GCN Circular 11949
Subject
GRB 110420B, GROND observations
Event
Date
2011-04-21T12:35:44Z (15 years ago)
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).