[vsnet-grb-info 20679] GRB 180613A GROND observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22777
SUBJECT: GRB 180613A GROND observations
DATE:    18/06/14 11:42:07 GMT
FROM:    Patricia Schady at MPE/Swift  <pschady at mpe.mpg.de>

P. Schady, T. Schweyer, T.-W. Chen (all MPE Garching) and J. Bolmer (ESO
Santiago) report:

We observed the field of GRB 180613A (Swift trigger 840853; Tohuvavohu et
al., GCN #22769) 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 23:30 UT on 2018-06-13, 7.9 hrs after the GRB
trigger. They were performed at an average seeing of 1.5" and at an
average airmass of 1.1.

We do not detect the HAWKI source reported by Thoene et al. (GCN #2275).
However, we note that the larger 4.9 arcsec uncertainty on the enhanced
XRT position (Beardmore et al., GCN #22773) now includes two, brighter
sources that are below the sensitivity of the DSS images.

Source #1 lies at the SW edge of the enhanced XRT 90% error circle, at

RA = 14:06:05.24
Dec = -43:04:21.6

with a preliminary magnitude r' = 23.1 +/- 0.1 mag.

Source #2 lies at the NW edge of the enhanced XRT error circle, at

RA = 14:06:05.24
Dec = -43:04:21.6

with r' = 22.9 +/- 0.1 mag. The above positions have an accuracy of 0.1"
in both directions.

Our g' band photometry is strongly affected by a nearby 5.6 mag star
located SW of the GRB position, but source #1 is detected in all other
GROND bands. Source #2 is detected in r', i' and z'. Our observations at
the position of the XRT afterglow have preliminary 3 sigma upper limits of

r' > 23.7 mag
i' > 22.8 mag
z' > 22.7 mag
J > 21.5 mag
H > 21.0 mag
K > 20.4 mag

All quoted magnitudes are in the AB system and are calibrated against
GROND zeropoints and 2MASS field stars. They are not corrected for the
Galactic foreground extinction corresponding to a reddening of E_(B-V)=
0.08 in the direction of the burst (Schlafly & Finkbeiner 2011).