GCN Circular 20640

T. Kruehler (MPE Garching) reports:

I observed the field of GRB 170208B (Swift trigger 737463; Siegel
et al. GCN 20632
Not found
 
 
) 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
Not found
 
 
, 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.