GRB 241209A: VLT optical observations

We observed the field of the short GRB 241209A (Parsotan et al., GCN 38476; Fermi GBM team, GCN 38474) using the ESO/VLT equipped with the FORS2 camera and the R filter. Observations started on 2024 9.264 UT (0.77 hr after the BAT trigger). Consistent with the location of the XRT counterpart (Page & Evans, GCN 38479) we detect an faint object at coordinates (J2000): RA = 10:21:35.22 Dec = +6:19:43.3 From a preliminary analysis, we measure a magnitude r = 23.80+/-0.09 (AB), calibrated against nearby sources from the Legacy Survey. This object is faintly visible in the Legacy Survey images of this field. From those images, we measure a magnitude r ~ 23.95 +- 0.15 (AB) (though we note that the magnitude reported in the Legacy Survey catalog is significantly fainter). At the current time, we cannot thus confirm the presence of transient emission in our data. Further observations are planned to establish fading. The underlying source might be the GRB host galaxy, or unrelated to the event. We also note the presence of a brighter galaxy (r = 21.6), about 2.9" to the NW, which has a photometric redshift of 0.78 +- 0.08 in the Legacy Survey (Zhou et al. 2021, MNRAS, 501, 3309). We acknowledge helpful support from the observing staff in Paranal, in particular Jonathan Smoker.