TITLE:   GCN CIRCULAR
NUMBER:  44835
SUBJECT: GRB 260604C: Kilonova-Catcher optical afterglow detection
DATE:    26/06/05 08:27:06 GMT
FROM:    Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>

R. Hellot, D. Christen, M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 260604C detected by Fermi/GBM (Fermi GBM team, GCN 44822) and SVOM (Götz et al., GCN 44823) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot and D. Cristen and the A1 CDK17 telescope located at the Alnitak Observatory operated by M. Freeberg. Our observations started at TGRB+1.0 hours and were taken with sdss-r and Rc filters. 

In our stacked frames, subtracted from the PanSTARRS DR2 template image, we clearly detect the optical counterpart (Lipunov et al., GCN 44825; GCN 44826; GCN 44827; Konno et al., GCN 44828; Zhu et al., GCN 44832; Becerra et al., GCN 44834).


We report part of our follow-up results in the table below :

| Tmid-TGRB (hrs) | Exp (s)   | Filter      | Magnitude       | Instrument  |
| ------------    | --------- |------------ |---------------- |------------ |
| 1.47            | 15 x 180s | Rc (Vega)   | 17.43 +/- 0.03  | A1/CDK17    | 
| 3.17            |  8 x 300s | r (AB)      | 18.41 +/- 0.05  | AITP/CDK17  | 
| 4.01            | 11 x 180s | Rc (Vega)   | 18.43 +/- 0.06  | A1/CDK17    | 

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. 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/).