GCN Circular 13840

S. Schmidl (TLS Tautenburg), D. A. Kann (MPE Garching), S. Klose (TLS
Tautenburg), and J.Greiner (MPE Garching) report on behalf of the GROND
team:

We observed the field of GRB 121001A (Swift trigger 535026, D'Elia et al.,
GCN # 13831) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla
Observatory (Chile).

Observations started on October 01, 2012, at 23:37 UT, 5.23 hr after the
GRB trigger. They were performed at an average seeing of 2".7 (in r') and
at an average airmass of 1.2.

We found a source in the z', J, H and K bands at coordinates (J2000):

RA   =  18:24:07.84

Dec. = -05:39:55.30

with an error of 0".3 in each coordinate.

This is consistent with the afterglow positions reported by Andreev et al.
(GCN # 13833) and Tello et al. (GCN # 13835), but slightly outside the
refined XRT position reported by Evans et al. (GCN # 18837).

Based on 1380 s of total exposures in g'r'i'z' (mean time 0.2415 days
after the trigger) and 1920 s in JHK (mean time 0.2363 days after the
trigger) we estimate preliminary AB magnitudes (upper limits are 3 sigma) of

g' > 23.6,

r' > 22.9,

i' > 22.2,

z' = 21.7 +/- 0.2,

J  = 19.5 +/- 0.4,

H  = 18.4 +/- 0.1 and

K  = 17.94 +/- 0.06.

Given magnitudes are calibrated against GROND zeropoints in the optical as
well as 2MASS field stars in the NIR and are not corrected for the high
Galactic foreground reddening of E_(B-V) ~ 1.3 mag (Schlegel et al. 1998).

Our r'- and i'-band non-detections vs. the detections reported by Andreev
et al. (GCN # 13833) and Tello et al. (GCN # 13835) confirm that this is
the afterglow of GRB 121001A. Furthermore, the bright source we detect in
K is not detected in 2MASS, while nearby sources of similar magnitude are
clearly visible in 2MASS.