Monitoring of X-Ray Sources in the Optical Spectral Region
(a) The optical light curve of HZ Her/Her X-1, composed of 807 observations obtained with the D 40 cm and D 17 cm astrographs of the Sonneberg Observatory. The intervals of inactive states are marked by the horizontal bars. Adapted from . (b) The orbital modulation of HZ Her. Open circles denote the extended active state between February 1959 and October 1993 ; the smooth line represents the moving averages of these data. Closed circles mark the orbital modulation in the low state observed by .