TITLE: GCN CIRCULAR NUMBER: 39767 SUBJECT: GRB 250317B: OHP/T193 optical observations DATE: 25/03/17 23:30:24 GMT FROM: Benjamin Schneider at MIT bschn@mit.edu
B. Schneider (LAM), C. Adami (LAM/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), A. Ugarte Postigo (LAM), M. Dennefeld (IAP/CNRS/Sorbonne U.) and D. Turpin (CEA/Irfu) report on behalf of a larger collaboration:
We observed the field of the GRB 250317B (Zhao et al., GCN 39753) using the T193cm telescope at the Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 9 exposures (8x360 s and 1x300 s) in the r-band starting at 21:56:41 UT on 2025-03-17 (19.74 hours after the trigger). In the stacked image, we clearly detected the optical counterpart reported by Watson et al., GCN 39754; Palmerio et al., GCN 39755; Pérez-Fournon et al., GCN 39757; Ghosh et al., GCN 39759 and Ferro et al., GCN 39764.
The preliminary magnitude derived for that source is: r = 21.87 +/- 0.08 mag (AB)
The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction.
We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen et Andres Carmona for the MISTRAL observations.
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