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

Abstract: We present the first broad-band spectral and timing study of the Be/X-ray pulsar XTE J0111.2$-$7317 (SXP31.0) during its first major outburst since its discovery in 1998. This giant Type II outburst, observed in April-June 2025, marks the source's return to activity after nearly three decades of quiescence. Utilizing our NuSTAR observations together with Swift/XRT and SRG/ART-XC ones, we trace the evolution of the outburst to a peak 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$\alpha$ line. No cyclotron absorption features are 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 quasi-periodic oscillation (QPO) at $0.8 \pm 0.1$ mHz, characterized by a fractional root-mean-square amplitude of 18% 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 is not detected. While the sub-mHz QPO is present, the pulsed fraction (PF) is low in soft X-rays, consistent with other super-Eddington pulsars exhibiting mHz QPOs, but rises above 20 keV reaching 35%. The QPO vanishes in subsequent observations coinciding with a sharp increase in the PF and a distinct change in pulse profile morphology. It is not observed in any follow-up observations at luminosities above or below its initial detection, suggesting it is a transient phenomenon.