A. Nicuesa Guelbenzu, S. Klose (both TLS Tautenburg), and A. Rau (MPE
We observed the field of GRB 210905A (Swift trigger #1071993; Sonbas et
al., GCN 30765) simultaneously in g'r'i'z'JHK with GROND mounted at the
2.2m MPG telescope at La Silla Observatory (Chile).
Observations started at 23:35 UT on September 5, about 23.2 hr after the
GRB trigger. They were performed at an average seeing of 1.7 arcsec and at
an average airmass of 1.2.
Based on combined images with 22 min total exposure in g'r'i'z' and 25 min
in JHK, at a mid-time of 23:55 UT on 2021-09-05 for the afterglow
(Strausbaugh & Cucchiara GCN 30769 and 30770; Tanvir et al., GCN 30771;
D'Avanzo et al., GCN 30772; Dobie et al., GCN 30773) we derive the
following preliminary AB magnitudes and 3-sigma upper limits:
g' > 24.3,
r' > 24.3,
i' > 23.8,
z' = 21.6 +/- 0.2,
J = 20.2 +/- 0.2,
H = 20.1 +/- 0.2,
K > 18.2.
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 and Regis Lachaume for excellent support and for
performing the observations.