TITLE:   GCN CIRCULAR
NUMBER:  25992
SUBJECT: GRB 191011A: GROND Detection of the Optical/NIR afterglow
DATE:    19/10/11 06:24:06 GMT
FROM:    Ana Nicuesa at TLS Tautenburg  <ana@tls-tautenburg.de>

A. Nicuesa Guelbenzu (TLS Tautenburg) and P. Schady (Univ. of Bath)
report:

We observed the field of GRB 191011A (Swift trigger 928924; Laha et al.,
GCN 25988) simultaneously in g'r'i'z'JHKs 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 04:38 UT on 2019-10-11, around 2 minutes after the
GRB trigger and lasted for about 5 minutes. They were performed at an
average seeing of 1.14" and at an average airmass of 1.1.

We detect the optical afterglow candidate reported by Laha et al. (GCN
25988) and measure the following preliminary magnitudes and upper limits
(AB magnitudes):

r' = 19.52 +/- 0.1 mag,
i' = 18.46 +/- 0.05 mag,
z' = 18.62 +/- 0.1 mag,
J  = 17.89 +/- 0.1 mag,
H  = 17.21 +/- 0.1 mag.

The optical afterglow is also visible in g' band.

The magnitudes are calibrated against GROND zeropoints and 2MASS field
stars and are not corrected for the Galactic foreground extinction
corresponding to a reddening of E_(B-V)= 0.02 mag in the direction of the
burst (Schlegel et al. 1998).

We acknowledge the excellent support from Regis Lachaume in La Silla
in acquiring these observations.