Beam pattern evolution of accreting X-ray pulsar 1A 0535+262 during its 2020 giant outburst

TL;DR

This study analyzes the pulse profile evolution of X-ray pulsar 1A 0535+262 during its 2020 outburst, revealing a transition from pencil to fan beam patterns linked to accretion regimes and a slightly distorted magnetic dipole.

Contribution

It provides the first detailed beam pattern reconstruction of 1A 0535+262 during a giant outburst, highlighting a transition between emission modes and a slightly offset magnetic dipole.

Findings

Evidence for a transition from pencil to fan beam patterns above the cyclotron energy.

Detection of a complex beam pattern with both fan and pencil components at lower energies.

Identification of a slightly distorted dipole magnetic field with a 12° offset.

Abstract

We report on pulse profile decomposition analysis of a bright transient X-ray pulsar 1A 0535+262 using the broadband Insight-HXMT observations during a giant outburst of the source in 2020. We show that the observed pulse profile shape can be described in terms of a combination of two symmetric single-pole contributions for wide range of energies and luminosities for a fixed geometry defining basic geometry of the pulsar. This corresponds to a slightly distorted dipole magnetic field, i.e., one pole has to be offset by $\sim 12^{\circ}$ from the antipodal position of the other pole. We reconstruct the intrinsic beam patterns of the pulsar assuming the geometry recovered from the decomposition analysis, and find evidence for a transition between "pencil" and "fan" beams in energy ranges above the cyclotron line energy which can be interpreted as transition from sub- to super-critical accretion regimes associated with onset of an accretion column. At lower energies the beam pattern appears, however, to be more complex, and contains substantial "fan" beam and an additional "pencil" beam component at all luminosities. The latter is not related to the accretion rate and is stronger in the fading phase of the outburst. We finally discuss results in context of other observational and theoretical findings earlier reported for the source in the literature.