UX UMa

Due to the longer orbital period the phase coverage of the presently available light curves of UX UMa is not as complete as for the other stars in this study. Therefore, the phase range for the present investigations is restricted to -0.25 < phi < 0.25. A representative individual light curve and the mean of all normalized curves are shown in Figs. 6a and 6b, respectively (note that the discontinuity in the mean curve close to phase phi = 0.17 is an artifact caused by a light curve with a particularly low light level after eclipse which contributes to the mean only at phases phi < 0.17. The broadness of the eclipse and the rounded bottom suggests that the eclipsed body is extended (the accretion disk) and that it is never fully eclipsed. This agrees with the low orbital inclination of determined by [Baptista et al. (1995)], which is just high enough to cause grazing eclipses of the white dwarf. Using UV light curves, [Baptista et al. (1995)] could also measure the white dwarf eclipse ingress and egress phases which are marked in Fig. 6 by dashed vertical lines. The eclipse contains an extended wing at egress, a feature introduced by the retarded eclipse egress of the hot spot. The presence of the latter is also visible in eclipse maps of UX UMa (Baptista et al. 1995).

  

Figure 6: a Representative light curve of UX UMa. b Mean light curve of UX UMa. c Mean scatter curve of UX UMa. The dashed vertical lines indicate the white dwarf eclipse ingress and egress phases as measured by [Baptista et al. (1995)]. A representative error bar is shown in the upper right corner.

The mean scatter curve, calculated in the same way as in the previous cases, is shown in Fig. 6c. The average mean error of the data points (upper right corner of Fig. 6c) is 0.125. Although the scatter curve is rather noisy there is no doubt about the presence of an eclipse. Once again, its boundaries agree remarkably well with those of the white dwarf eclipse, indicating that in UX UMa (as in HT Cas and V2051 Oph) the flickering light source is located very close to the central body. Using the orbital inclination of 70 degrees, a mass ratio of 1 (Baptista et al. 1995) and taking into account the distorted shape of the Roche-lobe filling secondary star it is found that the secondary star eclipses the accretion disk on the far side only out to about 4.4 white dwarf radii. Therefore, the flickering light source - at least a significant part of it - must be located within this distance for a sharply confined flickering eclipse to occur. In view of the scatter of the curve and the unknown contribution of residual noise (Bruch 1996) no attempt is made here to quantify this statement. However, the extremely rapid eclipse egress permits an even considerably narrower distribution of the flickering around the white dwarf. The apparently more gradual eclipse ingress and the slightly enhanced scatter before the eclipse (the Gauss fit to the histogram of all out-of-eclipse points yields a standard deviation of 0.185, significantly larger than the average mean error of 0.125) may also in this case indicate a hot spot contribution to the total flickering.