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Preprint 17/06

Long-term developments in Her X-1: Correlation between the histories of the 35 day turn-on cycle and the 1.24 sec pulse period

R. Staubert (1), S. Schandl (1), D. Klochkov (1), J Wilms (2), K. Postnov (3), N. Shakura (3)

(1) Institut für Astronomie und Astrophysik - Department of Physics,University of Tübingen, Germany
(2) Department of Physics, Univ. of Warwick, UK
(3) Sternberg Astronomical Institute, Lomonossov University, Moscow, Russia

AIP Conf. Proc. 840 65-70 (2006)

Abstract. We have studied the long-term (1971-2005) 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 and 2000). 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 spin-down 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.
Key words: Neutron stars, accretion disks, binary X-ray pulsars, spin-up/-down, free precession

Preprint (101 kb PDF file including figures)

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