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Preprint E/00

The 35-day cycle in Her X-1 as observational appearance of freely precessing neutron star and forcedly precessing accretion disk

N.A.Ketsaris (1), M.Kuster (2), K.A. Postnov (1), M.E. Prokhorov (1), N.I.Shakura (1), R.Staubert (2), J.Wilms (2)

(1) Sternberg Astronomical Institute, Moscow State University, 119899 Moscow, Russia
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

To appear in: Hot Points in Astrophysics, JINR (Dubna), Invited talk presented at workshop of Bogolyubov Theoretical Physics Laboratory, in press

Abstract. A careful analysis of X-ray light curves and pulse profiles of Her X-1 obtained over more than 20 years strongly evidences for free precession of a magnetized neutron star with rotational axis inclined to the orbital plane as a central clock underlying the observed 35-day period. Strong asymmetric X-ray illumination of the optical star atmosphere leads to the formation of gaseous streams coming out of the orbital plane and forming a tilted accretion disk around the neutron star. Such a disk precesses due to tidal forces and dynamical action of gaseous streams from the secondary companion. The locking of these torques with neutron star precession makes the net disk precession period to be very close to that of the neutron star free precession.

Key words: -

Paper (307k gzip'ed Postscript including figures)

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