TITLE:   GCN CIRCULAR
NUMBER:  45338
SUBJECT: GRB260809A: OHP/T193 optical observations
DATE:    26/08/10 03:11:58 GMT
FROM:    Christophe Adami at LAM <christophe.adami@lam.fr>

C. Adami (LAM/Pytheas/AMU),S. Basa (LAM/OHP/Pytheas/AMU),  A. Saccardi (CEA/Irfu), B. Schneider (LAM), F. Schussler (CEA Paris-Saclay) report on behalf of a larger collaboration:

We observed the field of GRB 260809A (Brunet et al., GCN45331) using the T193cm telescope at Observatoire de Haute-Provence (France) with the MISTRAL spectro-imager. We obtained 5x600 sec of exposure in the i-band under a ~2arcsec seeing, starting at 20260809_233302UT (~8.6 hr after the Brunet et al. trigger) and ending at 20260810_010555UT (~10.1 hr after the Brunet et al. trigger).
We obtained 3x600 + 4x1000 sec of exposure in the r-band under a ~2arcsec seeing, starting at 20260810_001106UT (~9.2 hr after the Brunet et al. trigger) and ending at 20260810_021627UT (~11.3 hr after the Brunet et al. trigger).
We obtained 900 sec of exposure in the Y-band under a ~2arcsec seeing at 20260809_234858UT (~8.9 hr after the Brunet et al. trigger), 1200 sec at 20260810_023727UT (~11.7 hr after the Brunet et al. trigger), and 1200 sec at 20260810_025838UT (~12 hr after the Brunet et al. trigger).

In the preliminary stacked images, we detect in r and i bands the afterglow at the position of Li et al. (GCN 45336) with preliminary magnitudes of: 

r = 23.36 +/- 0.10 mag
i = 22.03 +/- 0.06 mag

We do not detect the source in our Y-band image. This image has a preliminary estimated limiting magnitude of Y=21.2 (3sigma).

The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the STDWeb/STDPipe tools (Karpov 2025), is in the AB system, and is not corrected for Galactic extinction.

We acknowledge the excellent support from Observatoire de Haute-Provence and in particular Stephane Favard.