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GCN Circular 13840

GRB 121001A: GROND Afterglow Confirmation
2012-10-02T23:46:14Z (12 years ago)
Alexander Kann at TLS Tautenburg <>
S. Schmidl (TLS Tautenburg), D. A. Kann (MPE Garching), S. Klose (TLS
Tautenburg), and J.Greiner (MPE Garching) report on behalf of the GROND

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.
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