PRIME early detection and pre-outburst upper limit for the NIR counterpart to MAXI J1750-327

ATel #17988; A. Masegian (Columbia), R. Hamada (UOsaka), K. De (Columbia), D. Suzuki (UOsaka), T. Sumi (UOsaka), K. Nunota (UOsaka), T. Tamaoki (UOsaka), S. Makida (UOsaka), R. Oishi (UOsaka), R. Ogawa (UOsaka), S. Nakayama (UOsaka), H. Ose (UOsaka), K. Daimon (UOsaka)
on 19 Aug 2026; 10:11 UT
Credential Certification: Kishalay De (kd3038@columbia.edu)

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We report the NIR detection of the counterpart to the recently discovered X-ray transient MAXI J1750-327 / EP J175003.0-324244 with the PRime-focus Infrared Microlensing Experiment (PRIME). The X-ray transient was first reported on August 13 by MAXI (ATel #17963) and the Einstein Probe (ATel #17964). Subsequent postings have identified the source as a likely black hole X-ray binary (ATels #17969, #17974) and reported detections of optical (ATels #17971, #17977) and radio (ATel #17979) counterparts.

PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located at the South African Astronomical Observatory in Sutherland, South Africa (Sumi et al. 2025). As part of the PRIME Galactic bulge survey, the field containing MAXI J1750-327 has been observed repeatedly in the YJH bands since 2024. MAXI J1750-327 falls at the very edge of the PRIME FOV in the bulge survey grid, so its coverage varies between epochs depending on the random dither sequence.

Visual inspection revealed a bright source within the X-ray localization region in the J-band image taken on 2026-08-14 at 19:09 UTC (MJD 61266.7982). We performed forced photometry at the position of the optical counterpart (ATels #17971, #17977), yielding a measurement of J = 14.26 +/- 0.022 Vega mag. The source is not visible in earlier PRIME observations: in the most recent pre-outburst J-band observation obtained on 2026-07-22 at 20:02 UTC, forced photometry yields a 5-sigma upper limit of J > 17.30 Vega mag.

Comparison of the NIR photometry to contemporaneous optical photometry indicates a red color of g - J ~ 4 mag, suggesting likely foreground extinction. PRIME observations of this field are continuing.