TITLE:   GCN CIRCULAR
NUMBER:  37251
SUBJECT: GRB 240821A: Colibri Optical Upper Limit
DATE:    24/08/23 20:47:17 GMT
FROM:    Alan Watson at UNAM <alan@astro.unam.mx>

Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), S. Antier (OCA), Alan
M. Watson (UNAM), and D. Akl (AUS)

report on behalf of:

Colibri Science Team: Jean-Luc Atteia (IRAP), Nathaniel R. Butler (ASU),
Damien Dornic (CPPM), J.-G. Ducoin (CPPM), Francis Fortin (IRAP), Simona
Lombardo (LAM), Francesco Magnani (CPPM), and Margarita Pereyra (UNAM),

Colibri Engineering Team: François Dolon (UAR Pytheas), François Agneray
(LAM), Fernando Angeles (UNAM), Thomas Auphan (CPPM), Hafid Benamar (CPPM),
Jérémie Boy (IRAP), Edgar Cadena Zepeda (UNAM), Salvador Cuevas (UNAM),
Alejandro Farah (UNAM), Liliana Figueroa Carrilo (UNAM), Johan Floriot
(LAM), Jorge Fuentes-Fernandez (UNAM), Pascal Gallais (CEA), Rosalía 
Langarica (UNAM), Arthur Langlois (IRAP), Julien Lecubin (UAR Pytheas), 
Eduardo López Ángeles (UNAM), Erica E. Lugo-Ibarra (UNAM), Adrien Malgoyre 
(UAR Pytheas), Chrystel Moreau (LAM), José Luis Ochoa Abundis (UNAM), 
María H. Pedrayes-López (UNAM), Samuel Ronayette (CEA), Aurelia Secroun 
(CPPM), Jaime Ruiz Díaz-Soto (UNAM), Silvio Tinoco (UNAM), and 
Hervé Valentin (IRAP)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang
(IHEP), Stéphane Basa (LAM), Jean-Luc Atteia (IRAP), Arnaud Claret (CEA),
Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Andrea
Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC),
En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing
Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Bing Zhang (UNLV)

We imaged the field of GRB 240821A detected by SVOM/ECLAIRs/GRM, Fermi-GBM,
EIRSAT-1 GMOD (GCN circ. 37219, 37220, 37226, 37227, 37232, 37239)) during
the commissioning of the Colibri (SVOM/F-GFT) telescope at the Observatorio
Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.

The observations started 11.1 hours after the GRB trigger and finished 3.5
hour later. We obtained 7530 seconds of exposure with the engineering test
camera in a red filter that approximates SDSS r. The proximity of the
source to the moon made the limits relatively shallow.

The data have been reduced by a single data processing pipeline STDPIPE
(Karpov et al., 2022) and calibrated against the PS1 catalog.

We do not find any optical counterpart in a field of 13 arcmin to a side
centered on RA, Dec = 354.23, -10.18 (J2000).

We estimate an upper limit of 20 mag in r-band (5 sigma).

Note that EP-FXT sources and Swift-XRT sources are in the field of view of
Colibri (GCN 37230, GCN 37249)

This report is consistent other optical observations by GRANDMA (GCN
37225), GMG (GCN 37242), SVOM/VT (GCN 37243), Jinshan (GCN 37245), LCO (GCN
37246), REM (GCN 37247).

We warmly thank the COLIBRI engineering team, D. Turpin, A. Klotz, S.
Karpov, E. Jiménez Bailon, and the staff of the Observatorio Astronómico
Nacional on the Sierra de San Pedro Mártir.