On the Nature of the Tycho X-ray Source

Abstract

OF the five X-ray sources which are identified with remnants of supernovae, the Tycho 1572 source is of particular interest. In this case the X-ray emission cannot be considered as the continuation of the synchrotron radio spectrum because it can easily be shown that, independent of the mechanism of injection, the spectrum must be “bent” in the frequency region ν<10¹³ Hz. Also, it cannot be assumed that the observed X-radiation is thermal and caused by hot plasma behind the front of shock-wave produced by the expanding envelope of a remnant. This mechanism qualitatively explains X-radiation from Cassiopeia A if the undisturbed density n₀ of interstellar matter in the vicinity of the explosion is about 30 cm⁻³. This value of n₀ is in excellent agreement with the density of the HII region which is observed around Cas A (ref. 1). But in the case of the Tycho remnant one must assume that n₀ ∼5 cm⁻³ if the radiation is thermal. Such a value of n₀ contradicts the observed large dimensions of this remnant, which, according to all indications, has exploded in a relatively rare medium between the clouds of interstellar matter, where n₀≤0.1 cm⁻³. This excludes the possibility of explaining X-radiation from Tycho as thermal radiation from hot plasma.