TITLE: GCN CIRCULAR NUMBER: 45507 SUBJECT: NuSTAR Transient 260828: SOAR Optical Upper Limits DATE: 26/09/04 00:18:15 GMT FROM: Michael Davis at University of Minnesota <davi4614@umn.edu> Michael C. Davis (UMN), James Freeburn (UNC), Tomas Ahumada (NOIRLab), Michael Coughlin (UMN), Akash Anumarlapudi (UNC), Anirudh Salgundi (UNC) report: 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/45507. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...