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Published online by Cambridge University Press: 12 April 2016
The Her X-1/HZ Her system consists of a neutron star with a rotation period of 1.24s orbiting a ‘normal’ stellar companion every 1.7d. Variation in optical and ultraviolet magnitude is attributed to X-ray heating on the side facing the active neutron star with some contribution from X-ray heating of the disk. The system also has a 35-day period in which the X-ray emission displays so-called main and short ‘on’ states separated by relative X-ray quiescence. The 1.7-day optical flux variations continue throughout the 35-day period. No clear consensus exists as to the origin of the 35-day period; a popular interpretation is that of occultation of the compact X-ray source by a tilted, precessing, accretion disk.