GCN Circular 31000

B. O'Connor (UMD, GWU),S.B. Cenko (UMD,NASA-GSFC),
E. Troja (UMD, NASA-GSFC),A.Gottlieb (UMD,NASA-GSFC),
S.Dichiara (PSU),A. Kutyrev (UMD,NASA-GSFC),
S. Veilleux (UMD)

We observed the field of GRB 211023B (Dichiara et al., GCN 30960
Not found
 
 
)
using the Large Monolithic Imager (LMI) on the 4.3m Lowell Discovery
Telescope (LDT) at Happy Jack, AZ. Observations began on October 25,
2021 at 11:42:14 UT (~1.61 d after the GRB), and were performed in
r-band for a total of 18x150 s exposures under poor conditions.

We detect a source coincident with the position of the optical counterpart
reported by UVOT (Kuin et al., GCN 30975
Not found
 
 
) and CAHA (Kann et al., GCN 30992
Not found
 
 
).
We measure a brightness of r~23 AB mag, brighter than the archival object
reported by Pozanenko et al. (GCN 30972
Not found
 
 
). We do not detect any other source
within 5" radius of the UVOT position with a 3-sigma upper limit r>24.0 mag.

Given the non-detection at ~15 hours (Watson et al., GCN 30973
Not found
 
 
),
our detection implies a shallow temporal decay index beta<0.6,
assuming a power-law decline as F~t^-beta. While this may be due
in part to the underlying source (Pozanenko et al., GCN 30972
Not found
 
 
),
we note that the same flattening is also observed in X-rays by Swift/XRT.
Therefore, we strongly encourage continued follow-up of this source.

Magnitudes are calibrated against the SDSS catalog and are not corrected
for Galactic extinction.

We thank the staff of the Lowell Discovery Telescope for assistance
with these observations.