TITLE: GCN CIRCULAR NUMBER: 41444 SUBJECT: GRB 250818B: Kilonova-Catcher optical afterglow detection DATE: 25/08/20 13:19:56 GMT FROM: Damien Turpin at CEA-Saclay dturpin-astro@hotmail.com

R. Hellot, M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), D. Akl (NYUAD), M. Molham (NRIAG), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 250818B detected by SVOM/ECLAIRs and SVOM/GRM (Wang et al., GCN 41405) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a TEC160FL telescope operated by M. Freeberg and the CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at T0+5hr.

In our stacked frames, we detect an uncatalogued optical source consistent with the afterglow candidate reported by several teams: GOTO (Kumar et al., GCN 41406), Swift/XRT (Ferro et al., GCN 41407), SVOM/VT (Yao et al., GCN 41409), KAIT (Zheng et al., GCN 41417), Keck (Fong et al., GCN 41419), EP/FXT (Li et al.; GCN 41424), NOT (Bentdsen et al., GCN 41426), SAO/RAS (Moskvitin et al., GCN 41428), TRT (An et al., GCN 414430), the 1m Ledesi telescope (Kumar et al., GCN 41431), Swift/UVOT (Siegel et al., GCN 41435) and the Liverpool telescope (Dimple et al., GCN 41442).

We report some of our follow-up results in the table below: +---------------+-----------+------------+----------------+--------------+ | Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument | +===============+===========+============+================+==============+ | 4.97 | 15 x 300s | sdssr (AB) | 19.47 +/- 0.07 | CDK17-AITP | | 7.53 | 11 x 300s | sdssr (AB) | 20.17 +/- 0.16 | TEC160FL | +---------------+-----------+------------+----------------+--------------+

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 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/).

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