Accretion-disc precession in UX Ursae Majoris

Abstract

We report the results of a long campaign of time series photometry on the nova-like variable UX Ursae Majoris during 2015. It spanned 150 nights, with ~ 1800 h of coverage on 121 separate nights. The star was in its normal 'high state' near magnitude V = 13, with slow waves in the light curve and eclipses every 4.72 h. Remarkably, the star also showed a nearly sinusoidal signal with a full amplitude of 0.44 mag and a period of 3.680 ± 0.007 d. We interpret this as the signature of a retrograde precession (wobble) of the accretion disc. The same period is manifest as a ±33 s wobble in the timings of mid-eclipse, indicating that the disc's centre of light moves with this period. The star also showed strong 'negative superhumps' at frequencies ωorb + N and 2ωorb + N, where ωorb and N are, respectively, the orbital and precession frequencies. It is possible that these powerful signals have been present, unsuspected, throughout the more than 60 yr of previous photometric studies.