[vsnet-grb-info 15637] GRB 141017A: GROND Afterglow Confirmation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16926
SUBJECT: GRB 141017A: GROND Afterglow Confirmation
DATE:    14/10/19 12:57:11 GMT
FROM:    Alexander Kann at TLS Tautenburg  <kann at tls-tautenburg.de>

D. A. Kann, A. Nicuesa Guelbenzu (both TLS Tautenburg), and J. Greiner
(MPE Garching)  report on behalf of the GROND team:

We observed the field of GRB 141017A (Swift trigger 615672; Marshall et
al., GCN #16919) 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 La Silla
Observatory (Chile).

Observations started at 02:50 UT on 18 October 2014, around 8.5 hours
after the GRB trigger. They were performed at an average seeing of 1".7
and at an average airmass of 1.9.

Based on images with exposure times of 4130 s in g'r'i'z' (centered 0.459
days after the GRB) and 4800 s in JHK (centered 0.426 days after the GRB),
we detect a source within the Swift-XRT error circle reported by Goad et
al. (GCN #16921) at position

RA (J2000) = 06:14:31.28
Dec. (J2000) = -58:34:56.5

with an error of 0".5 and the following 3-sigma AB magnitudes:

g' = 24.3 +/- 0.2 mag,
r' = 23.6 +/- 0.2 mag,
i' = 23.4 +/- 0.2 mag,
z' = 23.2 +/- 0.3 mag,
J > 22.0 mag,
H > 21.4 mag, and
K > 20.3 mag.

The object has faded strongly compared to the early UVOT detection (Oates
et al., GCN #16924) and the Zadko Gingin detection (Klotz et al., GCN
#16920), confirming it as the afterglow. At this time, we cannot conclude
if this object might already be the host galaxy of GRB 141017A. Comparison
with an epoch taken 75 minutes earlier at lower S/N shows no significant
evidence for fading, but the spectrum is well-described by a spectral
slope with beta = 1 and a possible drop-out in g' (redshift z > 2.2).

Given magnitudes are derived based on calibrating the images against GROND
zeropoints (g'r'i'z') and 2MASS field stars (JHK) and are not corrected
for the Galactic foreground extinction corresponding to a reddening of
E_(B-V) = 0.04 in the direction of the burst (Schlegel et al. 1998).