GCN Circular 23111

Tassilo Schweyer (MPE Garching) and Patricia Schady (University of Bath) report:

We observed the field of GRB 180809B (Swift trigger 852553; Moss et al., GCN #23105) 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  23:00UT on 2018-08-09, 2.5 hrs after the GRB trigger. They were performed at an average seeing of 1.3" and at an average airmass of 2.

We detect a source in the JHK bands consistent with the position of the X-ray and optical afterglow (Mass et al., GCN #23105; Lipunov et al., GCN #23109). Based on a total exposure of time of 19 minutes in g'r'i'z' and 29 minutes in JHK we estimate the following AB magnitudes and upper limits at the position of the UVOT detection:

g' > 23.9 mag
r' > 23.9 mag
i' > 22.9 mag
z' > 22.4 mag
J = 21.0 +/- 0.3 mag
H = 19.9 +/- 0.2 mag
K = 18.7 +/- 0.1 mag

Given magnitudes are calibrated against PanSTARRS and 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.14 in the direction of the burst (Schlafly & Finkbeiner 2011).

We thank Markus Rabus for the excellent support from La Silla.