GCN 45507: NuSTAR Transient 260828: SOAR Optical Upper Limits
2026-09-04T00:18:15.714Z | rev 0
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.
GCN 45488: NuSTAR Transient 260828: LCO Optical Upper Limits
2026-09-01T13:48:16.344Z | rev 0
R. A. J. Eyles-Ferris (Leicester), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), J. A. Quirola-Vasquez (Radboud), A. Martin-Carrillo (UCD), J. Chacon (PUC), F. E. Bauer (SSI and UTA), M. Brightman (Caltech), P. G. Jonker (Radboud) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient detected by NuSTAR (Brightman, GCN 45478) denoted as NuSTAR 260828 using the LCO 1m telescope located at Cerro Tololo Interamerican Observatory (Chile) and equipped with the SINISTRO instrument. Observations consisted of a sequence of 6 x 300 s exposures in SDSS r-band and 5 x 300 s exposures in SDSS z-band.
We performed image subtraction on both stacked images against archival Pan-STARRS stacked images. We do not detect any new source in either band within the NuSTAR uncertainty (Brightman, GCN 45478), down to the following 3 sigma upper limits:
r > 22.22 (midtime: 2026-09-01 08:52:05 UT, ~3.59 days post detection );
z > 21.31 (midtime: 2026-09-01 09:23:30 UT, ~3.61 days post detection).
These upper limits are in AB magnitudes, calibrated using nearby stars from the Pan-STARRS catalogue, and are not corrected for the appreciable Galactic extinction (A_V = 1.74 mag).