V2051 Oph

Even disregarding the strong flickering activity, V2051 Oph has the most unstable light curve of all stars considered here. During most cycles, a hump is clearly present. However, its amplitude is quite variable, and in some cycles it all but vanishes. Moreover, its phase is not stable: While in most cycles its maximum occurs before the eclipse (as is expected for an orbital hump caused by the hot spot) sometimes it coincides with the eclipse or wanders to even later phases. Moreover, with a considerable frequency an intermediate hump appears roughly half a cycle before (or after) the principle hump.

The latter, however, disappears in the mean light curve which is shown together with a representative individual curve in of Figs. 4b and 4a, respectively. The mean hump is located at its canonical phase and has a moderate amplitude (as compared to e.g. Z Cha, Wood et al. 1986, or IP Peg, Sect. 4.3). Note also that in contrast to other eclipsing systems with prominent humps the end of the hump occurs even before the onset of the eclipse. However, this is only true for the mean curve and may be quite different in individual cycles. The mean eclipse profile differs from that of HT Cas (Sect. 4.1). It does not show the sudden eclipse ingress and egress of the white dwarf (although this may still be visible in individual light curves; Warner & Cropper 1983) but is more gradual. This, together with the round eclipse bottom, points towards a higher contribution of the accretion disk which is never totally eclipsed in this system. During an exceptionally low state of V2051 Oph [Baptista et al. (1998)] were able to measure the contact points of white dwarf eclipse ingress and egress. The corresponding phases are indicated by the dashed vertical lines in Fig. 4.

  

Figure 4: a Representative light curve of V2051 Oph. b Normalized mean orbital light curve of V2051 Oph. c Mean scatter curve of V2051 Oph. The dashed vertical lines indicate the contact phases of the white dwarf eclipse as measured by [Baptista et al. (1998)]. A representative error bar is shown in the lower right corner.

The scatter curves for the individual light curves were calculated as in the case of HT Cas. The resulting mean curve is shown in Fig. 4c. The average error of the data points of 0.064 is shown in the lower right corner. The eclipse is similar to that observed in HT Cas but - due to the much larger number of individual light curves and the absence of the problems with the steep eclipse ingresses and egresses - much better defined. In particular, the scatter eclipse is coincident with the white dwarf eclipse and definitely narrower than the disk eclipse. Thus, also in this system the flickering light source is located very close to the white dwarf while the outer parts of the accretion disk to not take part in the flickering to a perceptible degree.

In contrast to HT Cas, however, the scatter curve shows evidence that flickering occurs also to a certain degree at the location of the impact of the transferred matter onto the accretion disk: the scatter is clearly elevated at the phases when the orbital hump is visible. This is formally confirmed by the standard deviation of 0.080 of a Gauss fit to the histogram of the out-of-eclipse data points which is larger than their average mean error of 0.064. This hot spot flickering can also explain the apparent slight asymmetry of the scatter eclipse: it remains visible for a short time even after the white dwarf is already eclipsed, causing a slightly more gradual eclipse ingress, but is invisible when the central star emerges from the eclipse, explaining the steeper scatter eclipse egress. This behaviour of the flickering in V2051 Oph is identical to that of Z Cha during quiescence as measured by [Bruch (1996)].

The present results are in excellent agreement with those of [Warner & Cropper (1983)]. They concluded ``that in V2051 Oph the flickering is probably in general generated in the inner disk region with only a minor contribution from the hot spot''.