GCN Circular 19975

T.-W. Chen (MPE Garching), S. Klose, A. Nicuesa Guelbenzu (TLS), 
J. Bolmer (MPE Garching and ESO Vitacura), T. Kruehler and 
J. Greiner (both MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 161001A (Swift trigger 714404; Page et
al., GCN #19967) 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 02:48 UT on 2016-10-01, 1.71 hours
after the GRB trigger. They were performed at an average 
seeing of 2" and at an average airmass of 2.3.

We found a faint singular point source which is south-west 2.2 arcsec 
away from the photometric center of nearby bright DSS2 object
for which Kruehler et al. (GCN #19971) reported X-Shooter spectroscopy.
The position is within the 2.3" Swift-XRT error circle reported by 
Evans et al. (GCN #19968) and also within the 1.9" enhanced XRT position 
(Osborne et al., GCN 19969
Not found
 
 
), at
RA (J2000.0) = 04 h 47 m 40.57 s   
DEC (J2000.0) = -57d 15' 39.2
with an uncertainty of 0.5" in each coordinate.

Based on images with 36 min of total exposure in g'r'i'z'
and 30 min in JHK at a mid-time of 03:58 UT on 2016-10-01,
we derive the following preliminary magnitudes and upper limits
(all in the AB system):

g' = 23.1 +- 0.2 mag,
r' = 22.5 +- 0.1 mag,
i' = 21.6 +- 0.1 mag,
z' = 21.5 +- 0.1 mag,
J > 21.3 mag,
H > 20.3 mag, and
K > 18.7 mag.

Second-epoch observations were performed at a mean time 
of 09:19 UT at an average seeing 1.1" and at an average airmass of 1.1.
At that time the source seems to have faded to:

r' = 22.9 +- 0.1 mag

Given magnitudes are calibrated against USNO as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V) = 0.01 mag in the
direction of the burst (Schlafly & Finkbeiner 2011).