Identification of a candidate quiescent optical counterpart to MAXI J1750-327 / EP260812c

ATel #18069; E. S. Borowski, R. I. Hynes, M. T. Penny, M. L. Newman (Louisiana State University), D. M. Russell (NYU Abu Dhabi), P. Gandhi (University of Southampton), A. L. Crisp (Ohio State University)
on 22 Sep 2026; 00:58 UT
Credential Certification: Robert Hynes (rih@phys.lsu.edu)

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We report the identification of a candidate quiescent optical counterpart to the new X-ray transient MAXI J1750-327 (Atel #17963) / EP260812c (ATel #17964). We detect the quiescent counterpart blended with an interloper at the location of the optical counterpart identified by D. M. Russell et al. (ATel #17971).

We performed point spread function (PSF) photometry on good-seeing images from the DECam instrument at the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) which were obtained in 2024 and 2025 as part of the DECam Faint Source µlensing Survey (DEFSuS) following the method of A. L. Crisp et al. (2025). We obtain the following magnitudes from the DEFSuS images, calibrated using the DECaPS2 catalogue (A. K. Saydjari et al. 2023):

r = 21.69 +/- 0.09,
z = 20.25 +/- 0.06.

We also identify the candidate counterpart in coadded images spanning June to July 2025 from the Vera C. Rubin Observatory Data Preview 2 (DP2; Vera C. Rubin Observatory Team 2026). Data are available in Rubin g, r, i, and z bands, but the candidate is only detected in i and z. We perform a joint fit to the counterpart and all the nearby stars using the local PSF provided by the Rubin pipeline. We obtain the following magnitudes in the DP2 photometric system:

i = 20.95 +/- 0.19,
z = 20.13 +/- 0.27.

We can make a rough estimate of the orbital period of the system by applying equation 1 of T. Shahbaz & E. Kuulkers (1998) which relates the outburst amplitude and orbital period of low-mass X-ray binaries. We calculate the outburst magnitude by comparing our DEFSuS quiescent r band magnitude to a measurement obtained on MJD 61284.05, then correct back to the peak magnitude by adding in the difference between an r' band magnitude from an image taken with a 1-meter Las Cumbres Observatory (LCO) telescope at CTIO on MJD 61284.04 and one taken near the outburst peak with a 1-meter LCO telescope at the South African Astronomical Observatory on 61268.77. The LCO images were processed and magnitudes were extracted and calibrated using a real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see D. M. Russell et al. 2019, P. Saikia et al. 2026 for details). Based on the calculated r band outburst amplitude of 4.7 mag, we estimate an orbital period of about 18 hours. This relation relies on the amplitude in the V band, which is likely to be larger than the observed r band amplitude, implying a shorter orbital period than estimated here. However, see also K. M. López et al. 2019 for further discussion of the relation.

This material is based upon work supported in part by the National Science Foundation through Cooperative Agreements AST-1258333 and AST-2241526 and Cooperative Support Agreements AST-1202910 and 2211468 managed by the Association of Universities for Research in Astronomy (AURA), and the Department of Energy under Contract No. DE-AC02-76SF00515 with the SLAC National Accelerator Laboratory managed by Stanford University. Additional Rubin Observatory funding comes from private donations, grants to universities, and in-kind support from LSST-DA Institutional Members.