X-ray Emission from Old Novae

Abstract

THE strong X-ray source in Scorpius has been identified with a very blue stellar object which has many of the characteristics of an old nova1,2. The object varies irregularly by up to half an astronomical magnitude from day to day, but the average brightness appears to have been essentially the same for the past 70 years2. Superimposed on these variations is a fast flicker with a time scale of the order of a minute or so. The X-ray spectrum is rather complex; if the X-rays are assumed to result from thin-source bremsstrahlung, then the 1–10 A X-rays correspond to a temperature of about 5 × 107 °K (refs. 3–6), but the range from 44 to 60 A contains additional radiation which must come from a region of lower temperature6. The visible radiation probably comes predominantly from the same thin-source bremsstrahlung, although hydrogen emission lines and helium, carbon, nitrogen, and oxygen absorption lines appear superposed on the continuum2. Three orders of magnitude more energy is emitted as X-radiation than as visible light.