SIMON V., POLASEK C., STROBL J., HUDEC R. and BLAZEK M.
Abstract (from CDS):
We investigate the photometric activity of the cataclysmic variable (CV) V795 Her. We pay attention to the time evolution of the length of its superhump cycle Psh. We investigate how well the superhump profile is reproduced in the individual phases Φsh of Psh. We also analyze the mutual relation of the intensity of the superhump light and the superimposed rapid intensity variations. We used the photometric V-band CCD observations obtained between 2008 and 2010. The observing run on a given night consisted of a dense series of V-band images (exp. time of 20s), which enabled us to investigate the superhump profile and the superimposed rapid changes. We identified several time segments characterized by a relatively stable Psh and mean intensity level. We find that inside such a segment, the smoothed profile of the intensity curve folded with Psh displays the largest scatter of the residuals of the mean profile in Φsh of the peak intensity and during the decay from this peak. A zone of quiescence occurs at Φsh≃0.6-0.7. The profile of these residuals can be explained if the strength and luminosity of the spiral arms undergo large variations on a timescale much shorter than the segment's duration, as predicted in a previously developed model, while the conditions for the disk precession remain much more stable. The formation of these arms is considerably faster than their fading. We find that the emission of the superimposed short cycle (∼0.01-0.02d) in this CV depends on Φsh. We conclude that it does not come from pulsations of the white dwarf or, generally, from the close vicinity of this accretor. We argue that it is flickering with the site in the disk region that produces the superhump. This flickering is not located in the impact of the stream on the disk. V795 Her is a borderline case in the sense that its disk is close to the switch to the cool state. We explain the suppression of the thermal-viscous instability of the disk in V795 Her as caused by the tidal force that is also the reason for the superhumps. This tidal heating can also help to keep the accretion disks in other permanent superhumpers in the hot state.