TITLE:   GCN CIRCULAR
NUMBER:  11949
SUBJECT: GRB 110420B, GROND observations
DATE:    11/04/21 12:35:44 GMT
FROM:    Andrea Rossi at TLS Tautenburg  <rossi@tls-tautenburg.de>

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

We observed the field of GRB 110420B (Swift trigger 451794; Sbarufatti et
al. GCN 11944) 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 on La Silla.

Observations started at 7:00 UT on April 21, 8.15 hours after the GRB 
trigger. They were obtained under challenging sky conditions, with a 
seeing > 1.5 arcsec, an average airmass of 1.7, bright moonlight, and 
passing cirrus.

In a first epoch, centered on the refined BAT error circle (Markwardt et 
al., GCN 11946), we do not detect any new object 
down to the DSS2 limit. The XRT error circle (Sbarufatti et al., GCN 
11947) was not covered by these observations in g'r'i'z'.

A subsequent epoch started about 30 min later and was centered on the XRT 
afterglow candidate (Sbarufatti et al., GCN 11947). Here we note the 
presence of a relatively bright object at the border of the XRT error 
circle at coordinates RA, DEC (J2000) 21:20:25.389, -41:17:51.79 (+/- 0.5 
arcsec), which could be a galaxy seen nearly face-on. This object is also 
visible in the DSS2-red image.  Unfortunately, due to the bad seeing we 
cannot decide with certainty at the moment if this object is a galaxy or a 
star. Its r'-band magnitude is r'(AB)  = 19.5 +/- 0.1. No other source is 
detected inside the XRT error circle down to r'(AB) = 23.2.

Magnitudes are derived by calibrating the images against GROND zeropoints 
and 2MASS field stars. We note that the Galactic reddening along the line 
of sight is E(B-V) = 0.03 mag (Schlegel et al. 1998).