TITLE:   GCN CIRCULAR
NUMBER:  12770
SUBJECT: GRB 111228A: X-shooter spectroscopy
DATE:    11/12/29 19:19:59 GMT
FROM:    Steve Schulse at U. of Iceland  <steve@raunvis.hi.is>

S. Schulze (U Iceland), S. Covino (INAF-OABr), H. Flores (GEPI/Obs. de  
Paris), J. P. U. Fynbo (DARK/NBI), B. Milvang-Jensen (DARK/NBI), J.  
Sollerman (U Stockholm), D. Xu (WIS) report on behalf of the X-shooter  
GRB collaboration:

We observed the afterglow of GRB 111228A (Ukwatta et al. GCN 12737)  
with the ESO VLT equipped with the X-shooter spectrograph on 29  
December 2011, beginning 07:40 UT (15.93 hr after the GRB). The  
afterglow had an R-band brightness of 20.08 mag in a series of five  
acquisition images with an exposure time of 30 s each (mid-exposure  
time = 15.98 hours). The magnitude is not corrected for Galactic  
extinction.

We acquired four spectra with an exposure time of 600 seconds each,  
beginning at 07:47 UT. We detect several absorption lines

wave_obs wave_rest  Ion     redshift
(AA, vac)   (AA, vac)
3880.28     2260.78     FeII     0.71634
4023.15     2344.21     FeII     0.71620
4075.26     2374.46     FeII     0.71629
4089.47     2382.77     FeII     0.71627
4422.70     2576.88     MnII    0.71630
4439.44     2586.65     FeII     0.71629
4453.10     2594.50     MnII    0.71636
4462.57     2600.17     FeII     0.71626
4799.05     2796.35     MgII    0.71618
4811.32     2803.53     MgII    0.71616
4896.33     2852.96     MgI     0.71623
6753.24     3934.78     CaK    0.71630
6813.09     3969.59     CaH    0.71632

at a common redshift of z = 0.71627 +/- 0.00002. In addition we detect  
[OIII]5007 at z_em = 0.71635, similar to the absorption line redshift.  
Our measurements confirm the results by of Cucchiara et al (GCN 12761)  
and Palazzi et al (GCN 12763) and are similar to Dittmann et al (GCN  
12759) and Xu et al (GCN 12764).

The seeing conditions over the entire campaign were around 0.8 arcsec.

We acknowledge the support of the VLT staff, in particular Claudio  
Melo and Christophe Martayan.