GCN Circular 19625

D. A. Kann (TLS Tautenburg), T. Kruehler (MPE Garching),  S. Schmidl (TLS
Tautenburg), J. Bolmer, and J. Greiner (both MPE Garching) report on
behalf of the GROND team.

We observed the field of GRB 160630A (Swift trigger 702252; Krimm et al.,
GCN #19624) 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 04:10:58 UT, 6.8 minutes after the GRB trigger.
They were performed at an average seeing of 2".1 and at an average airmass
of 1.8.

In an image with exposure time 66 s in g'r'i'z' taken at a mid-time of 7.3
min after the trigger, we detect a source at the edge of the 1".8 XRT
error circle at position:

RA (J2000) = 13:13:02.04

Dec. (J2000) = -56:02:29.8

(with an error of 0".3) with (AB magnitudes):

g' > 21.0 mag,
r' = 20.4 +/- 0.1 mag,
i' = 19.5 +/- 0.1 mag,
z' = 18.7 +/- 0.1 mag.

The source faded by about 1.5 magnitudes within the next quarter hour. We
therefore propose this source to be the afterglow of GRB 160630A.

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction E_(B-V) = 0.43 mag in the direction of the burst (Schlafly &
Finkbeiner, 2011).