We model the visual light curve of CAL 87 based on the assumption that an accreting steadily burning white dwarf irradiates the accretion disk and the secondary star, as suggested by van den Heuvel et al. (1992). We use constraints on the geometry derived from the known orbital period. As sources of visual light we include the secondary star and an accretion disk with an optically thick, cold, clumpy spray at its riml presumably caused by an accretion stream of high mass flow rate impinging on the disk at the hot spot. This spray moving around the disk can account for the asymmetry in the light curve and the depth of the secondary minimum. It also might be the cause of the observed low X-ray luminosity if the white dwarf is permanently hidden by this disk.
Abstract.We have studied the long-term behaviour of the 1.24 sec pulse period and the 35 day precession period of Her X-1 and show that both periods vary in a highly correlated way (see also Staubert et al. 1997 and2000). When the spin-up rate decreases, the 35 day turn-on period shortens. This correlation is most evident on long time scales (∼2000 days), e.g., around four extended spindown episodes, but also on shorter time scales (a few 100 days) on which quasi-periodic variations are apparent. We argue that the likely common cause is variations of the mass accretion rate onto the neutron star. The data since 1991 allow a continuous sampling and indicate a lag between the turn-on behaviour and the spin behaviour, in the sense that changes are first seen in the spin, about one cycle later in the turn-on. Both the coronal wind model (Schandl & Meyer 1994) as well as the stream-disk model (Shakura et al. 1999) predict this kind of behaviour.
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