Detection of a puzzling dual-superorbital hard X-ray modulation in the X-ray binary GX 301-2

TL;DR

This study addresses the origin of wind-fed X-ray binary superorbital modulations by analyzing a long-term Swift/BAT hard X-ray light curve of GX 301-2 with Lomb-Scargle, WWZ, and Gaussian-process methods. It uncovers a rare dual superorbital signal at ~115 days (highly significant) and ~65 days (weaker), and argues that the 115-day period is the actual SM while the 65-day term is its beat with the orbital period. The authors favor a corotating interaction region (CIR) beat-frequency interpretation, which can naturally reproduce the observed linear relation between orbital and superorbital periods across wind-fed XRBs, while disk precession and triple-star scenarios face challenges. The work highlights CIRs as a unifying framework for wind-fed SMs and calls for extended observations and theoretical development to clarify the CIR–orbital period connection.

Abstract

The superorbital modulations (SMs) observed in wind-fed X-ray binaries remain a puzzling phenomenon in astrophysics. To investigate this behavior observationally, we analyzed the long-term hard X-ray light curve from the Swift/BAT 157-Month Hard X-ray Survey in X-ray binary GX 301-2. Using three timing analysis methods--the Lomb-Scargle periodogram, the weighted wavelet Ztransform, and Gaussian processes--we identify a rare dual-SM behavior in this source: the 115-day modulation exceeds the 5$σ$ global significance level, whereas the 65-day signal only marginally reaches the 4$σ$ level. Because the 115-day period is more consistent with the previously reported linear relation between orbital and superorbital periods, we interpret 115 days as the actual superorbital period, while the weaker and less stable 65-day period is its beat modulation with the orbital period.By assessing the applicability of different physical scenarios to our results, we suggest that this dual-SM behavior is most plausibly associated with corotating interaction regions (CIRs) in the stellar wind. This framework can also account for the observed linear orbital-superorbital relation, despite the unclear physical mechanism that sets the apparent ratio between the CIR and orbital periods across sources. Further long-term monitoring of this system, together with continued theoretical development of the CIR scenario, will be essential for clarifying the origin of wind-fed SMs.

Figures (9)

• Figure 1: LSP of the long-term Swift/BAT 14–195 keV light curve of GX 301-2. The periodograms are shown in terms of the $R$ statistic as a function of trial period. The left panel shows the full period range, while the right panel zooms into the yellow-shaded interval highlighted in the left panel. The dashed horizontal lines mark the global significance thresholds at 3$\sigma$ (green), 4$\sigma$ (blue), and 5$\sigma$ (red). Two candidate superorbital modulations are detected at $\sim$65 days and $\sim$115 days, with the $\sim$115 days peak exceeding the 5$\sigma$ level.
• Figure 2: WWZ analysis of the long-term Swift/BAT 14–195 keV light curve of GX 301-2. The left panel shows the WWZ power as a function of normalized time and trial period, highlighting the temporal evolution of periodic components. The right panel displays the time-averaged WWZ power spectrum as a function of period. The dashed horizontal lines mark the 115-day (red) and 65-day (blue) candidate superorbital periods; the $\sim$115-day component remains prominent across most of the baseline, whereas the $\sim$65-day signal is comparatively weaker.
• Figure 3: PSD of the best-fit GP model (SHO + SHO) for the long-term Swift/BAT 14–195 keV light curve of GX 301-2. The black curve shows the model PSD, and the shaded region indicates the uncertainty derived from the posterior distribution. Two clear periodic components are present, with peaks at $\sim$65 days and $\sim$115 days, marked by the blue and red dashed lines, respectively.
• Figure 4: Energy-resolved LSPs of GX 301-2 in selected Swift/BAT bands. The dual-SM signatures at $\sim$115 days and $\sim$65 days progressively weaken with increasing energy and become undetectable above 75 keV.
• Figure 5: Phase-folded Swift/BAT 14–195 keV light curves of GX 301-2 folded at $P=65$ days (left) and $P=115$ days (right). The black points with error bars show the folded data, while the red curves indicate the assumed sinusoidal trends. The red shaded regions highlight additional phase-localized excesses relative to a pure sinusoid, demonstrating departures from a simple sinusoidal modulation in both folded profiles.