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

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.

💡 Research Summary

This paper presents a comprehensive timing and spectral study of the Be/X‑ray binary EXO 2030+375 during its 2021 type‑II giant outburst, using data from the Insight‑HXMT satellite. The authors analyzed 80 observations (≈2.46 Ms exposure) covering the 1–150 keV band with the three onboard instruments (LE, ME, HE). Pulsations at the known 42 s spin period are detected throughout the outburst, and the pulse profiles are examined in several energy bands. Two main peaks (at pulse phases ≈0.60 and ≈0.35) dominate the profile; the relative strength of these peaks changes with luminosity. Below a luminosity of ~1 × 10³⁷ erg s⁻¹ the secondary peak is stronger, whereas above this value the primary peak dominates. This transition coincides with a change in the hardness‑intensity diagram (HID), where the 7–10 keV/4–7 keV hardness ratio shows two distinct excursions (hard→soft during the rise and soft→hard during the decay). The authors interpret these as signatures of the source crossing the critical luminosity (L_crit ≈ 1 × 10³⁷ erg s⁻¹), moving between a super‑critical regime (radiation‑pressure‑supported accretion column) and a sub‑critical regime (direct impact onto the neutron‑star surface).

The pulse fraction (PF) generally rises with energy but displays a non‑monotonic behavior near the outburst peak, with a dip around 10 keV. Moreover, PF anti‑correlates with luminosity across the whole outburst, reinforcing the idea that the beaming pattern changes as the accretion mode switches.

Spectral analysis focuses on the peak of the outburst (MJD 59460–59461). The authors combine seven exposures to improve statistics and fit the 2–50 keV spectra with an absorbed cutoff power‑law plus a blackbody component, supplemented by two Gaussian lines (a Fe Kα line at ~6.6 keV and an instrumental feature at ~5.7 keV). The best‑fit parameters are N_H ≈ 2.3 × 10²² cm⁻², photon index Γ ≈ 0.94, cutoff energy E_cut ≈ 18 keV, and blackbody temperature kT ≈ 0.74 keV. The model yields χ²/dof ≈ 0.75, and the derived 0.01–150 keV flux corresponds to a luminosity of ~1.4 × 10³⁷ erg s⁻¹. Flux‑resolved average spectra show only minor evolution with luminosity, indicating a relatively stable continuum shape throughout the outburst.

Phase‑resolved spectroscopy, performed by dividing the pulse into ten phase bins, reveals that the photon index and blackbody temperature vary with pulse phase, and the amplitude of these variations depends on the overall luminosity. At high luminosity the phase dependence is modest, while at lower luminosity the spectral parameters exhibit larger swings, consistent with a changing viewing geometry of the emission region (polar cap versus accretion column).

Although earlier works reported possible cyclotron resonant scattering features (CRSFs) at ~36 keV and ~63 keV, this study does not detect any statistically significant CRSFs in the Insight‑HXMT data, possibly due to limited signal‑to‑noise, line broadening, or unfavorable viewing geometry during the observations.

In summary, the paper provides strong observational evidence that EXO 2030+375 crosses its critical luminosity during the 2021 outburst, as manifested by concurrent changes in pulse profile morphology, pulse fraction, hardness‑intensity behavior, and phase‑dependent spectral parameters. These results support theoretical models of accretion onto highly magnetized neutron stars that predict distinct super‑critical and sub‑critical regimes. The work demonstrates the power of broadband, high‑time‑resolution observations for probing the geometry and physics of accretion columns, and sets the stage for future multi‑wavelength (including polarimetric) studies to further constrain magnetic field strength and emission geometry in Be/X‑ray binaries.