2022MNRAS.513....1C

The X-ray binaries (XRBs) Her X-1 and A 0535+26 have been extensively studied over the years during outbursts and in deep quiescence. Cyclotron resonance scattering features (CRSFs) have been observed in absorption correlating with X-ray luminosities, though recently, an anticorrelation was seen in the 2020 giant outburst of A 0535+26 at higher luminosities. These types of behaviour have been observed in several other XRBs, yet there are sources showing extended periods of no correlation at all. Such aperiodic changes and periods of no change in CRSFs can be explained by changes in the surface magnetic fields of the X-ray pulsars. In particular, we show here that CRSF variations occur in Her X-1 and A 0535+26 due to corresponding changes in the strong nondipolar components of their surface magnetic fields, while the magnetic dipoles reaching out to the accretion discs remain largely unchanged.