GCN Circular 13926

V. Sudilovsky (MPE Garching), D. A. Kann (MPE Garching/Universe Cluster),
T. Kr�hler (DARK, Copenhagen), and J. Greiner (MPE Garching) report on
behalf of the GROND team:

Following two days of bad weather delay, we observed the field of GRB
121027A (Evans et al., GCN 13906
Not found
 
 
) simultaneously in g'r'i'z'JHK with GROND
(Greiner et al. 2008, PASP, 120, 405) mounted on the 2.2 m MPG/ESO
telescope at La Silla Observatory (Chile).

Observations began at 08:10 on October 29th (mid-time ~2.05 days
post-trigger). They were performed at an average seeing of 1.6" (r'-band)
and at an average airmass of 1.2.

We detect the NIR afterglow reported by Starling et al. (GCN 13911
Not found
 
 
) and
Levan et al. (GCN 13920
Not found
 
 
) in all seven bands.

Based on a 460 sec integration in g'r'i'z' and 1200 sec integration in
JHK, we measure preliminary magnitudes (all in AB system) of

g' = 22.1 +/- 0.2 mag
r' = 21.6 +/- 0.1 mag
i' = 21.3 +/- 0.1 mag
z' = 20.9 +/- 0.1 mag
J  = 20.4 +/- 0.1 mag
H  = 19.7 +/- 0.1 mag
K  = 19.0 +/- 0.1 mag

We note an increase of brightness by ~ 1 magnitude relative to the
observations reported by Levan et al. (GCN 13920
Not found
 
 
) at 32 hours after the
GRB.

Our detection in all bands constrains the redshift of this burst to z < 3.5.

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