2009A&A...494.1025S

We present evidence for the existence of two ∼35day clocks in the Her X-1/HZ Her binary system. ∼35day modulations are observed 1) in the Turn-On cycles with two on- and two off-states and 2) in the changing shape of the pulse profiles which re-appears regularly. The two ways of counting the 35day cycles are generally in synchronization. This synchronization did apparently break down temporarily during the long Anomalous Low (AL3), which Her X-1 experienced in 1999/2000, in the sense that there must have been one extra Turn-On cycle. Our working hypothesis is that there are two clocks in the system, both with a period of about ∼35days: precession of the accretion disk (the less stable ``Turn-On clock'') and free precession of the neutron star (the more stable ``Pulse profile clock''). We suggest that free precession of the neutron star is the master clock and that the precession of the accretion disk is basically synchronized to that of the neutron star through a feedback mechanism in the binary system. However, the Turn-On clock can slip against its master when the accretion disk has a very low inclination, as is observed to be the case during AL3. We take the apparent correlation between the histories of the Turn-Ons, of the Anomalous Lows and of the pulse period evolution, with a 5yr quasi-periodicity, as evidence for strong physical interaction and feedback between the major components in the system. We speculate that the 5yr (10yr) period is due either to a corresponding activity cycle of HZ Her or a natural ringing period of the physical system of coupled components. The question of whether free precession really exists in neutron stars is very important for understanding matter with supra-nuclear density.