Luminosity dependence of the cyclotron line energy in 1A 0535+262 observed by Insight-HXMT during 2020 giant outburst

TL;DR

This study analyzes the spectral evolution of the X-ray pulsar 1A 0535+262 during its 2020 giant outburst, revealing a luminosity-dependent transition in cyclotron line energy and hysteresis effects linked to accretion geometry changes.

Contribution

First to observe a transition from positive to negative correlation of cyclotron line energy with luminosity in 1A 0535+262, indicating a change in accretion regime.

Findings

Anti-correlation of cyclotron line energy above critical luminosity

Hysteresis patterns in spectral parameters during outburst phases

Evidence for different emission region geometries in rising and fading phases

Abstract

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 November to December 2020 with a peak luminosity of $1.2$ $\times10^{38}\ \rm erg\ s^{-1}$. 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 $6.7\times10^{37}$ erg s$^{-1}$, the cyclotron absorption line energy anti-correlates with luminosity. Below the critical luminosity, a positive correlation is observed. The 1A~0535+262 becomes, therefore, the second source after V~0332+53, which clearly shows an anti-correlation above and transition between correlation and anti-correlation 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, similarly to V~0332+53, these changes might be related to 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.