Optical light curve of HZ Her / Her X-1
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. The binary system is seen edge on (our line of sight falls nearly in the orbital plane), leading to eclipses in the X-ray emission when the neutron star is eclipsed by its companion star.
The optical emission of the Her X-1 system shows regular variations with orbital phase due to two effects
- the tidal distortion of the companion star, affecting its visible surface
- intense X-ray heating of the companion by the neutron star
The optical light curve depicted here has been obtained from data taken by INTEGRAL's Optical Monitoring Camera (OMC) during twelve days of observations - covering about seven orbital periods of the binary system - in July and August 2005.
The insets show an artist's impression of the binary system (top right) and a sketch of the situation at different orbital phases (bottom left) - not to scale and omitting the accretion disk. The phases are also indicated along the light curve:
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 the Laboratorio de Astrofísica Espacial y física Fundamental (LAEFF). See the related links on the right-hand side of the page.
Image credit: light curve and sketch by Daniel Risquez (LAEFF/INTA).