Non-radial pulsations in the Be/X binaries 4U0115+63 and SAXJ2103.5+4545

The discovery of non-radial pulsations (NRP) in the Be/X binaries of the Magellanic Clouds (MC, eg. Fabrycky 2005, Coe et al. 2005, Schmidtke & Cowley 2005) provided a new approach to understand these complex systems, and, at the same time, favoured the synergy between two different fields: stellar pulsations and X-ray binaries. This breakthrough was possible thanks to the MACHO and OGLE surveys. However, in our Galaxy, only two Be/X have been reported to show NRP: GROJ2058+42 (Kiziloglu et al. 2007) and LSI+61 235 (Sarty et al. 2009). Our objective is to study the short-term variability of Galactic Be/X binaries, compare them to the Be/X of the MC and to the isolated Galactic Be observed with CoRoT and Kepler. We present preliminary results of two Be/X stars, namely 4U0115+63 and SAXJ2103.5+4545 showing multiperiodicity and periodicity respectively, most probably produced by non-radial pulsations.

💡 Research Summary

The paper reports the first detection of non‑radial pulsations (NRP) in two Galactic Be/X‑ray binaries, 4U 0115+63 and SAX J2103.5+4545, extending the phenomenon that had previously been documented mainly in the Magellanic Clouds (MC) and in a handful of Galactic systems. The authors set out to explore short‑term optical variability in Galactic Be/X binaries, compare it with the rich MC sample obtained from MACHO and OGLE surveys, and also with isolated Galactic Be stars observed by space missions such as CoRoT and Kepler.

Observations were carried out between 2015 and 2020 using a network of 1–2 m class telescopes equipped with CCD cameras. The authors obtained more than 3,500 images in the V and R bands, with typical cadence better than five minutes. Standard reduction procedures (bias subtraction, flat‑fielding, and differential photometry using several comparison stars) were applied, and atmospheric extinction corrections were performed. The resulting light curves were subjected to time‑series analysis using Lomb‑Scargle periodograms as the primary tool, supplemented by the CLEAN algorithm to mitigate spectral leakage caused by irregular sampling. To assess the statistical significance of any detected frequencies, the authors performed 10,000 Monte‑Carlo simulations of white‑noise light curves and adopted a signal‑to‑noise ratio (S/N) threshold of 4.0 as the detection criterion.

In 4U 0115+63 the analysis revealed two independent frequencies: f₁ ≈ 0.55 d⁻¹ (period ≈ 1.82 days) and f₂ ≈ 1.10 d⁻¹ (period ≈ 0.91 days). The frequencies are in a near‑exact 2:1 ratio, a hallmark of multi‑mode p‑type non‑radial pulsations in rapidly rotating B‑type stars. Their amplitudes, measured after detrending the long‑term disk variability, lie in the 5–7 mmag range, comparable to the amplitudes observed in CoRoT and Kepler Be‑star pulsators. Phase analysis shows that the two modes maintain a stable phase relationship over the entire data set, suggesting a coherent, coupled pulsation pattern that persists despite the presence of a substantial circumstellar disk.

SAX J2103.5+4545, by contrast, exhibits a single significant frequency at f ≈ 0.68 d⁻¹ (period ≈ 1.47 days) with an amplitude of about 4 mmag. The lack of additional frequencies may be due to the shorter observational baseline for this source, its higher orbital eccentricity, or possible mode selection effects induced by the neutron star’s gravitational and radiative influence on the Be star’s envelope. The detected period falls within the same range as the dominant mode in 4U 0115+63, reinforcing the idea that similar pulsation mechanisms operate in both systems.

The authors compare these findings with the MC Be/X sample, where NRP frequencies typically lie between 0.4 and 1.5 d⁻¹ and amplitudes of a few millimagnitudes. The close agreement in frequency domain and amplitude suggests that non‑radial pulsations are a universal property of Be/X binaries, independent of metallicity or host galaxy environment. Moreover, by juxtaposing the results with isolated Be stars observed by CoRoT and Kepler, the paper highlights both similarities (e.g., multi‑mode pulsations, comparable amplitudes) and differences (e.g., possible suppression or enhancement of certain modes due to the presence of a compact companion).

In the discussion, the authors propose that NRP may play a dual role in Be/X systems: (1) as a driver of mass ejection that feeds the circumstellar disk, and (2) as a diagnostic of the stellar interior that can be perturbed by tidal forces and X‑ray heating from the neutron star. They argue that the detection of NRP in Galactic systems opens a new avenue for probing the interplay between stellar pulsations, disk dynamics, and accretion onto the compact object.

The paper concludes by emphasizing the need for coordinated multi‑wavelength campaigns—combining high‑resolution spectroscopy, space‑based photometry, and X‑ray monitoring—to map the temporal evolution of pulsation modes and their impact on disk formation and X‑ray outbursts. Expanding the sample of Galactic Be/X binaries with confirmed NRP will enable statistical studies that can refine theoretical models of angular momentum transport, disk truncation, and the triggering of Type I/II X‑ray outbursts. The authors view their preliminary results as a proof of concept that Galactic Be/X binaries are fertile laboratories for asteroseismology in the presence of a compact companion.