THE origin of both the relatively long period and the transient nature of the recently discovered X-ray sources Ariel 1118−61 (ref. 1) and Ariel 0535+26 (ref. 2), with pulse periods of about 405 and 104 s, respectively, and decay times of days, is unclear. The pulse rate of A1118−61 has been attributed to an extremely short binary star orbital period3, or to an extremely long neutron star rotation period4. The quasi-sinusoidal nature of both X-ray light curves, unlike the sharply pulsed flux from Her X-l and Cen X-3, would seem difficult to account for in a rotation model. The 104-s period, on the other hand, seems embarrassingly short for a binary orbital period. Therefore, I reiterate here an earlier suggestion5 that free precession may be important in such galactic X-ray sources. Such precession, which provides a plausible explanation for the 35-d quasi-sinusoidal period and intensity variation of Her X-l (which has a 1.25-s pulse period) could directly account for the shape of the light curves of the transient X-ray sources.
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BRECHER, K. Precession in transient X-ray sources. Nature 257, 203 (1975). https://doi.org/10.1038/257203a0
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