TITLE:   GCN CIRCULAR
NUMBER:  40774
SUBJECT: GRB 250617B: NOT NIR observations of the afterglow
DATE:    25/06/18 09:27:29 GMT
FROM:    Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>

B. Schneider (LAM), X. Liu (NAOC), A. de Ugarte Postigo (LAM), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), B. P. Gompertz (U. Birmingham), A. A. Djupvik (NOT), S. Bijavara Seshashayana (NOT and Malmo Univ.), report on behalf of a larger collaboration:

We observed the field of GRB 250617B detected by Swift (Page et al., GCN 40760) and Fermi (Fermi GBM team, GCN 40763; Bala & Meegan, GCN 40766), using the NOTCam camera mounted on the Nordic Optical Telescope (NOT). We obtained 20x60 s exposures in the J-band starting at 01:02 UT on 2025-05-18 (~4 hr after the Swift trigger).

The optical counterpart reported by Page et al. (GCN 40760), Moskvitin et al. (GCN 40761), Jelinek et al. (GCN 40762), Grossan et al. (GCN 40771), and Ma et al. (GCN 40772) is well detected in our stacked image with a preliminary magnitude:

J = 19.08 +/- 0.10 (Vega), at mid time 4.24 hr after trigger.

The photometric calibration was performed using nearby stars from the 2MASS catalog and the magnitude is not corrected for Galactic extinction.