TITLE:   GCN CIRCULAR
NUMBER:  45887
SUBJECT: EP261006a: Kilonova-Catcher optical follow-up and detection 
DATE:    26/10/10 17:43:39 GMT
FROM:    Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>

R. Hellot, 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 EP261006a detected by Einstein Probe EP/WXT (Lao et al., GCN 45826) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a TEC160FL telescope operated by M. Freeberg and a CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at TGRB+3.3 days and were taken with the Johnson/Cousins BV and sloan gri filters. 

In our stacked frames, subtracted from the Pan-STARRS DR2 template image,  we clearly detect the optical counterpart reported by many teams (Eyles-Ferris et al., GCN 45831; Jiang et al. GCN 45833; Wu et al., GCN 45852; Bochenek et al., GCN 45853, GCN 45883; Chen et al. GCN 45853; Corcoran et al., GCN 45868; You et al., GCN 45874; Zhang et al., GCN 45877; Kang et al., GCN 45885).

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

| Tmid-TGRB (days) | Exp (s)     | Filter         | Magnitude     | Instrument       |
| ------------ | ------------ |------------ |------------ |------------ |
| 3.33           | 5 x 300s | g (AB)     | 18.64 +/- 0.04  |  CDK17    | 
| 3.37           | 15 x 300s | r (AB)     | 19.18 +/- 0.04  |  CDK17    | 
| 3.42           | 10 x 180s | B (Vega)     | 18.67 +/- 0.06  |  TEC160FL     | 
| 3.49           | 15 x 180s | g (AB)     | 18.44 +/- 0.03  |  TEC160FL     | 
| 3.52           | 15 x 180s | r (AB)     | 18.79 +/- 0.08  |  TEC160FL     | 
| 3.55           | 5 x 180s | i (AB)     | 19.34 +/- 0.38  |  TEC160FL     | 

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. Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog.

Our photometric measurements are in good agreement with previous magnitudes reported at similar epochs showing that the optical transient is still exhibiting a bright and blue colored (g-r = -0.35) emission.

The KNC network will keep monitoring this transient as long as it is detectable. Further updates will be reported in GCN Circulars.

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/).