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Quasi-periodic oscillations detected in X-ray binary SXP31.0

Astronomers from the University of Turku in Finland and elsewhere have performed a broadband spectral and timing study of an X-ray binary designated XTE J0111.2−7317, which resulted in the detection of quasi-periodic oscillations in this system. The finding was reported in a research paper June 24 on the arXiv pre-print server.

X-ray binaries (XRBs) consist of a normal star or a white dwarf transferring mass onto a compact neutron star or a black hole. During this accretion process, they emit energy, mostly in the form of X-rays. Based on the mass of the companion star, these systems can be divided into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs).

The largest subgroup of HMXBs are Be/X-ray binaries (Be/XRBs)—composed of Be stars and, usually, neutron stars, including pulsars. Observations show that most of these systems exhibit weak persistent X-ray emission that is interrupted by outbursts, which can last several weeks.

Discovered in 1998, XTE J0111.2−7317 (or SXP31.0) is a Be/XRB in the Small Magellanic Cloud (SMC). The system 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 of SXP31.0 have found that it experienced outbursts with luminosities reaching 100 undecillion erg/s, therefore approaching the Eddington for a neutron star. This makes it a promising candidate for studying accretion at high rates. To date, only a handful of super-Eddington pulsars have been detected, and their properties remain poorly understood.

That is why a team of astronomers led by the University of Turku's Alexander Salganik decided to employ the Nuclear Spectroscopic Telescope Array (NuSTAR) to conduct broadband observations of SXP31.0. Their study was complemented by data from the Swift spacecraft and the Spektr-RG (SRG) satellite.

The observations monitored SXP31.0 during its latest large outburst, which commenced in April 2025. The team managed to perform the first comprehensive spectral and timing characterization of this system, enabling a detailed investigation of both pulse-phase-averaged and pulse-phase-resolved properties.

The study found that during the outburst, SXP31.0 exceeded the Eddington limit of 180 undecillion erg/s for a canonical 1.4-solar mass neutron star. This places it among the most luminous outbursts ever observed in BeXRB systems.

Furthermore, the observations detected 0.8-mHz quasi-periodic oscillations (QPOs) in SXP31.0 at a bolometric luminosity of about 250 undecillion erg/s. This finding makes SXP31.0 the fourth known super-Eddington X-ray pulsar to exhibit millihertz low-frequency quasi-periodic variability.

It is generally assumed that QPOs arise from the interaction of matter in the accretion disk with the magnetosphere of a compact object such as a neutron star or a black hole. In the case of SXP31.0, Salganik's team noted that the QPO exhibits a transient nature, appearing only under specific physical conditions, as it is absent at both higher and lower luminosities after detection.