C. Adami, S. Basa (LAM), E. Le Floc’h, D. Götz, D.Turpin (CEA Paris-Saclay), B. Schneider (MIT), S. D. Vergani (GEPI, Obs. de Paris), report on behalf of a larger collaboration:
We observed the field of GRB 230506C (Tohuvavohu et al., GCN 22731) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. 5 exposures were obtained in the imaging mode with the r-band filter (4x300s + 1x600s) from 2023 6 May 20:46:12 UT to 2023 6 May 21:16:27 UT (mid time ~3.8h after trigger). In the combined frame, we detect a source consistent with the position of the X-ray counterpart reported by Kennea et al. (GCN 33736) and with the location of the optical afterglow reported by Pankov et al. (GCN 33734) and Jelinek et al. (GCN 33735). The r’ AB magnitude of this source is 20.9 +/- 0.06 mag. The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.
In addition, we exposed for 30min (2x15min) with the MISTRAL spectroscopic blue setting, covering from 4200A to 8000A, at R~750. We detect a faint continuum at the GRB position, clearly visible at wavelength redder than ~5100A, but the absence of continuum at Lambda bluer than 5100A may be due to the lack of efficiency of the instrument. The S/N is also too low to provide any significant emission or absorption line detection.
We acknowledge the excellent support from Observatoire de Haute-Provence, in particular Jean-Pierre Troncin.