Resolved Shot Noise in the AM Her System V834 Cen

Abstract

Cataclysmic variables of the AM Her type are close binary systems with accretion onto a highly magnetic (a few times 107 G) white dwarf. In these systems the magnetic field is strong enough to prevent the formation of an accretion disc. The accretion flow is instead funneled directly onto the polar caps where the plasma is shocked and radiates intense optical cyclotron radiation as well as bremsstrahlung at X-ray energies. The cyclotron radiation component, which dominates in the optical, is highly polarized (5-35%). The cyclotron emission exhibits intensity variations on a large range of time scales. This include quasi-periodic oscillations, flickering and long term changes between brightness states. The observed source intensity is also modulated by the white dwarf rotation which is synchronized with the orbital period (101 min for V834 Cen). The flickering is believed to be due to inhomogeneities in the accretion flow. These could either be density variations in a continous flow or it could be in the form of independent blobs accreting over the polar cap region. As blobs hit the white dwarf surface and radiate their accretion energy they should be seen as shot noise. For a theoretical discussion of the fate of such blobs see Frank et al. (1988). A modeling of the optical variability in AM Herculis, in terms of a shot noise process was made already by Panek (1980). It was found that a consistent model could be constructed by randomly occurring 708 – 908 rectangular shots, with on the average a few overlapping 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. The shot overlap meant that individual shots could not be resolved directly in the light curve. This limits the precision and uniqueness by which shot parameters can be determined. However, in our observational data of AM Her objects we have found occasions when the character of the flickering has changed to become much more “flarelike”, suggesting that individual shots might be seen in these cases. This change is also associated with a change in the shape of the power spectrum, in particular a higher power level and a steepening at the high frequency end. Most importantly, there is a distinct break in the power spectra around 0.03 Hz, indicative of a characteristic time scale in the flickering noise. In addition to a power spectrum analysis we have also made a direct modeling of individual flares in the light curve. The distribution and correlations of the fitted pulse parameters were studied and compared with simulated data. The primary aim was to investigate whether the flares are really individual shots or produced by a random superposition of more frequent smaller amplitude shots.