GCN Circular 30755

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


We observed the field of GRB 210901A (Swift trigger #1071488; Laha et al.,
GCN 30746
Not found
 
 
) simultaneously in g'r'i'z'JHK with GROND mounted at the 2.2m
MPG telescope at La Silla Observatory (Chile).

Observations started at 23:43 UT on September 1, about 8.5 hr after the
GRB trigger. They were performed at an average seeing of 2.3 arcsec and at
an average airmass of 1.5.

Inside the 2.5 arcsec enhanced XRT error circle (Osborne et al., GCN
30749
Not found
 
 
), we do not detect any source. Based on an observation with 23.4
minutes integration time in g'r'i'z' and 20 minutes in JHK, at a midtime
of 23:43 UT on September 1, we measure the following preliminary 3-sigma
upper limits (AB mags):

g' > 23.5,
r' > 23.6,
i' > 22.6,
J  > 20.1,
H  > 19.7, and
K  > 16.3.

We note the presence of an object at the north-western border of the XRT
error circle at coordinates RA, DEC (J2000) = 19:41:05.98, -76:55:33.0 for
which we measure r' = 21.33 +/- 0.05 mag (AB). Due to the relatively bad
seeing at present we cannot decide with certainty if this is a galaxy or a
star.

Given magnitudes are calibrated against the SkyMapper catalog (Wolf et
al., 2018, PASA 35, e010) in g'r'i'z' and 2MASS field stars in JHK
(Skrutskie et al., 2006, AJ 131, 1163).

We thank Maren Hempel for excellent support and for performing the
observations.