MISTRAL@OHP photometric and spectroscopic observations of GRB 241029A

We first performed imaging of the field of GRB 241029A (Fermi GBM team, GCN 37932; Schanne et al., GCN 37934; Chenwei Wang et al., GCN 37935; Evans et al., GCN 37936; Jiang et al., GCN 37937; Weikang Lin et al., GCN 37939; Jiang et al., GCN 37940; Francile et al., GCN 37941; Ackley et al., GCN 37943; Odeh et al., GCN 37944; Myers et al., GCN 37945; Bernardini et al., GCN 37946) with MISTRAL mounted on the 1.93 m telescope at Observatoire du Haut Provence (OHP, France). The observations consisted of 300s + 600s exposures in r-band plus 2x600s in g-band. We have very clear detections of the transient object both in g and r-band images: using as reference field stars from the Pan-STARRS catalogue, we determine preliminar magnitudes of r(AB) = 21.6 +/- 0.1 mag and g(AB) = 21.4 +/- 0.1 mag at a mean date of 2024-10-29T18:12:00 UT, ~16 hours after the burst. We immediately after took a spectrum of the object using the MISTRAL blue grism (end of data collection at 2024-10-29T19:45:21UT, ~17h30 after ther burst). Our spectrum cover the wavelength range 4200-8000 AA and consist of one exposure of 1500 seconds. We detect a weak but significant continuum between ~5700/5900 and ~6500AA and this suggests an upper limit of 3.7/3.9 for the redshift. We acknowledge the excellent support from Observatoire de Haute-Provence and in particular Yoann Degot-Longhi for the MISTRAL observations and Thomas Baycroft.