GRB 210504A: Lowell Discovery Telescope optical observations

We observed the field of the GRB 210504A (Beardmore et al., GCN 29929; Lien et al., GCN 29930) using the Large Monolithic Imager (LMI) on the 4.3m Lowell Discovery Telescope (LDT) at Happy Jack, AZ. Observations started on May 5, 2021 at 06:32:14 UT (about 16.6 hours after the Swift trigger) using the SDSS g, r, i, and z filters. Observations were taken at an airmass of 2.5 and seeing of about 1.5". We detect the optical counterpart identified by the Swift/UVOT (Breeveld et al., GCN 29933), the NOT (Heintz et al., GCN 29937), and VLT (Xu et al., GCN 29944). The afterglow has magnitude r ~ 21.66 +/- 0.04 AB mag and i ~ 21.43 +/- 0.03 AB mag. Magnitudes are calibrated against the PanSTARRS catalog and are not corrected for Galactic extinction. We thank the staff of the Lowell Discovery Telescope for assistance with these observations.