TITLE:   GCN CIRCULAR
NUMBER:  29528
SUBJECT: Subject: ZTF21aakruew/AT2021cwd: GROND observations
DATE:    21/02/18 12:18:01 GMT
FROM:    Ana Nicuesa at TLS Tautenburg  <ana@tls-tautenburg.de>

A. Nicuesa Guelbenzu and S. Klose (both TLS Tautenburg) report:

We observed the field of the optical transient ZTF21aakruew/AT2021cwd (Yao
et al., GCN 29508) that was located inside the error box of GRB 210212B
(Svinkin et al., GCN 29511) with GROND (Greiner et al. 2008, PASP 120,
405) mounted at the 2.2m MPG telescope at the ESO La Silla Observatory
(Chile).

Observations were performed at 5:00 UT (midtime) on February 15, 2021,
about 3.1 days after the GRB 210212B trigger. They were executed at an
average seeing of 1.15 arcsec and at an average airmass of 1.3.

The optical transient is detected in the optical but not in the NIR bands.
Based on a 33 min exposure in g'r'i'z' and 30 min in JHK, and we measure
the following preliminary AB magnitudes and upper limits (3 sigma):

g' = 24.16 +/- 0.14,
r' = 23.63 +/- 0.12,
i' = 23.41 +/- 0.21,
z' > 23.3,
J  > 21.7,
H  > 21.1,
K  > 18.1,

calibrated against SDSS and 2MASS field stars. The observed r'-band
magnitude is formally consistent with the observations reported by
Pozanenko et al. (GCN 29512).

After correction for Galactic extinction (E(B-V)=0.04 mag; Ho et al., GCN
29508), the data are in agreement with an SED that follows a power law. We
consider this as evidence that the transient was indeed afterglow light,
confirming its suggested association with GRB 210212B (Svinkin et al., GCN
29511; Svinkin et al., GCN 29514).


We thank R. Lechaume (PUC) for performing the observations.