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

TL;DR

This study analyzes the timing and spectral properties of the Be/X-ray binary EXO 2030+375 during a 2021 outburst, revealing state transitions, pulse profile variability, and spectral evolution related to accretion regimes.

Contribution

It provides detailed timing and spectral analysis of EXO 2030+375 during an outburst, highlighting state transitions and accretion regime changes with new observational insights.

Findings

Detection of pulsations in 1-150 keV energy band.

Observation of pulse profile shape changes during outburst phases.

Identification of state transitions linked to critical luminosity.

Abstract

We report the X-ray spectral and timing analysis of the high mass X-ray binary EXO 2030+375 during the 2021 type-II outburst based on the Insight-HXMT observations. Pulsations can be detected in the energy band of 1-150 keV. The pulse profile shows 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 exhibits an anti-correlation with 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) suggest state transitions 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 reaching the critical luminosity and transitioning between super-critical and sub-critical accretion regimes. We performed the 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 different luminosities.