TITLE:   GCN CIRCULAR
NUMBER:  37882
SUBJECT: GRB 241025A: GRANDMA and Kilonova-Catcher optical afterglow detections and upperlimits
DATE:    24/10/25 17:49:36 GMT
FROM:    Emma de Bruin at University of Minnesota <debru045@umn.edu>

I. Abdi (AUS), E. de Bruin (UMN), C. Andrade (UMN), T.Hussenot (IJCLAB), M. Masek(FZU),
S. Antier (OCA), S. Karpov (FZU), M. Coughlin (UMN), I. Tosta e Melo (UniCT-DFA), P. Hello, N. Leroy (IJCLAB), P-A Duverne (APC), T. Pradier (Unistra/IPHC), N. Guessoum (AUS), M. Tanasan (NARIT), K. Noysena (NARIT), D.Turpin (CEA-Saclay/Irfu), M. Freeberg (KNC), D. St-Gelais (KNC) on behalf of the GRANDMA and Kilonova-Catcher collaborations:

We observed the Swift/UVOT localisation of GRB 241025A, detected by Swift/BAT (GCN 37859) with the GRANDMA network and its citizen science project Kilonova-catcher (KNC). Our observations started 70 min after the trigger.

We detect at the Swift/UVOT coordinates the afterglow already observed by TRT (GCN 37862) DDOTI (GCN 37865) and SVOM/VT (GCN 37871), and we obtain the following magnitudes and 5-sigma upperlimits:

| Tstart (UTC)  | Telescope | Exposure | Filter | Magnitude (Vega) |
	 	 	 	
| 2024-10-25T02:46:16 | KNC-iT11 | 4x180s | R | >18.5 (U.L) |
| 2024-10-25T03:28:43 | KNC-iT21 | 10x180s | V | >18.5 (U.L) |
| 2024-10-25T04:04:36 | KNC-iT21 | 10x180s | R | 19.29 +- 0.15 |
| 2024-10-25T04:58:39 | TRT-SRO | 5x300s | R | 19.66 +- 0.05  |
| 2024-10-25T06:05:10 | KNC-iT11 | 20x180s | R | 19.97 +- 0.22 |
| 2024-10-25T06:35:00 | TRT-SRO | 5x300s | I | 18.90 +- 0.08 |
| 2024-10-25T07:25:25 | TRT-SRO | 5x200s | B | > 21.4 (U.L) |
| 2024-10-25T07:50:17 | TRT-SRO | 5x200s | V | >21.1 (U.L) |
| 2024-10-25T08:04:33 | TRT-SRO | 5x200s | R | 20.23 +- 0.08 |
| 2024-10-25T08:25:49 | TRT-SRO | 5x200s | I | 19.08 +- 0.17 |

The V-band image of TRT-SRO contains a marginal 4-sigma detection at V=21.11+-0.23, just under the V>21.1 5-sigma upperlimit.

TRT-SRO images were calibrated using the PanSTARRS catalog. KNC images were calibrated using the Gaia DR3 Synphot catalog.

All the data have 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). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).