GCN Circular 41497
Subject
GRB 250821A: Kilonova-Catcher optical upper limits
Event
Date
2025-08-22T15:19:10Z (2 days ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We observed the field of GRB 250821A detected by Swift/BAT (Dichiara et al., GCN 41462) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope T32 telescope located at Siding Spring Observatory and operated by M. Freeberg. Our observations started at T0+17 min.
In our stacked frames, subtracted from the PanSTARRS DR2 template image, we do not detect any optical counterpart inside the Swift/XRT enhanced position (Beardmore et al., GCN 41472).
We report our follow-up results in the table below:
+---------------+-----------+-----------+----------------------+-------------+
| Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument |
+===============+===========+===========+======================+=============+
| 0.86 | 20 x 180s | Ic (Vega) | 19.4 (U.L., 5 sigma) | T32 |
+---------------+-----------+-----------+----------------------+-------------+
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
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/).