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

TL;DR

This study uses eclipse mapping to analyze flickering and brightness variations in UU Aqr, revealing asymmetric spiral shock waves in the accretion disk that influence flickering and long-term brightness states.

Contribution

It provides the first detailed eclipse maps of flickering components in UU Aqr, identifying spiral shock waves as key features affecting variability.

Findings

Flickering maps show two asymmetric arcs similar to those in IP Peg.

Spiral shock waves account for 25% of steady-light flux.

Long-term brightness changes linked to variations in mass transfer rate.

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.