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

TL;DR

This study reports the discovery of a rare dual-superorbital modulation in the X-ray binary GX 301-2, with one period likely representing the true superorbital cycle and the other being a beat frequency, suggesting wind-related phenomena.

Contribution

First observational evidence of dual-superorbital modulations in an X-ray binary, interpreted through the lens of corotating interaction regions in stellar winds.

Findings

115-day superorbital period confirmed with high significance.

65-day period identified as beat modulation with orbital period.

Dual modulation likely linked to stellar wind structures.

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$\sigma$ global significance level, whereas the 65-day signal only marginally reaches the 4$\sigma$ 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.