Optical flickering and shot noise in AM Her systems

Abstract

The emission from AM Her systems show a variety of time-variable phenomena, including flickering with a typical power spectrum slope of −1 to −2. The optical flickering is usually modeled as, or assumed to be, a shot noise process with many overlapping simultaneous shots. Panek (1980) found that, for AM Her, a consistent model could be constructed with randomly occurring 70… 90 s rectangular shots. The power spectrum had a v−2 shape above 0.02 Hz as expected, although it is not clear whether a break in the slope of the power spectrum was actually seen around 0.01 Hz. There are many additional reports in the literature of ‘characteristic time-scales’ on the order of tens of seconds. However we are not aware of any case where the reported time-scale has been supported by e.g. a break in the power density spectrum. It is easy to show that the usual method of estimating a ‘characteristic time-scale’ from the auto correlation function of de-trended data will suggest such a characteristic time-scale even if none is present in the original time series. The evidence for shot noise in AM Her light curves therefore needs a re-examination. In this paper we give a short summary of our analysis of optical flickering in V834 Cen, based on high speed photometry obtained at ESO in 1987. Details will be reported elsewhere.