Insight-HXMT observed the Galactic transient EP260812c/EP J175003.0-324244/MAXI J1750-327 and detected a low-frequency quasi-periodic oscillation signal

ATel #17983; Z. X. Li, G. L. Huang, Q. C. Zhao, L. Tao, S. N. Zhang, H. Feng, S. M. Jia, S. Zhang, X. B. Li, Y. P. Chen, Q. C. Shui, F. J. Lu, L. M. Song, S. J. Zheng, C. K. Li, B. Y. Chen, L. Zhang, W. Z. Li (IHEP) on behalf of the Insight-HXMT team
on 18 Aug 2026; 02:27 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yu-Peng Chen (chenyp@ihep.ac.cn)

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The Wide-field X-ray Telescope (WXT) onboard the Einstein Probe (EP) and The MAXI detected a new X-ray transient EP260812c/MAXI J1750-327 in 2026-08-12T20:10:43 and August 13, 2026 (ATel #17964, ATel #17963). Insight-HXMT performed a follow-up observation of this transient at 04:18:57 UT on 2026 August 15. A significant quasi-periodic oscillation (QPO) at ~0.45 Hz was previously reported from EP Follow-up X-ray Telescope (FXT) data (ATel #17969). We detect a QPO in all three instruments onboard Insight-HXMT: the Low Energy X-ray telescope (LE; 1-10 keV), the Medium Energy X-ray telescope (ME; 10-30 keV), and the High Energy X-ray telescope (HE; 27-200 keV). The centroid frequencies and fractional rms amplitudes are 0.744 (+0.015/-0.013) Hz and 11.2 (+1.0/-1.2)% for LE, 0.725 (+/-0.006) Hz and 14.8 (+/-0.6)% for ME, and 0.725 (+/-0.006) Hz and 11.8 (+0.4/-0.5)% for HE, respectively. Notably, a clear QPO signal is also detected with HE at energies above 100 keV. These QPO properties are consistent with a type-C low-frequency QPO, suggesting that EP260812c/EP J175003.0-324244/MAXI J1750-327 is a candidate black hole X-ray binary. Compared with the QPO detected with the FXT on 2026 August 14, the QPO observed by Insight-HXMT shows clear evolution in both its centroid frequency and fractional rms amplitude.

In our preliminary broadband X-ray spectral analysis, we used the LE data in the 2-10 keV range, the ME data in the 10-30 keV range, and the HE data in the 40-90 keV range. We initially fitted the spectrum with the model constant*tbabs*(diskbb+cutoffpl). The fit yielded a hydrogen column density of 1.5 (+/-0.2) e22 cm-2. The diskbb component had an inner-disk temperature of 0.26 (+/-0.02) keV and a normalization of 1.1 (+1.1/-0.5)e+6. The cutoffpl component had a photon index of 1.70 (+/-0.01), a cutoff energy of 40.9 (+0.9/-0.8) keV, and a normalization of 7.1 (+/-0.2). The absorbed 2-90 keV flux derived from the spectral fit was 2.974 (+/-0.008) e-8 erg/cm2/s.

Insight-HXMT continues to monitor EP260812c/EP J175003.0-324244/MAXI J1750-327 during the ongoing outburst. Further follow-up observations are encouraged to identify the nature of this source.

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).

EP-FXT observation of the Galactic transient EP260812c/EP J175003.0-324244/MAXI J1750-327 and detection of a low-frequency quasi-periodic oscillation signal