latter object is probably the result of a supernova explosion some 50000 years ago. The remnant now has a diameter of about 30pc and is composed “mainly of interstellar matter that has been swept up by the blast. The X-rays from the Cygnus Loop are of lower photon energy than those from Cas A. The reason for this may be that the expansion rate of the Cygnus Loop has been slowed down over the years by its interaction with the interstellar medium; as a result the heating produced currently by the collision is weaker than in Cas A, and the resulting temperature is insufficient to lead to the emission of any photons with energy greater than 1 keV.
Two supernova remnants appear to be sources of gamma-ray emission in the energy range around l00MeV; these are the Crab Nebula and the Vela supernova remnant, a comparatively old SNR. As it happens, these are the only two SNRs which are definitely associated with pulsars, but this may be only a coincidence. We do not yet know what causes the gamma-ray emission; a possible mechanism involves the production and decay of pions as a result of the interaction of the cosmic ray protons produced in the SNR with the nearby interstellar medium. Current gamma-ray telescopes, however, have insufficient resolution to allow measurement of the angular diameters of these gamma-ray sources, and the possibility remains that the gamma rays are produced very close to the pulsar rather than in the shell. A synchrotron origin tor the gamma rays is also possible.