Radio flux density monitoring of MAXI J1750-327 with the Yamaguchi Interferometer

ATel #18016; Masanori Akimoto, Shogo Nohara, Hibiki Fujii, Natsuki Maekawa, Kotaro Ninuma, Kenta Fujisawa (Yamaguchi University)
on 28 Aug 2026; 00:13 UT
Credential Certification: Kenta Fujisawa (kenta@yamaguchi-u.ac.jp)

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MAXI J1750-327 was discovered as a new X-ray transient by MAXI on 2026-08-13, and was also detected by Einstein Probe as EP 260812c (ATel #17963, #17964). The X-ray spectral and timing analysis suggests that MAXI J1750-327 is a galactic black hole candidate X-ray binary (ATel #17969). Subsequent MAXI observations suggested that the source was undergoing a hard-to-soft transition around August 14.1 UT (ATel #17974), although a clear state transition had not yet been established. MeerKAT detected a radio counterpart at the optical position with a flux density of approximately 14 mJy at 1.28 GHz (ATel #17979). Subsequent radio observations with ALMA and VLA measured flux densities of 12.5 mJy at 233 GHz and 84.6 mJy at 10 GHz, respectively. We carried out the radio flux density monitoring of MAXI J1750-327 with the Yamaguchi Interferometer between August 19 and 24. The observations were conducted simultaneously at central frequencies of 6856 and 8448 MHz, with a bandwidth of 512 MHz for each frequency band. We used 3C48 and J1744-312 as the flux and gain calibrators, respectively. Observations of MAXI J1750-327 and J1744-312 were interleaved, with integration times of 600 and 120 s, respectively. We investigated the short-variation of the flux density on hour timescales as well as daily timescales. We summarize our measurements of MAXI J1750-327 for each observing date, including the minimum, maximum, mean, and standard deviation of the flux densities as well as the mean 1-sigma thermal-noise error. 6856 MHz Date Start (UT) End (UT) Minimum (mJy) Maximum (mJy) Mean (mJy) Stdev (mJy) Mean 1-sigma thermal noise error (mJy) 8/19 11:20:00 12:29:00 23.3 26.4 25.0 1.57 0.778 8/20 9:35:00 10:39:00 19.7 20.7 20.2 0.460 0.874 8/21 9:31:00 12:35:00 19.5 27.3 23.3 3.11 0.865 8/22 9:27:00 12:31:00 20.2 27.5 23.6 3.11 0.842 8/23 9:23:00 12:33:00 6.95 62.8 17.4 20.3 0.872 8/24 9:19:00 12:23:00 66.3 73.0 69.2 2.54 0.849 8448 MHz Date Start (UT) End (UT) Minimum (mJy) Maximum (mJy) Mean (mJy) Stdev (mJy) Mean 1-sigma thermal noise error (mJy) 8/19 11:20:00 12:29:00 17.8 19.9 19.1 1.16 1.73 8/20 9:35:00 10:39:00 20.1 24.8 20.2 1.94 1.96 8/21 9:31:00 12:35:00 18.5 25.6 22.0 2.31 2.00 8/22 9:27:00 12:31:00 18.8 25.1 21.2 2.24 1.89 8/23 9:23:00 12:33:00 <9.35 70.3 18.5 22.5 1.94 8/24 9:19:00 12:23:00 52.0 61.8 57.6 3.13 1.93 We observed mean flux densities of ~20 mJy at both 6856 and 8448 MHz between August 19 and 22. The flux densities then decreased from ~20 mJy to ~10 mJy in both bands between August 22 and 23. On August 23, during the first hour of the observation, we measured a flux density of ~7 mJy at 6856 MHz and an upper limit of 10 mJy at 8448 MHz. The flux densities then increased approximately linearly over the following two hours, reaching 62.8 mJy at 6856 MHz and 70.3 mJy at 8448 MHz. On August 24, we measured flux densities of 69.2 mJy at 6856 MHz and 57.6 mJy at 8448 MHz. We will continue the radio flux density monitoring of MAXI J1750-327 with the Yamaguchi Interferometer.