THE 35-D MODULATION OF THE X-RAY-EMISSION OF HER X-1 IN THE FRAMEWORKOF THE SOD MODEL - RESULTS OF A 3-DIMENSIONAL SPH SIMULATION

Several models have been proposed to explain the 35-d X-ray periodicit y 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 ligh t of Roberts' slaved orienting disc model (SOD) with the intention of finding some limits to the inclination of the rotation axis of the sec ondary and to the value of the polytropic index gamma. These results s how that a gamma value between 1.05 and 1.1 and an inclination angle p hi of the order of 45-degrees are the most suitable for enabling the S OD model to work in three-dimensional space. The simulated disc is rat her small and thin, and its precessional motion is reproduced in a sat isfactory way. Nevertheless, our simulations seem to be unable to expl ain the 35-d period shadowing effect of Her X-1, because of the too-un iform temperature distribution in the disc. This does not preclude the validity of the SOD model, as the results concerning the temperature distribution are obtained in the quasi-polytropic approximation, altho ugh temperature profiles in good agreement with the analytical models have already been obtained in our previous simulations. Our results ma y, however, suggest alternatively that a non-precessing disc whose opt ical depth is subject to a long-period variation may account for the o bserved 35-d period X-ray modulation. In any case, implementation of t he radiative terms should be taken into account in future more stringe nt tests of the model.