TITLE:   GCN CIRCULAR
NUMBER:  44633
SUBJECT: GRB 260515A: OHP/T193 optical observations
DATE:    26/05/16 03:39:29 GMT
FROM:    Christophe Adami at LAM <christophe.adami@lam.fr>

M. Dennefeld (IAP), B. Schneider (LAM), C. Adami (LAM/Pytheas/AMU), S. Basa (LAM/OHP/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), A. Saccardi (CEA/Irfu) report on behalf of a larger collaboration:

We observed the field of the SVOM/ECLAIRs GRB 260515A (Brunet et al., GCN 44622; He et al., GCN 44623; Saccardi et al., GCN 44624; Sosnovskij et al., GCN 44625; He et al., GCN 44626; Lipunov et al., GCN 44627; Wu et al., GCN 44628; Malesani et al., GCN 44629) using the T193cm telescope at Observatoire de Haute-Provence (France) with the MISTRAL spectro-imager. We obtained two epochs: 2x5 minutes of exposure in the r-band starting at 2026-05-15T21:29:45 UT (2.36 hr after the trigger), and 3x10 minutes of exposure in the r-band starting at 2026-05-16T001045UT (5.04 hr after the trigger).

In the stacked images, the optical counterpart is visible with preliminary magnitudes of:

r = 20.42 +/- 0.13 mag (1st visit)
r = 21.34 +/- 0.12 mag (2nd visit)

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 Yoann Degot-Longhi.