Timing and spectral studies of the Be/X-ray binary EXO 2030+375 using Insight-HXMT observations

We report the results of a X-ray spectral and timing analysis of the high mass X-ray binary EXO 2030+375 during the 2021 type-II outburst based on Insight-HXMT observations. We confirm that pulsations can be detected in the energy band of 1−150 keV. The pulse profile shows both energy and luminosity dependence and variability. We observed transitions in the pulse profile shape during the rising and the decaying phase of the outburst. The pulse fraction (PF) exhibits an anti-correlation with the luminosity and a non-monotonic energy dependence, with a possible dip near 30 keV during the outburst peak. The hardness-intensity diagrams (7−10 keV/4−7 keV) suggests transitions in state during the early and late phases of the outburst. These transitions are consistent with the luminosity at which the pulse profile shape changes occur, revealing the source reached a critical luminosity and transitioned between super-critical and sub-critical accretion regimes. We performed an average and phase-resolved spectral analysis, where the flux-resolved average spectra show a stable spectral evolution with luminosity. The phase-resolved spectral analysis reveals that the dependence of spectral parameters on the pulse phase varies with changing luminosities.