The X-ray light curves of AM Herculis systems

We reconsider the X-ray light curves of AM Herculis systems using the inhomogeneous accretion picture suggested by Kuijpers & Pringle and developed by Frank, King & Lasota. We show that the soft X-ray light curves of AM Her (and other systems) at both ‘normal’ and ‘anomalous’ epochs can be readily understood: the form depends on the separation and number of dense, soft X-ray producing blobs accreting instantaneously. In the anomalous state of AM Her, this number is only ∼15, falling on a sparsely populated accretion zone; the individual footpoints are seen in soft X-rays as protruding splashes and give rapid transitions from the low to high flux levels as the accretion zone (<10⁻² of the white dwarf surface) rotates into view. The high level occupies ∼50 per cent of the cycle, as observed. In the normal state, there are many more dense blobs, and the curves become quasi-sinusoidal. In all cases, there is a strict hierarchy between the instantaneous area A_acc accreting dense blobs, the effective area A_eff radiating most of the accretion luminosity as soft X-rays, the total zone area A_zone over which the blobs are scattered, and the white dwarf surface area 4πR², namely $$A_\text{acc}\ll A_\text{eff}\lt,\enspace\ll A_\text{zone}\ll 4\pi R^2$$. At normal epochs, A_eff may be almost as big as A_zone, whereas at anomalous epochs, A_eff≪A_zone. Diffuse material contributing only a minor fraction of the accretion rate falls over much of A_zone, and produces both the observed hard X-rays and near-UV. Thus, UV and soft X-rays need not correlate, as observed. The hard X-ray region may have a large vertical extent, accounting for the long duty-cycle in the AM Her anomalous state, and the quasi-sinusoidal hard X-ray light curve observed in e.g. EF Eri.