C. Adami (LAM), T. Adami (ENS Paris-Saclay), B. Schneider (MIT), A. Saccardi (GEPI, Obs. de Paris), A. de Ugarte Postigo (OCA, CNRS), E. Le Floc’h, F. Schüssler, D. Turpin (CEA-Saclay), S. D. Vergani, J. P. Palmerio (GEPI, Obs. de Paris), S. Basa (LAM), D. Götz (CEA-Saclay), S. Antier (OCA) report, on behalf of a larger collaboration:
We observed the field of GRB 230812B (Fermi GBM Team GCN, 34386; Roberts et al., GCN 34391; Scotton et al., GCN 34392; Page et al., GCN 34394; Zheng & Filippenko, GCN 34395; Kuin et al., GCN 34399; Beardmore et al., GCN 34400; Casentini et al., GCN 34402; de Ugarte Postigo et al., GCN 24409, 34410) using the T120 telescope of Observatoire de Haute Provence (OHP) in imaging mode. We obtained 10x360s exposures in the V-band with a mid-epoch of 2023-08-13 22:00 UT (~1 day after trigger), 2x300s exposures in the R-band with a mid-epoch of 2023-08-14 00:00 UT, and 10x360s exposures in the I band with a mid-epoch of 2023-08-13 20:40 UT.
We derive the following photometry, calibrated against the Pan-STARRS catalog and not corrected for Galactic dust reddening:
V = 20.87 +/- 0.08 mag
R = 20.64 +/- 0.09 mag
I = 19.77 +/- 0.04 mag
We also observed the field of GRB 230812B using the T193cm equipped with the MISTRAL spectro-imager. We obtained 4x300s + 1x60s exposures in the r’ band at a mid-epoch of 2023-08-13 20:15 UT, leading to the following preliminary photometry estimate:
r’ = 20.32 +/- 0.06 mag
We finally obtained 1h of exposure (2x15min + 1x30min) in spectroscopic mode, using the blue setting of MISTRAL. We clearly detect the continuum associated with the transient emission, leading to a redshift upper limit (z<2.5) consistent with the redshift determination obtained by the GTC and the NOT (de Ugarte Postigo et al., GCN 24409, 34410). The signal to noise in the final combined spectrum is however not sufficient to securely identify any absorption line.
We acknowledge Claire Moutou as well as the excellent support from Jean Balcaen and Yoann Degot-Longhi (Observatoire de Haute Provence).