A1 Refereed original research article in a scientific journal
Discovery of a 0.8-mHz quasi-periodic oscillation in the transient X-ray pulsar SXP31.0 and associated timing transitions
Authors: Salganik, Alexander; Tsygankov, Sergey S.; Molkov, Sergey V.; Lapshov, Igor Yu.; Lutovinov, Alexander A.; Tkachenko, Alexey Yu.; Mushtukov, Alexander A.; Poutanen, Juri
Publisher: EDP Sciences
Publication year: 2026
Journal: Astronomy and Astrophysics
Article number: A141
Volume: 705
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202556076
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.1051/0004-6361/202556076
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/508342212
We present the first broadband spectral and timing study of the Be/X-ray pulsar XTE J0111.2−7317 (SXP31.0) during the first major outburst since its discovery in 1998. This giant type II outburst, observed between April and September 2025, marks the source’s return to activity after nearly three decades of quiescence. Using NuSTAR observations together with data from Swift/XRT and SRG/ART-XC, we followed the outburst’s evolution, with the source reaching a bolometric luminosity of Lbol = 3.6 × 1038 erg s−1. The broadband spectra are well described by an absorbed cutoff power law, two blackbody components (hot and soft), and a narrow Fe Kα line. No cyclotron absorption features were detected in either the phase-averaged or phase-resolved spectra in the 5–50 keV band. Most notably, we report the discovery of a previously undetected quasiperiodic oscillation (QPO) at 0.8 ± 0.1 mHz, characterized by a fractional root-mean-square (rms) amplitude of 14% at a super-Eddington bolometric luminosity of Lbol = 2.5 × 1038 erg s−1. In contrast, the previously reported 1.27 Hz QPO was not detected. While the 0.8 mHz QPO is present, the pulsed fraction (PF) is low in soft X-rays, which is consistent with other super-Eddington pulsars exhibiting mHz QPOs; however, it rises above 20 keV to reach 35%. The QPO vanishes in subsequent observations coinciding with a sharp increase in the PF and a distinct change in pulse profile morphology. It was not observed in any follow-up observations at luminosities above or below its initial detection, suggesting it is a transient phenomenon.
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Funding information in the publication:
AS acknowledges support from the EDUFI Fellowship and Jenny and Antti Wihuri Foundation (grant no. 00240331). SST acknowledges financial support from the Centre for Astrophysics and Relativity at Dublin City University during his visit. AAM acknowledges UKRI Stephen Hawking fellowship. SVM, IYL, AAL and AYT acknowledge support from the Ministry of Science and Higher Education of RF grant 075-15-2024-647. We thank the anonymous referee for their careful reading of the paper and thoughtful suggestions, which helped improve the clarity of the results. AS also thanks Vadim Kravtsov for helpful discussions during the preparation of this work. This work is partially based on observations with the Mikhail Pavlinsky ART-XC telescope, hard X-ray instrument on board the SRG observatory. The SRG observatory was created by Roskosmos in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI) in the framework of the Russian Federal Space Program, with the participation of Germany. The ART-XC team thanks the Roscosmos State Corporation, the Russian Academy of Sciences, and Rosatom State Corporation for supporting the ART-XC telescope, as well as the JSC Lavochkin Association and partners for manufacturing and running the Navigator spacecraft and platform. This work made also use of data supplied by the UK Swift Science Data Centre at the University of Leicester and data obtained with NuSTAR mission, a project led by Caltech, funded by NASA, and managed by JPL. This research also has made use of the NuSTAR Data Analysis Software (NUSTARDAS) jointly developed by the ASI Science Data Centre (ASDC, Italy) and Caltech. We are grateful to the Swift and NuSTAR teams for approving and rapid scheduling of the monitoring campaign. This work made use of Astropy: (http://www.astropy.org) a community-developed core Python package and an ecosystem of tools and resources for astronomy (Astropy Collaboration 2013, 2018, 2022). This research has made use of data and software provided by the High Energy Astrophysics Science Archive Research Centre (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory.
