On the origin of X-ray dips in Her X-1

TL;DR

This paper proposes that matter flows caused by asymmetric X-ray illumination and disk wobbling in Her X-1 lead to specific X-ray dips, explaining observed pre-eclipse, anomalous, and post-eclipse features.

Contribution

It introduces a new model linking asymmetric illumination, matter flows, and disk wobbling to the formation of various X-ray dips in Her X-1.

Findings

X-ray dips are caused by matter flows crossing the line of sight.

Disk wobbling enhances the shielding of the X-ray source.

Different types of dips occur at specific orbital phases.

Abstract

A strong X-ray illumination of the optical star atmosphere in Her X-1, asymmetric because of a partial shadowing by the tilted twisted accretion disk around central neutron star, leads to the formation of matter flows coming out of the orbital plane and crossing the line of sight before entering the disk. We suggest that the absorption of X-ray emission by this flow leads to the formation of pre-eclipse and anomalous dips of type I. These dips are observed during several orbits after turn-on both in the main-on and short-on state. Almost coherent action of tidal torques and matter streams enhances the disk wobbling which causes the disk edge to shield the X-ray source after the turn-on. Anomalous dips of type II and post-eclipse recovery appear due to this process only on the first orbit after turn-on.