TITLE:   GCN CIRCULAR
NUMBER:  37738
SUBJECT: GRB 241006A: GRANDMA optical upper limits
DATE:    24/10/08 19:18:37 GMT
FROM:    Thomas Hussenot-Desenonges at IJCLab <thomas.hussenot@ijclab.in2p3.fr>

T. Hussenot-Desenonges (IJCLAB), Y. Rajabov (UBAI), M. Tanasan (NARIT), I. Abdi (AUS), R.Strausbaugh (EIU), S. Antier (OCA), S. Karpov (FZU), M. Coughlin (UMN), C. Andrade (UMN), I. Tosta e Melo (UniCT-DFA), P. Hello, N. Leroy (IJCLAB), P-A Duverne (APC), T. Pradier (Unistra/IPHC), N. Guessoum (AUS), L. De Almeida, W. Corradi, N. Sasaki, F. Navarete, L. Braga (LNA), K. Noysena (NARIT)
on behalf of the GRANDMA collaboration.

We performed observations with the GRANDMA network on the field of GRB 241006A, detected by Swift-BAT (GCN 37722).
The first images were taken at OPD starting from 2024-10-07T01:01:42, 3 hours after the GRB trigger, in the I and R filters. TRT-SBO also observed the field in R and I filters starting from 2024-10-07T09:26:52, 11.5 hours after the trigger.

We did not detect any optical counterparts of the Swift-XRT afterglow position (RaDec 304.74263 -40.80500, GCN 37724), with the following 5-sigma upperlimits at its coordinates:

+-------------+-------------+--------+----------+-----------+-------------+
| MJD (start) | T-T0 (days) | Filter | Exposure | Telescope | U.L. (Vega) |
|-------------+-------------+--------+----------+-----------+-------------+
| 60590.0429  | 0.1318      | I      | 16x360s  | OPD       | 19.6        |
|-------------+-------------+--------+----------+-----------+-------------+
| 60590.0556  | 0.1445      | R      | 9x360s   | OPD       | 20.1        |
|-------------+-------------+--------+----------+-----------+-------------+
| 60590.3937  | 0.4826      | R      | 5x300s   | TRT-SBO   | 21.6        |
|-------------+-------------+--------+----------+-----------+-------------+
| 60590.4079  | 0.4968      | I      | 5x300s   | TRT-SBO   | 20.0        |
+-------------+-------------+--------+----------+-----------+-------------+

We also searched the Swift-BAT field (RA, Dec 304.794, -40.796, uncertainty 3 arcmin) and did not detect any optical counterpart. The 5-sigma upperlimits on the field are:
+-------------+-------------+--------+----------+-----------+-------------+
| MJD (start) | T-T0 (days) | Filter | Exposure | Telescope | U.L. (Vega) |
|-------------+-------------+--------+----------+-----------+-------------+
| 60590.0429  | 0.1318      | I      | 16x360s  | OPD       | 19.4        |
|-------------+-------------+--------+----------+-----------+-------------+
| 60590.0556  | 0.1445      | R      | 9x360s   | OPD       | 20.0        |
|-------------+-------------+--------+----------+-----------+-------------+
| 60590.3937  | 0.4826      | R      | 5x300s   | TRT-SBO   | 21.5        |
|-------------+-------------+--------+----------+-----------+-------------+
| 60590.4079  | 0.4968      | I      | 5x300s   | TRT-SBO   | 19.9        |
+-------------+-------------+--------+----------+-----------+-------------+



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

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 497, 5518)