High-speed photometry of Z Chamaleontis during outbursts

Abstract

High-speed while-light photometry of the dwarf nova Z Cha at time resolutions of 4 or 5s is given for six supermaxima and one ordinary maximum observed during the period 1973–84. Measurements of the superhump amplitudes and eclipse parameters enable correlations to be found with time after the appearance of supcrhumps and with phase Φ0 in orbit of the superhump maximum. The depth of eclipse increases systematically during superoutburst, and the width of eclipse decreases. However, both depth and width of eclipse are functions of Φ0. Eclipse asymmetries occur which are also a function of Φ0. From an analysis of the correlations we deduce that at early stages of the superoutburst the source of the superhump light is partially eclipsed by the secondary star, which implies that the source is not a bright spot on the secondary. Synthetic light curves arc computed which, by comparison with the observed eclipse profiles, assist in locating the source of the superhump light. We are able to eliminate locations in the vicinity of the quiescent bright spot or along the mass-transfer stream as it penetrates into (or flows over) the accretion disc. The only successful simulations were obtained by modulating the intensity of a bright ring at the outer edge of the disc at the superhump period. This ring is brightest near the line of centres of the two stars, and falls off downstream, with the result that the trailing lune of the disc is brighter than the leading lune. In discussing the structural changes that take place during a superoutburst we make some limited use of Maximum Entropy Method (MEM) deconvolution of eclipse light curves. At maximum light, before superhumps have appeared, the observed light curve shows unusual dips at phases 0.25 and 0.75; at this time the accretion disc is large and possesses a bright rim. As the outburst proceeds the bright rim and the outer regions fade relative to the inner regions of the disc. At the end of superoutburst. the disc has almost completely disappeared (in visible light) leaving a central source only a few white dwarf radii in size. Our MEM deconvolution of the eclipse observed during a normal maximum also shows a bright rim to the disc.