TITLE:   GCN CIRCULAR
NUMBER:  20640
SUBJECT: GRB 170208B: GROND observations
DATE:    17/02/09 14:08:32 GMT
FROM:    Thomas Kruehler at MPE Garching  <kruehler@mpe.mpg.de>

T. Kruehler (MPE Garching) reports:

I observed the field of GRB 170208B (Swift trigger 737463; Siegel
et al. GCN 20632) simultaneously in g'r'i'z'JHK with GROND
(Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG
telescope at the ESO La Silla Observatory (Chile).

Observations started at 01:22 UT on 2017-02-09, 2.8 hr after the
GRB trigger and reveal a faint optical source centered at:

RA (J2000): 08:28:34.55
Dec (J2000): -09:01:38.1

which is 2.0" away from the center of the XRT error circle
(Goad et al. GCN 20635, current 90% error radius of 1.6").

Based on combined images with 66 min of total exposures in g'r'i'z'
and 60.0 min in JHK at a mid-time of 02:39 UT on 2017-02-09, this
source has the following preliminary magnitudes and upper limits
(all in the AB system):

g' = 24.5 +- 0.4 mag
r' = 24.4 +- 0.3 mag
i' = 23.4 +- 0.2 mag
z' = 23.1 +- 0.2 mag
J > 21.3 mag
H > 21.0 mag
K > 19.0 mag

Given magnitudes are calibrated against Pan-STARRS and 2MASS field
stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=0.042 mag in the
direction of the burst (Schlafly & Finkbeiner 2011).

This source is marginally detected already in the archival Pan-STARRS
imaging of the field, so it is not the GRB 170208B afterglow. It is,
however, a possible host galaxy candidate (the chance coincidence
probability of finding a similarly bright object at 2" from the XRT
position is around 6%).

I acknowledge excellent help in obtaining these data from the
supporting astronomers on La Silla, Sam Kim and Simon Steinmassl.