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Discovery of a 0.8-mHz quasi-periodic oscillation in the transient X-ray pulsar SXP31.0 and associated timing transitions

Alexander Salganik, Sergey S. Tsygankov, Sergey V. Molkov, Igor Yu. Lapshov, Alexander A. Lutovinov, Alexey Yu. Tkachenko, Alexander A. Mushtukov, Juri Poutanen

Abstract

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 $L_{\rm bol} = 3.6 \times 10^{38}$ 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 K$α$ 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 $0.8 \pm 0.1$ mHz, characterized by a fractional root-mean-square (rms) amplitude of 14% at a super-Eddington bolometric luminosity of $L_{\rm bol} = 2.5 \times 10^{38}$ 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.

Discovery of a 0.8-mHz quasi-periodic oscillation in the transient X-ray pulsar SXP31.0 and associated timing transitions

Abstract

We present the first broadband spectral and timing study of the Be/X-ray pulsar XTE J0111.27317 (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 erg s. The broadband spectra are well described by an absorbed cutoff power law, two blackbody components (hot and soft), and a narrow Fe K 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 mHz, characterized by a fractional root-mean-square (rms) amplitude of 14% at a super-Eddington bolometric luminosity of erg s. 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.

Paper Structure

This paper contains 13 sections, 9 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. NuSTAR observatory
  4. SRG/ART-XC
  5. Swift observatory
  6. Spectral data approximation
  7. Results
  8. Timing analysis during QPO-active interval
  9. Detection of the 0.8 mHz QPO
  10. Pulse profiles
  11. Timing analysis during QPO-free interval
  12. Spectral analysis
  13. Discussion and conclusions

Figures (9)

  • Figure 1: Evolution of SXP31.0 during the 2025 outburst based on Swift/XRT, NuSTAR, and SRG/ART-XC observations. The light curve in the 4--12 keV energy range is shown at the top panel. The gray points show the CGRO/BATSE 20--50 keV pulsed luminosity from the 1998 outburst, overplotted for comparison. Along the time axis it was placed arbitrarily, and the luminosity was scaled by a factor of 0.45. The spin period (see Table \ref{['table:periods']}) is shown at the bottom panel.
  • Figure 2: Smoothed dynamic power spectrum (top) and light curve (bottom) of NuObs1 in the 3--79 keV energy band. The dynamic power spectrum is computed using a sliding window of 8192 s with a step of 512 s. The hatched region indicates the part of the time range where the sliding window cannot be applied without extending beyond the data boundaries. The light curve is binned with a 10 s time resolution.
  • Figure 3: Rms-normalized PDS of SXP31.0 during the QPO-active interval for NuObs1 (3--79 keV) computed from the first 32.8 ks of the observation where the QPO is most prominent. The best-fit model is shown in red, with the green dashed line for the QPO component and the blue dashed line for the shoulder component. Panel (b) shows residuals to the model with only the QPO and spin frequency, while panel (c) shows residuals after adding the shoulder component.
  • Figure 4: Same as Fig. \ref{['fig:powerspec-nuobs1']} but for ArtObs1 (4--25 keV) observation. Here the orange dashed line shows the red-noise component. Panel (b) shows the residuals for the model with the red noise only, while panel (c) shows residuals after adding the QPO component.
  • Figure 5: Energy-resolved pulse profiles of SXP31.0. Gray dashed vertical lines mark the boundaries of the pulse peaks. The phase corresponding to the maximum of the profile is set to zero. Two full periods are shown per profile, each vertically offset for clarity. The flux in each energy band is normalized to its mean value. Energy ranges (in keV) are labeled next to each profile.
  • ...and 4 more figures