TITLE: GCN CIRCULAR NUMBER: 41331 SUBJECT: GRB 250812A: Kilonova-Catcher optical afterglow detection DATE: 25/08/12 17:32:39 GMT FROM: Damien Turpin at CEA-Saclay dturpin-astro@hotmail.com

M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), M. Molham (NRIAG), M. Mašek (Institute of Physics, Prague, FZU, CZ), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 250812A (Xin et al., GCN 41322) detected by SVOM/ECLAIRs with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope T72 telescope operated by M. Freeberg. Our observations started at T0+1.8hr.

In our stacked frames, subtracted from the Legacy Survey DR10 template image, we detect an uncatalogued optical source at a position consistent with the TRT (He et al., GCN 41324), SVOM/VT (Xin et al., GCN 41326) and SVOM/COLIBRÍ (FM-GFT) (Rakotondrainibe et al., GCN 41328) optical afterglow candidate as well as the Swift/XRT (Sbarrato et al., GCN41325) x-ray afterglow candidate .

We report our follow-up results in the table below: +---------------+-----------+-----------+----------------+------------+ | Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument | +===============+===========+===========+================+============+ | 2.3 | 10 x 300s | Rc (Vega) | 18.60 +/- 0.06 | iT72 | | 12.4 | 20 x 180s | Rc (Vega) | 19.12 +/- 0.11 | iT72 | +---------------+-----------+-----------+----------------+------------+

Our observations clearly show a fading behavior as also shown by the TRT and SVOM/VT observations (He et al., GCN 41324; Xin et al., GCN 41326). Thus we also suggest this source to be the afterglow of GRB 250812A.

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Johnson-cousins 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/).

View this GCN Circular online at https://gcn.nasa.gov/circulars/41331. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...