HZ Her / Her X-1 is a well known X-ray binary system, consisting of a neutron star which is an accreting X-ray pulsar and a companion star of 2.3 solar masses circling each other every 1.7 days on an almost circular orbit.

The companion star is distorted by the gravitation of the neutron star and filling its Roche lobe, thus fueling the X-ray emission. The orbital plane of the binary is almost in the same plane as the observer leading to eclipses in the X-ray emission when the neutron star is eclipsed by its companion.

The optical emission of the Her X-1 system shows regular variations with orbital phase due to two effects: (1) the tidal distortion of the companion star, affecting its visible surface, and (2) intense X-ray heating of the companion by the neutron star.

The orbital lightcurve above has been obtained from data taken by Integral's Optical Monitoring Camera (OMC) during 12 days of observations - about 7 orbital periods - in July-August/2005.

The insets show an artist's impression of the system and a sketch of the situation at different orbital phases - not to scale and omitting the accretion disk:

• (A) Phase zero corresponds to maximal radial velocity.
• (B) At phase 0.25 we have the maximum of the optical emission, observing the hot face of the X-ray heated companion.
• (D) At phase 0.75 we have minimal optical emission as we observe the cold face of the companion star and the X-ray eclipse.

Public data for Her X-1 and many other variable systems can be accessed via the OMC Archive at LAEFF.

Read more in the ESA Science News entitled "Astronomers may have glimpsed tiny star's surface".