TITLE:   GCN CIRCULAR
NUMBER:  35716
SUBJECT: GRB 240209A: NOT photometry of the AT2024cew / GOTO24pw counterpart candidate
DATE:    24/02/13 17:19:17 GMT
FROM:    Antonio de Ugarte Postigo at OCA <deugarte@oca.eu>

A. de Ugarte Postigo (CNRS/OCA & LAM), D. B. Malesani (DAWN/NBI & Radboud Univ.), M. I. Andersen (NBI), J. Sollerman (SU) report:

We observed AT2024cew / GOTO24pw (Belkin et al. GCN 35715), the optical counterpart candidate of GRB 240209A (Fermi GBM team, GCN 35712), using AlFOSC imaging on the 2.5m Nordic Optical Telescope (NOT), at the Roque de los Muchachos Observatory (La Palma, Spain).

The observation consisted of 5x300 s using r-band filter, with a mean time 3.990 days after the burst. The image shows a strong detection of the proposed counterpart at a preliminary magnitude of r~19.80+/-0.05, as compared to PanSTARRS field stars.

This is brighter than the last reported GOTO magnitude observed a day before, corresponding to a behaviour unlike typical GRB afterglows. However, we do note that there is a difference in the filters that could account for some inconsistency in the magnitudes of the two instruments.

If the object is at the same redshift of the nearby galaxy (photo-z = 0.16), then its absolute magnitude would be M_V ~ -19.9. This would be similar to some luminous supernovae (f.ex. a SN Ia) close to peak light. However, the fast decay and subsequent re-brightening in a day-scale, would be peculiar for a SN near peak.