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

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

X-ray binaries (XRBs) include a standard star or a white dwarf transferring mass onto a compact neutron star or a black gap. Throughout this accretion course of, they emit power, largely within the type of X-rays. Based mostly on the mass of the companion star, these techniques may be divided into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs).

The most important subgroup of HMXBs are Be/X-ray binaries (Be/XRBs)—composed of Be stars and, normally, neutron stars, together with pulsars. Observations present that the majority of those techniques exhibit weak persistent X-ray emission that’s interrupted by outbursts, which might final a number of weeks.

Found in 1998, XTE J0111.2−7317 (or SXP31.0) is a Be/XRB within the Small Magellanic Cloud (SMC). The system has an orbital interval of 90.5 days and consists of a pulsar and a companion star of spectral kind B0.5–1Ve.

Earlier observations of SXP31.0 have discovered that it skilled outbursts with luminosities reaching 100 undecillion erg/s, subsequently approaching the Eddington for a neutron star. This makes it a promising candidate for finding out accretion at excessive charges. Up to now, solely a handful of super-Eddington pulsars have been detected, and their properties stay poorly understood.

That’s the reason a group of astronomers led by the College of Turku’s Alexander Salganik determined to make use of the Nuclear Spectroscopic Telescope Array (NuSTAR) to conduct broadband observations of SXP31.0. Their research was complemented by knowledge from the Swift spacecraft and the Spektr-RG (SRG) satellite tv for pc.

The observations monitored SXP31.0 throughout its newest massive outburst, which commenced in April 2025. The group managed to carry out the primary complete spectral and timing characterization of this method, enabling an in depth investigation of each pulse-phase-averaged and pulse-phase-resolved properties.

The research discovered that through the outburst, SXP31.0 exceeded the Eddington restrict of 180 undecillion erg/s for a canonical 1.4-solar mass neutron star. This locations it among the many most luminous outbursts ever noticed in BeXRB techniques.

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

It’s typically assumed that QPOs come up from the interplay of matter within the accretion disk with the magnetosphere of a compact object akin to a neutron star or a black gap. Within the case of SXP31.0, Salganik’s group famous that the QPO reveals a transient nature, showing solely beneath particular bodily situations, as it’s absent at each greater and decrease luminosities after detection.

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