A two-armed pattern in flickering maps of the nova-like variable UU Aquarii

Abstract

We report the analysis of a uniform sample of 31 light curves of the nova-like variable UU Aqr with eclipse mapping techniques. The data were combined to derive eclipse maps of the average steady-light component, the long-term brightness changes, and low- and high-frequency flickering components. The long-term variability responsible for the 'low' and 'high' brightness states is explained in terms of the response of a viscous disk to changes of 20-50 per cent in the mass transfer rate from the donor star. Low- and high-frequency flickering maps are dominated by emission from two asymmetric arcs reminiscent of those seen in the outbursting dwarf nova IP Peg, and are similarly interpreted as manifestation of a tidally-induced spiral shock wave in the outer regions of a large accretion disk. The asymmetric arcs are also seen in the map of the steady-light aside of the broad brightness distribution of a roughly steady-state disk. The arcs account for 25 per cent of the steady-light flux and are a long-lasting feature in the accretion disk of UU Aqr. We infer an opening angle of 10+/-3 degrees for the spiral arcs. The results suggest that the flickering in UU Aqr is caused by turbulence generated after the collision of disk gas with the density-enhanced spiral wave in the accretion disk.