TITLE:   GCN CIRCULAR
NUMBER:  30695
SUBJECT: GRB 210820A: GROND observations
DATE:    21/08/23 13:50:31 GMT
FROM:    Ana Nicuesa at TLS Tautenburg  <ana@tls-tautenburg.de>

A. Nicuesa Guelbenzu, S. Klose (both TLS Tautenburg) and
A. Rau (MPE Garching) report:

We observed the field of GRB 210820A (Swift trigger 1069551); Beardmore et
al., GCN 30664) simultaneously in g'r'i'z'JHK with GROND  mounted at the
2.2 m  MPG/ESO telescope at La Silla Observatory (Chile).

Observations were performed at 9:03 UT (midtime) on August 21, 2021,
about 15.5 hours  after the GRB trigger. They were executed at an
average seeing of 1.3 arcsec and at an average airmass of 1.2.

We  detect a source  within the XRT error circle (Beardmore, et al.,
GCN 30667) in the i' band with the following preliminary AB
magnitude

i'=22.46 +/- 0.28

in  agreement with the  RATIR detection (Butler et al., GCN 30672).

We do not detect the source in the other bands down to the following
limiting AB magnitudes:

g' > 22.6,

r' > 22.9,

z' > 22.3,

J > 20.9,

H > 20.4, and

K > 17.4.


The given limits and magnitude are derived based on calibrating the images
against SDSS in g'r'i'z' and 2MASS field stars in JHK.

We thank  Paul Eigenthaler  for excellent support and for performing the
observations.