NuSTAR observations of wind-fed X-ray pulsar GX 301–2 during unusual spin-up event

Abstract

We report on NuSTAR observations of the well-known wind-accreting X-ray pulsar GX 301–2 during a strong spin-up episode that took place in January–March 2019. A measurement of high luminosity of the source in the most recent observation allowed us to detect a positive correlation of the cyclotron line energy with luminosity. Beyond that, only minor differences in spectral and temporal properties of the source during the spin-up, presumably associated with the formation of a transient accretion disk, and the normal wind-fed state could be detected. Finally, we discuss conditions for the formation of the disk and possible reasons for lack of any appreciable variations in most of the observed source properties induced by the change of the accretion mechanism, and conclude that the bulk of the observed X-ray emission is still likely powered by direct accretion from the wind.