[vsnet-grb-info 15424] GRB140818B : GROND afterglow candidate

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16713
SUBJECT: GRB140818B : GROND afterglow candidate
DATE:    14/08/19 02:45:09 GMT
FROM:    Karla Varela at MPE  <kvarela at mpe.mpg.de>

F. Knust, K. Varela, and J. Greiner (all MPE Garching) report on behalf of
the GROND team:

We observed the field of GRB 140818B (Swift-60985; Vargas et al., GCN
16709) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,PASP
120, 405) mounted at the 2.2m MPG telescope at ESO La Silla Observatory
(Chile). Observations started at 23:43 UT on 2014-08-18, 5 hours after the
GRB trigger. They were performed at an average seeing of  1.23"  and at an
average airmass of 1.2.

Based on images with exposure times of  340s in g’r'i'z’ and 370s in JHKs,
we detect a source inside the 1.9” error circle reported by Beardmore et
al. (GCN 16712) at

RA (J2000): 18h 04m 32.47s
Dec (J2000): -01d 23’ 09.7"

with an uncertainty of 0.5” and magnitudes

g’ > 24.1 mag,
r' = 22.1 +/- 0.1 mag,
i' = 21.8 +/- 0.1 mag,
z' = 21.4 +/- 0.1 mag.
J > 20.9 mag,
H > 20.7 mag, and
K > 19.2 mag.

The afterglow candidate reported by Aceituno et al. (GCN #16711) is also
detected inside the 3.9” XRT error circle (GCN 16711) but not inside the
1.9” XRT error circle (GCN 16712) with magnitudes :

r' = 22.4 +/- 0.1 mag,
i' = 21.6 +/- 0.1 mag,
z' = 20.9 +/- 0.1 mag.

The given magnitudes are derived based on calibrating the images against
the GROND zeropoints and 2MASS field stars are not corrected for the
Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.73
in the direction of the burst (Schlegel et al. 1998).