TITLE:   GCN CIRCULAR
NUMBER:  37225
SUBJECT: GRB 240821A: GRANDMA/LesMakes-T60 Upper Limit
DATE:    24/08/21 23:01:16 GMT
FROM:    Cristina Andrade at UMN <andra104@umn.edu>

A. Klotz (CNRS-OMP-IRAP), P. Thierry (AGORA), S. Antier (OCA), D. Akl (AUS), P.A. Duverne (APC), C. Andrade (UMN), S. Karpov (FZU), M. Coughlin (UMN), P. Hello (IJCLAB) , T. Pradier (Unistra/IPHC), N. Guessoum (AUS), S. Schanne (CEA-Saclay/Irfu), D. Turpin (CEA-Saclay/Irfu), S. Guillot (IRAP) on behalf of the GRANDMA collaboration: 

We imaged the field of GRB 240821A detected by SVOM/ECLAIRs (GCN 37220) and Fermi-GBM (GCN 37219) with LesMakes-T60 at La Réunion.

The observations started ~1.4 hours after the GRB trigger (in Sloan r) and finished 1 hour later. Proximity to the moon made the limits relatively shallow.

The data have been reduced by a single data processing pipeline STDPIPE (Karpov et al., 2022). We do not find any optical counterpart within the ECLAIRs localization error box (7 arcmin centered on RA, Dec = 354.23, -10.18 (J2000)). We estimate an upper limit of 18.4 mag in r-band (5 sigma).

We thank SVOM for the useful communication. We use the SkyPortal application (skyportal.io) to monitor our observational campaign.

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 49, 5518).