GRB241120A: OHP/T193 optical observations

We observed the field of GRB241120A (Gotz et al., GCN 38280; Fermi GRM team, GCN 38279; Hamburg et al., GCN 38285; Pankov et al., GCN 38291; Cherry et al., GCN 38295) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. Four exposures were obtained in the r-band (300s + 3x420s) from 2024 21 November 04:39 UT to 2024 21 November 05:05 UT (~+23h after detection) with two positions on the sky (position "1" for exposures 1 and 4, and position "2" for exposures 2 and 3) separated by a dithering of ~10arcsec. The moon was at an illumination of ~68% and at a distance of 56deg from target. The combined frame has a detection upper limit of r~22.5+/-0.6 (5sigma limit). The photometric calibration was performed using objects from the PanSTARRS catalog. The magnitudes are not corrected for Galactic extinction. There is no clear additional objects in the MISTRAL image as compared with the PanStarrs r-band image except a source at 12:57:06.0 +52:08:26.6 (r~22.2+/-0.2). This source (apparently extended) is however only present in two of the four exposures we performed (position "2" exposures) and is possibly an internal reflection within the instrument due to the intense Moon illumination. More observations are requested to check this point. We also checked the candidate of Pankov et al. We clearly detect this object and measured a magnitude of r~20.40+/-0.08 23h after the original detection. This object is also present in the PanStarrs r-band catalog at r~20.39+/-0.02. If this was the transient, it was back to its initial magnitude 0.96 days after detection. This would be in good agreement with the light curve proposed by Pankov et al. We acknowledge the excellent support from Observatoire de Haute-Provence, in particular Jean Pierre Troncin and the SOPHIE observer Aleyna Adamson.