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NuSTAR observation of the new black hole transient MAXI J1750-327/EP260812c

ATel #18004; S. Mandel, K. Mori, C. Hailey (Columbia U.), M. Parra (Ehime U.), A. Marino (ICE-CSIC, USACH), N. Degenaar, R. Wijnands (U. Amsterdam), J. Tomsick (UCB/SSL), M. Bachetti (INAF), P. Draghis (MIT), J. Grindlay, J. Hong (Harvard CfA), G. Ponti (INAF, MPE), M. Reynolds (OSU)
on 25 Aug 2026; 01:43 UT
Credential Certification: Shifra Mandel (ss5018@columbia.edu)

Subjects: X-ray, Binary, Black Hole, Transient

We report on a follow-up NuSTAR observation of the new X-ray transient MAXI J1750-327, which was detected by MAXI on Aug. 13, 2026 (ATel #17963), as well as by Einstein Probe WXT on Aug. 12-13 (ATel #17964). Low-frequency quasi-periodic oscillations (QPOs) detected by Einstein Probe FXT on Aug. 14 (ATel #17969) and by Insight HXMT on Aug. 15 (ATel #17983) are indicative of a black hole low-mass X-ray binary (BH-LMXB). Additional observations were conducted with the Las Cumbres Observatory on Aug. 13-14 (ATel #17971), with MeerKAT on Aug. 15 (ATel #17979), and with PRIME on Aug. 14 (ATel #17988), identifying the optical, radio and NIR counterparts, respectively.

We triggered a NuSTAR ToO observation, which was performed on Aug. 18, 2026 with an exposure of 22.6 ks. MAXI J1750-327 is clearly visible as a bright source in the NuSTAR images. We extracted FPMA and FPMB spectra from a circular region of radius 60″. The FPMA and FPMB combined 3—79 keV net count rate is ~1050 cts/s.

An absorbed cut-off power-law model yielded a good fit (reduced χ² ~1.2 for 219 d.o.f.). The best-fit model parameters are n_H = (1.4 +/-0.5)*10²² cm^-2, photon index Γ = 1.79 +/-0.02 and cut-off energy E_cut = (31.8 +/-1.0) keV (all errors are quoted at the 90% C.L.) Residuals at energies ≳50 keV indicate the presence of additional hard non-thermal emission. The 2-10 keV unabsorbed flux is estimated at ~3.3*10^-8 erg/s/cm². These spectral parameters are similar to those reported by SVOM (ATel #17993), and are representative of a BH-LMXB in the hard state. A QPO is apparent at ~1.4 Hz, marking an increase in frequency from the ~0.45 and 0.7 Hz QPOs measured by Einstein Probe FXT (ATel #17969) and Insight HXMT (ATel #17983), respectively, several days earlier.

We thank the NuSTAR team for their quick response in scheduling this observation.