Long uninterrupted 4–25 keV observations of the X-ray pulsar Her X-1 with a total exposure of about two days around the 35-day cycle beginning were performed by the Mikhail Pavlinsky ART-XC telescope of the Spectrum-Röntgen-Gamma observatory. The main turn-on of the 35-day cycle was found to occur at the orbital phase 0. 25, which is confirmed by simultaneous measurements from SWIFT/BAT and Fermi/GBM monitors. The form of rapid X-ray flux increase after the turn-on is similar in all instruments and can also describe the IXPE measurements of another turn-on and can be attributed to scattering by accretion disk opening the source. The quasi-sine like X-ray light curve before the turn-on suggests the nutation of a tilted precessing accretion disk with a period of 0. 87 days.
Revnivtsev et al. (2025) studied this question.