Modeling of 35-day superorbital cycle of $B$ and $V$ light curves of IMXB HZ Her/Her X-1

TL;DR

This paper models the 35-day superorbital cycle of HZ Her/Her X-1, explaining optical variability through neutron star free precession and disk dynamics, offering new insights into the system's precession mechanisms.

Contribution

It introduces a model linking neutron star free precession with optical variability, incorporating disk tilt and luminosity parameters, advancing understanding of superorbital cycles.

Findings

Optical variability can be explained by neutron star free precession.

Disk tilt and luminosity significantly influence the superorbital cycle.

Potential synchronization mechanisms between neutron star precession and gas streams are discussed.

Abstract

The X-ray binary Her X-1 consists of an accreting neutron star and the optical component HZ Her. The 35-day X-ray superorbital variability of this system is known since its discovery in 1972 by the Uhuru satellite and is believed to be caused by forced precession of a warped accretion disk tilted to the orbital plane. We argue that the observed features of the 35-day optical variability of HZ Her can be explained by free precession of the neutron star with a period close to that of the forced disk. The model parameters include a) the X-ray luminosity of the neutron star; b) the optical flux from the accretion disk; c) the tilt of the inner and outer edges of the accretion disk. A possible synchronization mechanism based on the coupling between the neutron star free precession and the dynamical action of non-stationary gas streams is discussed.