Activity of the Polar AM Her (RX J1816.2+4952): A Short Review

Abstract

We show that AM Her displays the transitions between the high and low states with an intermittently existing dominant cycle with length between 400 and 800 days. Moreover, these transitions accumulate in clusters, which produces an additional long cycle after smoothing; a single isolated short episode of the low state does not suggest a break of this cycle. The seasons of existence of the cycle can be controlled by the lifetime of the active regions (e.g. prominences, spots) on the donor. In some high-state episodes, a higher luminosity of the bremsstrahlung emission is not accompanied by a higher optical (cyclotron+stream) emission. Part of the bremsstrahlung emission can be buried in some episodes. Changes of the structure of the accretion region(s) are necessary to explain the variations of the optical and X-ray activity in the high-state episodes of AM Her.