TITLE: GCN CIRCULAR NUMBER: 38543 SUBJECT: GRB 241209B: GRANDMA/TRT upper limit DATE: 24/12/12 15:00:34 GMT FROM: marion.pillas@ligo.org

M. Pillas (ULiege), M. Tanasan, K. Noysena (NARIT), S. Antier (OCA), O. Pyshna (Caltech), N. Guessoum (AUS), A. Klotz (IRAP), C. Andrade (UMN) S. Karpov (FZU), M. Coughlin (UMN), , P. Hello (IJCLAB), P-A Duverne (APC), T. Pradier (Unistra/IPHC), D.Turpin (CEA-Saclay/Irfu), on behalf of the GRANDMA collaboration:

We observed the field of GRB 241209B, detected by SVOM/ECLAIRs and GRM (GCN 38478) using TRT-SRO. Observations began 0.209 days after ECLAIRs T0.

We didn't detect any optical afterglow candidate with an upper limit of 22 mag in Johnson-R (3 sigma, Vega Mag)

Further analysis is required to check consistency with the afterglow candidate of SVOM/VT (GCN 38516).

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022).

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

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