Transient X-ray source A1118—61

Abstract

THE transient X-ray source Ariel 1118—61 was discovered late in 1974 (refs 1, 2). It has been suggested³ that this source may be associated with the long period Mira-type variable RS Cen, which is located within the error box for the X-ray source. The proposed mechanism of X-ray generation was accretion on to a compact object in orbit around the variable star. The periodic variation in the radius of the star would lead to a variable accretion rate at the orbit of the compact object, and give rise to a variable X-ray flux. This suggestion has two important consequences. First, it raises the possibility that some, at least, of the apparently transient X-ray sources can be explained by similar phenomena. For instance, Barnden and Francey⁴ and Shukla and Wilson⁵ reported the presence of two presumably ‘transient’ X-ray sources in the constellation Cetus. The error box for each source (Cetus X-1 (ref. 4) and Cetus X-2 (ref. 5)) was large, ∼ 10–15° diameter, but they did just overlap. Of significance here is the fact that the overlap area is centred on O Ceti (Mira), the type star of this class of variables. The two observations were at similar phases (0.12 and 0.22) of the variable, but separated in time by ∼1 cycle. The difference in phase of the Centaurus and Cetus transients is easily explained by differences in orbit size and expansion velocity.