TITLE:   GCN CIRCULAR
NUMBER:  43543
SUBJECT: GRB 260127A: NOT optical observations
DATE:    26/01/28 16:53:18 GMT
FROM:    Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>

D. B. Malesani (DAWN/NBI and Radboud), J. An (NAOC), L. B He (NAOC), G. Corcoran (UCD), K. Valeckas (NOT & NBI), L. Izzo (INAF/OAC and DARK/NBI), A. Martin-Carrillo (UCD), A. Saccardi (CEA/Irfu), B. Schneider (LAM) report on behalf of a larger collaboration:

We observed the optical counterpart (Saccardi & Malesani, GCN 43530; Wu et al., GCN 43537; Hagio et al., GCN 43540) of GRB 260127A (Klingler et al., GCN 43529; Fermi GBM team, GCN 43528) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out in the SDSS i, z, and r filters (with exposure times 3x300, 5x200, and 3x300 s, respectively).

The optical counterpart is detected in the stacked images for all bands. We report the following magnitudes:

| Filter | Epoch (UT) | Time since GRB (hr) | Exp time (s) | Magnitude (AB) |
| ------ | ---------- | ------------------- | ------------ | -------------- |
| i      | Jan 28.197 | 10.88               | 3x300        | 22.30 +- 0.08  |
| z      | Jan 28.209 | 11.18               | 5x200        | 22.05 +- 0.18  |
| r      | Jan 28.243 | 12.00               | 3x300    	   | 22.30 +- 0.06  |


The above magnitudes are calibrated against the Pan-STARRS catalog and are not corrected for Galactic extinction. We note that at the coordinates of the optical counterpart a faint source is reported in the Legacy Survey catalogue. Considering its tabulated magnitudes (r = 23.3, i = 23.2, z = 22.3), the transient emission is thus significantly contaminated by the host flux.