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The 35-d modulation of the X-ray emission of Her X-1 in the framework of the SOD model: results of a three-dimensional SPH simulation

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Several models have been proposed to explain the 35-d X-ray periodicity observed in Her X-1. We present the results of six three-dimensional quasi-polytropic smoothed particle hydrodynamics (SPH) simulations of the tilted, twisted accretion disc of Her X-1 carried out in the light of Roberts' slaved orienting disc model (SOD) with the intention of finding some limits to the inclination of the rotation axis of the secondary and to the value of the polytropic index γ. These results show that a γ value between 1.05 and 1.1 and an inclination angle φ of the order of 45° are the most suitable for enabling the SOD model to work in three-dimensional space. The simulated disc is rather small and thin, and its precessional motion is reproduced in a satisfactory way