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.
