Light-curve dip production through accretion stream-accretion disc impact in the HZ Her/Her X-1 binary star system

Abstract

We discuss the production of Hercules X-1's (Her X-1) light-curve dips through accretion stream impact on the disc. We model the binary system geometry, the position of the line of sight relative to the impact site and the effect of several binary system parameters on the occurrence phases of dips. Our input accretion disc models have been validated by observations. The accretion stream geometry is calculated from first principles and it is consistent with theoretical studies and observations. Our model explains the marching dip effect and several other features of the 35-d phase–orbital phase graph. The outer accretion disc radius has the greatest effect on the timing of dips, while the mass ratio, L1's coordinates, the orbital inclination and separation, and the inner accretion disc radius have negligible effects. We find that dip production occurs for distances from impact to the line of sight of under 2 per cent of the orbital separation. We consider the possibility of disc penetration by the stream and find that it is required to reproduce correctly the marching dip effect and the complex trends of the dips observed by the Rossi X-ray Timing Explorer/Proportional Counter Array. In particular, the model explains the properties of pre-eclipse, anomalous, post-eclipse, main-high-state, short-high-state and low-state dips, and shows that they are all produced by the same mechanism.