We have performed phase-resolved spectral analysis of the accreting pulsar 1A~0535+262 based on observations of Insight-HXMT during the 2020 Type-II outburst of the source. We focus on the two-dimensional dependence of the cyclotron resonance scattering features (CRSFs) along the outburst time and at different phases. The fundamental CRSF line (f-CRSF) shows different time- and phase-dependent behaviors. At higher luminosity, the phase profile of the f-CRSF energy changes from a single peak to double peaks, with the transition occurring at MJD 59185. On the contrary, the first harmonic CRSF (1-st CRSF) at ∼ 100 keV is only detected within a narrow phase range (0.8−1.0) accompanied by a shallow f-CRSF line. Based on these results, we speculate that when the source enters the supercritical regime, the higher accretion column can significantly enhance the harmonic line at a narrow phase through an "anti-pencil" beam at a higher energy band. At the same time, it will also affect the behavior of the fundamental line.
@article{arxiv.2204.11222,
title = {Phase dependent evolution within large luminosity range of 1A 0535+262 observed by Insight-HXMT during 2020 giant outburst},
author = {Ling-Da Kong and Shu Zhang and Long Ji and Victor Doroshenko and Andrea Santangelo and Mauro Orlandini and Filippo Frontera and Jian Li and Yu-Peng Chen and Peng-Ju Wang and Zhi Chang and Jin-Lu Qu and Shuang-Nan Zhang},
journal= {arXiv preprint arXiv:2204.11222},
year = {2022}
}