Patricia Schady and Thomas Kruehler (MPE Garching) report:
We observed the field of the Fermi GRB 170214A (trigger 508779271/170214649, Mailyan et al., GCN #20675) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile).
Observations started at 08:53 UT on 2017-02-15, 17.3 hrs after the GRB trigger, and they were performed at an average seeing of 1.5" and at an average airmass of 1.6. We detect an uncatalogued source within the XRT error circle of Source 1 (Beardmore et al., GCN #20679), at the same position reported by Malesani et al., (GCN #20683).
Based on combined images with 16 min of total integration time in g�r�i�z� and 19 min in JHK at a mid-time of 09:05 UT on 2017-02-15, this source has the following preliminary magnitudes (all in the AB system):
g' = 22.6 +/- 0.3 mag
r� = 21.3 +/- 0.1 mag
i� = 20.8 +/- 0.1 mag
z' = 20.3 +/- 0.1 mag
J = 19.6 +/- 0.1 mag
H = 19.3 +/- 0.1 mag
K = 18.8 +/- 0.3 mag
We re-observed the target on 2017-02-16 at a mid-time of 08:45 UT, and the r�-band magnitude has decayed by a further 0.1 mag compared to the NOT measurement (Malesani et al., GCN #20683).
The given magnitudes are derived based on calibrating the images against SDSS in the optical and 2MASS field stars in the NIR bands. They are not corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.30 in the direction of the burst (Schlafly & Finkbeiner 2011).
We acknowledge the excellent help provided by the observer on the 2.2 m telescope at La Silla, Simona Ciceri, in obtaining these data.