Astronomers Discover Quasi-Periodic Oscillations in X-ray Binary SXP31.0

Astronomers from the University of Turku, Finland, and their collaborators have detected quasi-periodic oscillations (QPOs) in the X-ray binary system designated SXP31.0 (XTE J0111.2−7317) during a significant outburst event. This groundbreaking discovery, reported in a paper published on June 24, 2025, on the arXiv pre-print server, sheds light on the dynamics of this unique astrophysical system, which is located in the Small Magellanic Cloud.

X-ray binaries, which consist of a compact object such as a neutron star or black hole accreting material from a companion star, have been a focal point of astrophysical research due to their extreme environments and energetic emissions. Based on their mass, X-ray binaries are categorized into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs). Among HMXBs, Be/X-ray binaries, which include Be stars and typically neutron stars, are of particular interest due to their high luminosities and dynamic behavior.

SXP31.0, discovered in 1998, has an orbital period of 90.5 days and consists of a pulsar and a companion star of spectral type B0.5–1Ve. Previous observations indicated that SXP31.0 experiences outbursts with luminosities reaching astonishing levels, approaching the Eddington limit for neutron stars—specifically, 100 undecillion erg/s. This makes SXP31.0 a prime candidate for investigating the phenomena associated with accretion at super-Eddington rates.

The recent study, led by astronomer Alexander Salganik from the University of Turku, utilized the Nuclear Spectroscopic Telescope Array (NuSTAR) in conjunction with data from NASA's Swift spacecraft and the Spektr-RG satellite. The observations, which monitored SXP31.0 during a major outburst starting in April 2025, allowed the research team to conduct a comprehensive spectral and timing analysis. The findings revealed that the system exceeded the Eddington limit of 180 undecillion erg/s for a typical 1.4-solar mass neutron star, marking it as one of the most luminous outbursts recorded in Be/XRB systems.

Moreover, the detection of 0.8-mHz quasi-periodic oscillations during this outburst is particularly significant. These oscillations were observed at an impressive bolometric luminosity of approximately 250 undecillion erg/s, positioning SXP31.0 as the fourth known super-Eddington X-ray pulsar exhibiting millihertz low-frequency QPOs. According to Salganik, the QPOs appear to be transient phenomena, emerging only under specific conditions, as they were absent at both higher and lower luminosities.

Theoretical models suggest that quasi-periodic oscillations are likely the result of interactions between material within the accretion disk and the magnetosphere of the compact object. This study not only enhances the understanding of the physical processes occurring in X-ray binaries but also opens avenues for further research into the behaviors of super-Eddington pulsars.

This discovery has significant implications for the field of astrophysics, particularly in the study of accretion physics and the characteristics of neutron stars. As researchers continue to explore the complexities of systems like SXP31.0, they may uncover new insights into the fundamental processes governing stellar evolution and the dynamics of extreme environments in the universe.

In summary, the detection of quasi-periodic oscillations in SXP31.0 presents a significant advancement in the study of X-ray binaries, underscoring the intricate interactions between compact objects and their surroundings. With ongoing observations and analyses, the astrophysics community looks forward to further elucidating the mysteries held within such extraordinary celestial systems.