Timing analysis of EXO 2030+375 during its 2021 giant outburst observed with Insight-HXMT

TL;DR

This study analyzes the 2021 giant outburst of EXO 2030+375 using Insight-HXMT data, revealing pulse profile evolution, accretion mode transition, and magnetic field estimates supporting multipole magnetic fields.

Contribution

It provides detailed timing analysis during the outburst, linking pulse profile changes to accretion modes and estimating magnetic fields with implications for neutron star magnetic structure.

Findings

Pulse profile evolves significantly with luminosity.

Transition between super-critical and sub-critical accretion modes observed.

Magnetic field estimates support the presence of multipole magnetic fields.

Abstract

We report the evolution of the X-ray pulsations of EXO 2030+375 during its 2021 outburst using the observations from \textit{Insight}-HXMT. Based on the accretion torque model, we study the correlation between the spin frequency derivatives and the luminosity. Pulsations can be detected in the energy band of 1--160 keV. The pulse profile evolves significantly with luminosity during the outburst, leading to that the whole outburst can be divided into several parts with different characteristics. The evolution of the pulse profile reveals the transition between the super-critical (fan-beam dominated) and the sub-critical accretion (pencil-beam dominated) mode. From the accretion torque model and the critical luminosity model, based on a distance of 7.1 kpc, the inferred magnetic fields are $(0.41-0.74) \times 10^{12}$ G and $(3.48-3.96) \times 10^{12}$ G, respectively, or based on a distance of 3.6 kpc, the estimated magnetic fields are $(2.4-4.3) \times 10^{13}$ G and $(0.98-1.11)\times 10^{12}$ G, respectively. Two different sets of magnetic fields both support the presence of multipole magnetic fields of the NS.