Luminosity Dependence of the Cyclotron Line Energy in 1A 0535+262 Observed by Insight-HXMT during the 2020 Giant Outburst

We report on a detailed spectral analysis of the transient X-ray pulsar 1A 0535+262, which underwent the brightest giant outburst ever recorded for this source from 2020 November to December with a peak luminosity of 1.2 × 1038 erg s−1. Thanks to the unprecedented energy coverage and high-cadence observations provided by Insight-HXMT, we were able to find for the first time evidence for a transition of the accretion regime. At high luminosity, above the critical luminosity 6.7 × 1037 erg s−1, the cyclotron absorption line energy anticorrelates with luminosity. Below the critical luminosity, a positive correlation is observed. Therefore, 1A 0535+262 becomes the second source after V0332+53, which clearly shows an anticorrelation above and transition between correlation and anticorrelation around the critical luminosity. The evolution of both the observed CRSF line energy and broadband X-ray continuum spectrum throughout the outburst exhibits significant differences during the rising and fading phases; that is, for a similar luminosity, the spectral parameters take different values, which results in hysteresis patterns for several spectral parameters including the cyclotron line energy. We argue that, similar to V0332+53, these changes might be related to the different geometry of the emission region in rising and declining parts of the outburst, probably due to changes in the accretion disk structure and its interaction with the magnetosphere of the neutron star.