Characteristics of the photometric modulation of HZ Her/ Her X-1 in active states over a very long time interval

Abstract

The properties of the photometric orbital modulation of the X-ray binary HZ Her/ Her X-1 and its variations over a long time interval (decades) are studied by the statistical methods using photographic plates of Sonneberg Observatory. The moving averages of the light curve in the long-lasting active state (1959–1993) show that both the smoothed orbital light curve and the smoothed scatter of its residuals σ mag display striking asymmetries with respect to the orbital phase φ orb=0.5; they are apparent both in the whole data set and in the segments of the 35 day cycle. The light curve near the orbital phase 0.5 can be characterized by the maximum either flat-topped and symmetric or sharp and lagging behind φ orb=0.5. The course of σ mag is largely asymmetric with respect to the primary minimum, usually with a smooth decrease of σ mag within φ orb=0.2–0.6 and a steep rise within φ orb=0.6–0.85. All these facts suggest the presence of an additional source of variations, in addition to the geometric effects of the precessing disk. The role of the interaction of the mass stream with the precessing warped disk is suggested as a possible explanation. Our data for the extended active state within the years 1959–1993 revealed that the mean brightness at all orbital phases remains stable and displays at most marginal secular trends. The brightness during the short active states (1934–1937 and 1941–1949) is shown to be lower than in the active state within 1959–1993 and is attributed to a lower degree of heating of the secondary star.