Insight-HXMT observations of a soft X-ray flare and a possible QPO-type transition in MAXI J1750-327 / EP260812c (Format Corrected)

ATel #18026; B. Y. Chen, W. Z. Li, Q. C. Shui, Y. P. Chen, S. Zhang, S. N. Zhang, H. Feng, L. Tao, L. Zhang, S. M. Jia, X. B. Li, F. J. Lu, L. M. Song, S. J. Zheng, C. K. Li, Z. X. Li, G. L. Huang, Q. C. Zhao, on behalf of the Insight-HXMT team
on 1 Sep 2026; 13:41 UT
Credential Certification: Yu-Peng Chen (chenyp@ihep.ac.cn)

Tweet

MAXI J1750-327 / EP260812c / EP J175003.0-324244 was discovered by MAXI/GSC and Einstein Probe (ATels #17963 and #17964) and was subsequently identified as a black-hole X-ray binary candidate based on its X-ray spectral and timing properties (ATels #17969 and #17974). Extensive X-ray and multiwavelength follow-up observations have since been reported (ATels #17971, #17977, #17978, #17979, #17983, #17988, #17990, #17993, #18004, #18007, #18010, and #18017). Insight-HXMT began monitoring the source on 2026 August 15 at 04:18:57 UTC and has continued high-cadence observations throughout the outburst.

On 2026 August 23, Insight-HXMT detected a rapid soft X-ray flare beginning around exposure P080461700901, consistent with the contemporaneous rise in the MAXI/GSC 2-4 keV light curve. We used LE 2-10 keV, ME 10-30 keV, and HE 30-120 keV for the light-curve and timing analyses. Between exposures 0901 and 0904, the exposure-averaged LE count rate increased from 871.88 +/- 0.97 to 1397.24 +/- 1.22 count s^-1, while the HE count rate decreased from 260.59 +/- 0.32 to 203.69 +/- 0.28 count s^-1. Preliminary joint fitting of the 2-120 keV spectrum with constant*TBabs*(diskbb+cutoffpl), allowing the hydrogen column density to vary freely, showed that the best-fit column density changed from 1.170 (-0.114, +0.038) to 1.774 (-0.010, +0.128) x 10^22 cm^-2. The inner-disc temperature increased from 1.011 (-0.034, +0.030) to 1.066 (-0.015, +0.005) keV, while the photon index increased from 2.340 (-0.029, +0.018) to 2.636 (-0.005, +0.027), confirming substantial spectral softening.

The ME 10-30 keV dynamic power spectrum shows a rapid shift of the QPO ridge from approximately 4.5 Hz to the 8-9 Hz range across exposures 0901-0904. The ME 10-30 keV dynamic power spectrum is available in the linked figure. The higher-frequency feature is intermittent rather than continuously present, appearing only during limited intervals while the low-frequency broad-band noise is markedly reduced. At the onset of the flare, Insight-HXMT detected a strong, narrow QPO with properties consistent with a type-C QPO. In exposure 0901, its ME 10-30 keV centroid frequency was 4.510 +/- 0.009 Hz, with Q = 9.08 +/- 0.54 and a fractional rms amplitude of 15.57 +/- 0.36%; it was accompanied by strong broad-band noise. During the soft X-ray flare, ME detected a broader QPO in exposure P080461700903 at 8.260 +/- 0.073 Hz, with Q = 4.60 +/- 0.49 and a fractional rms amplitude of 12.54 +/- 0.61%; the accompanying broad-band noise was substantially weaker. The evolution from a strong, high-rms type-C-like QPO to a broader, lower-rms feature at approximately 8 Hz suggests a possible transition toward a type-B QPO during the flare. The QPO parameters are reported with 1-sigma uncertainties, and a more detailed time-resolved analysis is ongoing.

Insight-HXMT is continuously monitoring MAXI J1750-327 / EP260812c. Given the source's rapid spectral and temporal evolution, we invite teams possessing multiwavelength data on this source to contact us at bychen@ihep.ac.cn to facilitate a comprehensive analysis.

Insight-HXMT, China's first X-ray astronomy satellite, was launched on 2017 June 15. It carries the Low Energy X-ray Telescope (LE; 1-15 keV), Medium Energy X-ray Telescope (ME; 5-30 keV), and High Energy X-ray Telescope (HE; 20-250 keV), providing broad-band timing and spectral capabilities (Zhang et al. 2020, Sci. China Phys. Mech. Astron., 63, 249502).

Insight-HXMT/ME 10-30 keV dynamic power spectrum of MAXI J1750-327 / EP260812c