TITLE:   GCN CIRCULAR
NUMBER:  13256
SUBJECT: GRB 120422A GROND observations
DATE:    12/04/23 07:42:24 GMT
FROM:    Marco Nardini at MPE  <nardini@mpe.mpg.de>

M. Nardini (Uni. Milano-Bicocca), S. Schmidl (TLS Tautenburg),
J.Greiner (MPE Garching), and D.A. Kann  (TLS Tautenburg) report on behalf of the GROND team:

We observed the field of GRB 120422A (Swift trigger 520658, Troja et al.,
GCN #13243) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla
Observatory (Chile).

Observations started on April 22, 2012, at 23:39 UT, 16.5 hr after the GRB
trigger, and continued for 3 hours. They were performed at an average seeing of
1.45" and at an average airmass of 1.45.

Based on 49 min of total exposures  in g'r'i'z' and 100 min in JHK
we found a blue source consistent with the afterglow position
reported by Kuin et al. (GCN #13248) and  Cucchiara et al. (GCN #13245). 

In g'r'i'z'J we measure a separation of about 1.6" from the SDSS (DR8) galaxy
SDSS J090738.51+140108.3 cited by Cucchiara et al., even if we notice that this value is just slightly larger than the measured seeing.


At the position of afterglow candidate we measure the following magnitudes (in the AB system)

g' = 21.68 +- 0.05 AB mag,

r' = 21.59 +- 0.05 AB mag,

i' = 21.3 +- 0.1AB mag

calibrated against SDSS field stars and not corrected for Galactic foreground reddening of E(B-V)=0.035 (Schlegel et al. 1998). 
We note that the i'-band measurement is significantly contaminated by the underlying candidate host galaxy and thus has to be considered preliminary. Nonetheless, a comparison with the earlier observations reported by Cucchiara et al. (GCN #13245) indicate a fading of at least 2 mag and thus confirms the afterglow nature of the source.