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.

Hercules X-1 (Her X-1): New results from the past decade of observations

“…The likely-correct model for dips was presented and calculated by [19], which is the impact point of the accretion stream and the disk, which periodically moves into the line-of-sight from observer to the neutron star. Further spectral observations with better instruments have improved the measurement of the spectrum and included partial covering absorption (e.g.…”

A Broadband X-Ray View of the Precessing Accretion Disk and Pre-eclipse Dip in the Pulsar Her X-1 with NuSTAR and XMM-Newton

“…The unabsorbed emission spectrum is well described by a power law, suggesting that this persistent emission results from X-ray scattering in the obscuring matterial (Vrtilek & Halpern 1985;Choi et al 1994;Leahy et al 1994;Reynolds & Parmar 1995). Modeling of long term X-ray light curves suggest that these dips are caused by obscuration of material at the impact region of the accretion stream and accretion disk (e.g., Igna & Leahy 2012).…”

A broad-band X-ray view of the precessing accretion disk and pre-eclipse dip in the pulsar Her X-1 with NuSTAR and XMM-Newton

“…The unabsorbed emission spectrum is well described by a power law, suggesting that this persistent emission results from X-ray scattering in the obscuring matterial (Vrtilek & Halpern 1985;Choi et al 1994;Leahy et al 1994;Reynolds & Parmar 1995). Modeling of long term X-ray light curves suggest that these dips are caused by obscuration of material at the impact region of the accretion stream and accretion disk (e.g., Igna & Leahy 2012).…”