[vsnet-grb-info 12452] GRB 120927A: GROND upper limits

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13827
SUBJECT: GRB 120927A: GROND upper limits
DATE:    12/09/28 14:57:53 GMT
FROM:    Patricia Schady at MPE/Swift  <pschady at mpe.mpg.de>

A. Rossi (TLS Tautenburg), P. Schady and  J. Greiner (MPE Garching)  
report on behalf of the GROND team:

We observed the field of GRB 120927A (Swift trigger 534815; Maselli et  
al., GCN 13822) simultaneously in g'r'i'z'JHK with GROND (Greiner et  
al. 2008, PASP 120, 405) mounted at the 2.2m MPG/ESO telescope at La  
Silla Observatory (Chile).

Observations started at  09:05 UT on Sep 28, 10.4 hrs after the GRB  
trigger, as soon the target became visible from La Silla. They were  
performed at an average seeing of 2.6 arcsec and at an average airmass  
of 2.3. It was only possible to observe for a total exposure time of  
7.2 min in the g'r'i'z' bands during morning twilight, whereas in the  
JHK bands we were able to observe for a total of 32 min.

Using our full data set, we do not detect any source within the  
refined XRT error circle (Beardmore et al., GCN 13824) in our stacked  
GROND images down to the following 3 sigma limiting magnitudes (AB)  
mags:

g' > 22.4
r' > 22.5
i' > 21.9
z' > 21.9
J  > 21.3
H  > 21.0
K  > 19.4

The g'r'i'z and JHK bands are calibrated against SDSS and 2MASS,  
respectively, and the above limits are not corrected for the Galactic  
foreground reddening of E_(B-V)= 0.04 mag in the direction of the  
burst (Schlegel et al. 1998).

We note the presence of a source south of the enhanced XRT position,  
which is already present in DSS2 images.