Pulse-phase resolved variability of the X-ray pulsar RX J0440.9+4431 with <i>SRG</i> /ART-XC

ABSTRACT We present a new method of accurate measurements for the pulse-phase-resolved power spectrum. We apply this method to the observation of X-ray pulsar RX J0440.9+4431 obtained during its supercritical accretion episode by the Mikhail Pavlinsky ART-XC X-ray telescope onboard the SRG observatory. We demonstrate that the power spectrum is composed of low- and high-frequency components which are not only well separated in frequency, but also show a very different dependence on the pulse phase. The behaviour of the low-frequency stochastic variability can be explained by the effect of changes in the rate of feeding the accretion columns near the pulsar surface from the disc far away. The high-frequency stochastic variability component should be associated with a different source of instability, plausibly operating at the base of accretion columns.