Optical features of the accretion disk in the HZ Her/Her X-1 system during the anomalous low state in 1999

Abstract

We present the results of UBV photoelectric and CCD observations of Her X-1/HZ Her near its primary minimum in 1999, when the object was in an anomalous low X-ray state, together with UBV observations accumulated since 1988. We compare the orbital light curves in the normal and anomalous low state, which differ considerably. The differences can be interpreted as being due to changes in the temperature and size of the hot spot on the stellar surface illuminated by the X-ray pulsar. The observations show that the accretion-disk luminosity was a factor of six to ten lower in 1999 than in the normal state. We used all available photoelectric data to compute the power spectrum in the B band. The only significant peaks are those corresponding to the orbital period and to beating of the orbital period with the periods characterizing gaps in the observations (one year and one day) and with the 35-day period. We conclude that it is impossible to determine the direction of the disk precession from the power spectrum alone.