TITLE:   GCN CIRCULAR
NUMBER:  6655
SUBJECT: GRB 070721B: further observations and analysis
DATE:    07/07/24 15:00:42 GMT
FROM:    Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center  <malesani@astro.ku.dk>

J.P.U. Fynbo (DARK), P. Jakobsson (Univ. Hertfordshire), D. Malesani, J. 
Hjorth (DARK), I. Ilyn (AIP), report:

We have obtained further observations of GRB 070721B (Ziaeepour et al., 
GCN Report 73). We also performed further analysis on our images of the 
optical afterglow of GRB 070721B (Malesani et al., GCN 6651).

Our first observation was carried out with the Nordic Optical Telescope 
(mean time July 22.173 UT, i.e. 17.6 hr after the trigger). Close to the 
UVOT position, we detect a single source at the following coordinates 
(J2000, against USNO-B1):

RA = 02:12:33.00
Dec = -02:11:41.4

This position is consistent with source #2 reported by Malesani et al. 
(GCN 6651).

We have also obtained further imaging with the ESO-VLT on July 23.409 UT 
(47.3 hr after the trigger). We find that source #2 has faded compared 
to the two previous epochs. We hence confirm that object #2 is the 
optical afterglow of GRB 070721B. Object #1, reported by Malesani et al. 
(GCN 6651), is most likely a spurious source caused by a reflection from 
the bright star 0877-0042643 in the field (R~6.5).

In our 2-dimensional spectrum we also note the presence of bright 
high-redshift galaxy with a redshift z=3.62, very close to the redshift 
of GRB 070721B (Malesani et al., GCN 6651). This object is 21" away of 
the afterglow, which corresponds to ~150 kpc at z=3.62.

We acknowledge the ESO staff for excellent support.