Dramatic changes in the timing behavior of SXP31.0 as seen by NuSTAR and SRG/ART-XC during the ongoing type II outburst

ATel #17216; Alexander Salganik (Turku U.), Sergey S. Tsygankov (Turku U.), Sergey Molkov (IKI RAS), Alexander Lutovinov (IKI RAS), Alexander Mushtukov (Oxford U.), Juri Poutanen (Turku U.)
on 2 Jun 2025; 17:04 UT
Credential Certification: Alexander Salganik (alsalganik@gmail.com)

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The Be/X-ray pulsar SXP31.0 (Chakrabarty et al. 1998) is currently exhibiting a Type II outburst in April--June 2025 (ATel 17147, ATel 17163), reaching a bolometric luminosity of ~4E+38 erg/s, the brightest one since its discovery in 1998. The outburst evolution closely resembles that of the 1998 event. Two NuSTAR and two SRG/ART-XC observations were triggered by our team during the outburst and revealed significant timing changes. At the time of the first NuSTAR and ART-XC observations (MJD 60796-60798) the source flux was around (1.2-1.4)E-10 erg/s/cm2 in the 4-12 keV band, the pulsed fraction (PF) was ~10% below 20 keV, rising to ~30% at higher energies. Significant low-frequency QPO was detected by both instruments at frequencies near 0.8 +/- 0.1 mHz (see also ATel 17204). The energy spectra were described by a power law with an exponential cutoff (Gamma = 0.55+/- 0.04, Efold = 8.6+/-0.1 keV) plus a blackbody component with the temperature of kT=1.31+/-0.03 keV. An iron line at 6.4 keV was also detected in all observations. In the second ART-XC observation on MJD 60813, we found that QPO was dissipated. At the same time, the PF increased to ~25% in the 4-25 keV energy range, and the flux rose to 1.9E-10 erg/s/cm2 in 4-12 keV. The spectrum was described by the same model with Gamma = 1.1 +/- 0.3, Efold=7+/-1 keV, kT=1.44+/-0.05 keV. During the second NuSTAR observation on MJD 60826, the 4-12 keV flux was 1.5E-10 erg/s/cm2, comparable to the first NuSTAR observation. No QPO was detected in this observation, and the PF remained consistent with the second ART-XC observation when luminosity was 1.2 times higher. The spectrum was described by Gamma = 0.28 +/- 0.04, Efold=9.5+/-0.2 keV, kT=0.96+/-0.04 keV. Also during our observations the spin period decreased from 30.451 to 30.394 s over 30 days (MJD 60796--60826). Thanks to joint fits with Swift/XRT, we also consistently detect a soft excess (Yokogawa et al. 2000) with the temperature kT approximately 0.2 keV and blackbody radius of R~100-200 km. We continue to monitor the source using the XRT, NICER and ART-XC telescopes. We also thank the NuSTAR and SRG/ART-XC teams for accepting the TOO observations and for the quick execution of the observing program. We are also grateful to the Swift and NICER teams for rapid scheduling of the monitoring program.