Phase dependent evolution within large luminosity range of 1A 0535+262 observed by Insight-HXMT during 2020 giant outburst

TL;DR

This study analyzes phase-dependent spectral features of the accreting pulsar 1A 0535+262 during a 2020 outburst, revealing how cyclotron lines vary with luminosity and phase, indicating changes in accretion geometry.

Contribution

It provides the first detailed phase-resolved analysis of cyclotron lines during a giant outburst, highlighting the evolution of line profiles with luminosity and phase in 1A 0535+262.

Findings

Fundamental CRSF energy profile shifts from single to double peaks with luminosity.

Harmonic CRSF detected only in a narrow phase range at high energy.

Accretion column structure influences cyclotron line behavior during supercritical regime.

Abstract

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 $\sim$ 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.