Insight-HXMT detects transient spectral softening and QPO suppression during an X-ray flare in GS 1354-64

ATel #17673; Bo-yan Chen (IHEP), L. D. Kong (Nankai University), J. Q. Peng (IHEP), Q. C. Shui (IHEP), S. Zhang (IHEP), R. C. Ma (University of Southampton), G. B. Zhang (YNAO), H. H. Liu (University of Tuebingen), Y. P. Chen (IHEP), L. Ji (Sun Yat-sen University), P. J. Wang (Qinghai University), H. X. Liu (Qinghai University), W. Z. Li (IHEP), S. N. Zhang (IHEP) on behalf of the Insight-HXMT team
on 12 Feb 2026; 02:57 UT
Credential Certification: Yu-Peng Chen (chenyp@ihep.ac.cn)

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The MAXI instrument detected a new outburst of the black hole X-ray binary GS 1354-64 in December 2025 (ATel #17563). Since the trigger, the source has been followed up across multiple wavebands, including EP/WXT, Swift/XRT, NuSTAR, and XRISM in the X-ray band, MeerKAT in the radio band, and near-infrared/optical observations (e.g., ATel #17582, #17583, #17586, #17612, #17625, #17638, #17650). Insight-HXMT monitoring reveals clear spectral evolution during the rising phase. These results are based on publicly available Target of Opportunity (ToO) observations. The broadband spectra are fitted with an absorbed disk blackbody plus power-law model, with the hydrogen column density fixed at N_H = 1.4e22 cm^-2 (ATel #17583). On January 18, 2026, the 2-100 keV spectrum can be described with kT = 0.20 (-0.01, +0.01) keV and Gamma = 1.85 (-0.01, +0.01), corresponding to an unabsorbed 1-100 keV flux of F ~ 1.46e-8 erg cm^-2 s^-1. A QPO at 0.27 Hz is detected simultaneously in the LE, ME, and HE detectors and remains visible up to 50-70 keV. We note, however, that in this earliest observation the simple disk blackbody plus power-law model does not adequately describe the high-energy residuals. Preliminary tests indicate that replacing the power-law component with a thermal Comptonization model (e.g., thcomp convolved with diskbb) provides an improved fit to the hard X-ray emission. A more detailed spectral analysis will be presented elsewhere. By January 27, the spectrum softens further to kT = 0.50 (-0.02, +0.02) keV and Gamma = 2.43 (-0.02, +0.01), with a flux of F ~ 1.54e-8 erg cm^-2 s^-1. The QPO centroid frequency increases to 3.5 Hz and is detected in all three detectors up to 30-50 keV. During the pronounced X-ray flare between January 28 and January 31, the previously detected QPO is no longer observed in the power density spectra of the LE, ME, or HE detectors. In the observation at the flare peak on January 29, the spectrum becomes significantly softer, with kT = 1.15 (-0.01, +0.02) keV and Gamma = 2.72 (-0.02, +0.03), and the 1-100 keV flux rising to F ~ 3.53e-8 erg cm^-2 s^-1. In post-flare observations on January 31, the spectrum hardens again to kT = 0.50 (-0.03, +0.01) keV and Gamma = 2.42 (-0.02, +0.03), with the flux decreasing to F ~ 1.50e-8 erg cm^-2 s^-1. The QPO reappears with a centroid frequency of about 3.6 Hz and is again detected in the LE, ME, and HE detectors up to 30-50 keV. These Insight-HXMT observations demonstrate that the transient spectral softening during the flare is accompanied by a temporary suppression of the QPO, followed by a recovery of both spectral and timing properties after the flare. Insight-HXMT is China's first X-ray astronomy satellite, launched on 2017 June 15. It carries three instruments: the Low Energy telescope (LE, 1-15 keV), the Medium Energy telescope (ME, 5-30 keV), and the High Energy telescope (HE, 20-250 keV), providing broadband timing and spectral capabilities (Zhang et al. 2020, Sci China Phys Mech Astron, 63, 249502). Insight-HXMT continues to monitor GS 1354-64 during the ongoing outburst. Continued multi-wavelength observations are encouraged.