[vsnet-grb-info 20564] GRB 180418A GROND observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22662
SUBJECT: GRB 180418A GROND observations
DATE:    18/04/19 10:03:15 GMT
FROM:    Patricia Schady at MPE/Swift  <pschady at mpe.mpg.de>

The field of GRB 180418A (Swift trigger 826428; D'Elia et al., GCN #22646)
was observed 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 01:27 UT on 19th April, 18.7 hours after the GRB
trigger. They were performed at an average seeing of 1.5" and at an
average airmass of 1.7.

A source is detected in the g'r'i' bands at a position consistent with the
X-ray and optical afterglow (Zheng et al., GCN #22647; Goad et al., GCN
#22650). The source is not clearly point-like, and it is therefore
possible that some host galaxy light is contributing to the flux. Based on
22 min of exposure in g'r'i'z' and 20 min in JHK, the estimated AB
magnitudes are:

g' = 23.60 +/- 0.14 mag
r' = 23.24 +/- 0.13 mag
i' = 22.88 +/- 0.21 mag
z' > 22.96 mag
J > 21.16 mag
H > 20.67 mag
K > 18.82 mag

Given magnitudes are calibrated against SDSS for g'r'i'z' and 2MASS field
stars for JHK, and they 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).

I thank Sam Kim for his excellent support from La Silla.