Temporal and spectral variations of the X-ray pulsar Cen X-3 observed by NuSTAR

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: qi.liu@mnf.uni-tuebingen.de
² Department of Astronomy, School of Physics and Technology, Wuhan University, Wuhan 430072, PR China
e-mail: wangwei2017@whu.edu.cn
³ School of Physics and Astronomy, Sun Yat-Sen University, Zhuhai 519082, PR China

Received: 24 December 2023
Accepted: 23 April 2024

Abstract

We report a time-resolved analysis of the accreting X-ray pulsar Cen X-3 using observations carried out by NuSTAR, which cover approximately two binary orbits in different intensity states. The pulse profile is relatively stable over the orbital phase and shows energy dependence. It has an obvious double-peaked shape in the energy band below 15 keV –with the second pulse peak decreasing as energy increases– and is gradually dominated by a single peak in higher energy bands. We find that the pulse profile in the energy band of 3–5 keV at high-intensity states shows a subtle triple-peaked shape, with the main peak divided into two subpeaks. We also find a positive correlation between the pulse fraction and both energy and flux. Our spectral analysis reveals that the spectra can be well described by the continuum of Fermi-Dirac cutoff and NPEX models, and the cyclotron line is detected with the centroid energies varying from 26 keV to 29 keV, along with the iron emission line around 6.4 keV. We investigated the dependence between the cyclotron resonant scattering feature (CRSF) centroid energy and luminosity and discuss the theoretical critical luminosity. Although the variation of E_cyc − L_X is not distinct, there is a possibility that the critical luminosity lies within the range of ∼(0.5 − 4)×10³⁷ erg s⁻¹ in the band of 4–78 keV. The photon index shows a strong positive correlation with luminosity. Our orbital-phase analysis reveals that the spectral parameters show orbital variability, and the highly variable photoelectric absorption may indicate the existence of clumpy stellar winds.