TITLE:   GCN CIRCULAR
NUMBER:  45488
SUBJECT: NuSTAR Transient 260828: LCO Optical Upper Limits
DATE:    26/09/01 13:48:16 GMT
FROM:    Gregory Corcoran  at University College Dublin  <gregory.corcoran@ucdconnect.ie>

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).