Traces of wobbling accretion disk in X-ray pulsar Her X-1 from observations of the ART-XC telescope of the SRG observatory

Abstract

Long uninterrupted observations of the X-ray binary system Her X-1 were performed with the Mikhail Pavlinsky ART-XC telescope of the Spectrum-Röntgen-Gamma (SRG) X-ray Observatory in the 4--25 keV energy range with a total exposure of about two days around the main turn-on of the X-ray source. We present the results of timing and spectral analysis of these observations. The opening of the X-ray source is determined to occur at the orbital phase $\ddot{I}{•}_b\approx 0.25$. The analysis of the X-ray light curve reveals a first direct observational evidence of the nutation of a tilted precessing accretion disk with a period of $\simeq 0.87$ days. The appearance of X-ray pulsations near the orbital phase $\ddot{I}{•}_b\simeq 0.77$ prior to the main turn-on at the maximum of the nutation variability has been also detected. During the X-ray eclipse, a non-zero X-ray flux is measured, which is presumably associated with scattering of an X-ray emission in a hot corona around the optical star illuminated by the X-rays from the central neutron star. An increase in the X-ray flux after the main turn-on can be described by the passage of the radiation from the central source through a scattering corona above the precessing accretion disk.