TITLE:   GCN CIRCULAR
NUMBER:  45403
SUBJECT: GRB 260818B: NOT optical upper limits
DATE:    26/08/19 16:13:31 GMT
FROM:    d.s.oneill@bham.ac.uk

D O’Neill (U. Birmingham), B. Schneider (LAM), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), D. Xu (NAOC), A. L. Bouquin (NOT), A. Schenone Zanuzzi (NOT), report on behalf of a larger collaboration:

We observed the field of GRB 260818B detected by SVOM/ECLAIRs (Cordier et al., GCN  45398), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 4x200 s exposures in the SDSS r-band starting on 2026-08-19 at 03:43:55 UT (7.55 hr post trigger) and 5x200 s in the SDSS z-band starting on 2026-08-19 at 03:59:24 UT (7.80 hr post trigger). 

Because of the size of the ALFOSC field of view, only 58% of the ECLAIRs error circle could is covered in our stacked images.

In the stacked images we identify no new source and obtain the following upper limits:

r > 23.7 AB mag
z > 22.6 AB mag

Magnitudes are calibrated against nearby stars from the Pan-STARRS catalog, and not corrected for Galactic extinction.