SIMBAD references

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 total exposure of about two days around the main turn-on of the X-ray source. We present the results of the timing and spectral analysis of these observations. The opening of the X-ray source is determined to occur at the orbital phase ϕ_b ≈ 0.25. The analysis of the X-ray light curve reveals the first direct observational evidence of the nutation of a tilted precessing accretion disk with a period of ≃0.87 days. The appearance of X-ray pulsations near the orbital phase ϕ_b ≃ 0.77 prior to the main turn-on at the maximum of the nutation variability has also been detected. During the X-ray eclipse, a non-zero X-ray flux is measured, which is presumably associated with the scattering of 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 radiation from the central source through a scattering corona above the precessing accretion disk.