TITLE: GCN CIRCULAR NUMBER: 41267 SUBJECT: GRB 250805A: Kilonova-Catcher optical upper limits DATE: 25/08/07 15:27:58 GMT FROM: Damien Turpin at CEA-Saclay dturpin-astro@hotmail.com

P. Jaquiery, R. Hellot (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 250805A (Ambrosi et al., GCN 41233) detected by Swift/BAT with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a Celestron C14 telescope located at Beverly-Begg Observatory and operated by P. Jaquiery and the CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at T0+1.2hr.

In our stacked frames, subtracted from the Legacy Survey DR10 template image, we do not detect any optical counterpart inside the Swift/XRT position (Kennea et al., GCN 41234; Beardmore et al., GCN 41238) and at the position of the optical afterglow candidate reported by the VLT/X-shooter (Izzo et al., GCN 41241) and SVOM/VT (Li et al., GCN 41242) teams.

We report our follow-up results in the table below: +---------------+-----------+------------+----------------------+-------------+ | Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument | +===============+===========+============+======================+=============+ | 1.9 | 28 x 120s | sdssr (AB) | 18.3 (U.L., 5 sigma) | C14-BBO | | 26.1 | 5 x 600s | sdssr (AB) | 19.5 (U.L., 5 sigma) | CDK17-AITP | +---------------+-----------+------------+----------------------+-------------+

These upper limits are compatible with the magnitudes and upper limits reported by (Liu et al., GCN 41236; Izzo et al., GCN 41241; Li et al., GCN 41242; Siegel et al., GCN 41265)

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 SkyMapper DR4 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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