GCN Circular 45507

We observed the field of the fast X-ray transient NuSTAR 260828 (Brightman, GCN 45478

) with the 4.1-m Southern Astrophysical Research (SOAR) Telescope at Cerro Pachón, Chile, using the Goodman High-Throughput Spectrograph in imaging mode. Observations consisted of 3 × 300 s exposures in the SDSS r band, reduced using the Goodman Spectroscopic Pipeline (Torres-Robledo et al. 2020).

We performed image subtraction using SFFT (Hu et al. 2022) on the stacked image using an archival Pan-STARRS1 stacked image as a reference. We find no new source within the NuSTAR localization down to a 3-sigma limiting magnitude of r > 22.25 (midtime of 2026-09-01 09:57:29 UT, ~3.64 days after the NuSTAR detection).

Our nondetection is consistent with the contemporaneous LCO observations reported by Eyles-Ferris et al. (GCN 45488

), who obtained r > 22.22 approximately one hour earlier.

The upper limit is reported in the AB magnitude system and was calibrated using nearby stars from the Pan-STARRS1 catalog. The magnitude has not been corrected for Galactic extinction.