A. Nicuesa Guelbenzu, S. Klose, S. Schmidl (all TLS Tautenburg)
and A. Rau (MPE Garching) report:
We observed the field of GRB 210731A (Swift trigger 1062336; Troja et
al., GCN 30568) simultaneously in g'r'i'z'JHK with GROND mounted at
the 2.2m MPG telescope at ESO La Silla Observatory (Chile).
Observations started on 01-AUG-2021 at 02:33 UT, about 4.2 hours after
the GRB trigger. They were performed at an average seeing of 1.0
arcsec and at an average airmass of 1.1.
At a meantime of 2:46 UT, for the GRB afterglow (de Wet et al,
GCN 30570; Kuin et al., GCN 30572) we measure the following AB
g' = 18.71 +/- 0.01 mag,
r' = 18.44 +/- 0.01 mag,
i' = 18.19 +/- 0.01 mag,
z' = 18.01 +/- 0.01 mag,
J = 17.66 +/- 0.02 mag,
H = 17.38 +/- 0.02 mag, and
K = 17.14 +/- 0.15 mag.
Optical magnitudes are calibrated against the Pan-STARRS catalog,
NIR magnitudes against 2MASS stars in the field.
After correcting for a Galactic foreground reddening of E(B-V)=0.09
mag (Schlegel et al. 1998), the SED is a power-law with no evidence
for extinction by dust in the GRB host galaxy.
We thank Sam Kim for excellent support and for performing the