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

Yu-Cong Fu, L. M. Song, G. Q. Ding, M. Y. Ge, Y. L. Tuo, S. Zhang, S. N. Zhang, X. Hou, J. L. Qu, J. Zhang, L. Zhang, Q. C. Bu, Y. Huang, X. Ma, Z. X. Yang, X. F. Lu, T. M. Li, Y. C. Xu, P. J. Wang, S. H. Xiao, H. X. Liu, X. Q. Ren, Y. F. Du, Q. X. zhao, Y. X. Xiao

Published 2023-02-04Version 1

We report the evolution of the X-ray pulsations of EXO 2030+375 during its 2021 outburst using the observations from 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 and this divides the whole outburst 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, we calculate the magnetic fields of $(0.41-0.74) \times 10^{12}$ G and $(3.48-3.96) \times 10^{12}$ G based on a distance of 7.1 kpc, or the magnetic fields of $(2.4-4.3) \times 10^{13}$ G and $(0.98-1.11)\times 10^{12}$ G based on a distance of 3.6 kpc. Two different sets of magnetic fields both support the presence of multipole magnetic fields of the neutron star.