XSPECT observation of the newly discovered black hole transient MAXI J1750-327 / EP260812c: Detection of variable low frequency QPOs

ATel #18010; Rwitika Chatterjee (URSC, ISRO), Vivek Kumar Agrawal (URSC, ISRO), Biswajit Paul (Raman Research Institute), Anuj Nandi (URSC, ISRO)
on 25 Aug 2026; 18:30 UT
Credential Certification: Rwitika Chatterjee (rwitika@ursc.gov.in)

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MAXI J1750-327 was discovered as a new X-ray transient by the MAXI/GSC nova alert system on 2026 August 13 (ATel #17963), with the source also detected by the EP/WXT approximately five hours earlier and designated EP260812c (ATel #17964). Subsequent X-ray observations revealed an evolving low-frequency Quasi-periodic Oscillation (QPO), with EP/FXT detecting a feature at ~0.45 Hz on August 14 (ATel #17969), followed by an increase to ~0.7 Hz in Insight-HXMT observations on August 15 (ATel #17983), and further to ~1.4 Hz in NuSTAR observations on August 18 (ATel #18004). These features, tentatively identified as type-C QPOs, suggested that the source is a candidate black hole X-ray binary. Optical, near-infrared and radio counterparts were also identified by follow-up observations (ATels #17971, #17977, #17979, #17988, #17990, #18007).

XPoSat, consisting of the co-aligned XSPECT and POLIX instruments, initiated Target of Opportunity observations on 2026 August 20 to monitor the source. We performed preliminary timing and spectral analysis of the XSPECT data. In the first four days of XSPECT observations, the source count rate remained approximately constant at ~170 counts/s during August 20-21, followed by a sharp increase to ~330 counts/s by August 23.

Power spectral analysis revealed significant (> 3σ) and variable low frequency QPOs during the observation period. On August 20, a broad feature was detected at 2.11 +/- 0.05 Hz. Subsequent days showed strong QPO features with centroid frequencies ranging from ~1.87 Hz to ~2.84 Hz, and fractional rms amplitudes ranging from 5.2 to 8.0%. Interestingly, a subset of the observations on August 22-23 show double-peaked narrow features, with the frequency of the second peak at ~4.2 Hz.

We fitted the spectra with an absorbed diskbb + cutoff power-law model. The hydrogen column density was found to be ~0.9-1.0 × 10²² cm^-2. The inner disk temperature showed a gradual increase from ~0.5 keV to ~1.0 keV. The power-law index varied between 1.3-2. The cutoff energy was found to be in the range ~15-35 keV (with large uncertainties) during the first three days, followed by a dramatic decrease to ~ 6 +/- 1 keV on the fourth day. The unabsorbed 0.8-15 keV flux increased from ~6.8 × 10^-8 to ~1.1 × 10^-7 erg s^-1 cm^-2 during the observations.

These preliminary results demonstrate significant spectral and timing evolution of MAXI J1750-327 during its ongoing outburst, and XPoSat continues to monitor the source.

XSPECT (X-ray SPECtroscopy and Timing) instrument on-board XPoSat is a soft X-ray spectrometer operating in the energy band 0.8-15 keV. XSPECT employs Swept Charge Devices and has a net geometric area of 60 cm². XSPECT carries out long-term, pile-up free observations of bright X-ray sources. The XPoSat project is managed and facilitated by the Indian Space Research Organisation.