TITLE:   GCN CIRCULAR
NUMBER:  29946
SUBJECT: GRB 210504A: Lowell Discovery Telescope optical observations
DATE:    21/05/05 14:36:48 GMT
FROM:    Brendan O'Connor at UMD  <oconnorb@umd.edu>

B. O'Connor (UMD, GWU), S.Dichiara (UMD, NASA-GSFC), E. Troja
(UMD, NASA-GSFC), P. Gatkine (Caltech), J.M. Durbak (UMD), S.B. Cenko
(NASA-GSFC), A. Kutyrev (UMD, NASA-GSFC), S. Veilleux (UMD) report:

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.