TITLE:   GCN CIRCULAR
NUMBER:  32648
SUBJECT: GRB 221009A: Redshift from X-shooter/VLT
DATE:    22/10/10 09:39:41 GMT
FROM:    Antonio de Ugarte Postigo at OCA  <deugarte@oca.eu>

A. de Ugarte Postigo (OCA), L. Izzo (DARK/NBI), G. Pugliese 
(Amsterdam Univ. and Leiden Observatory), D. Xu (NAOC), 
B. Schneider (CEA Paris-Saclay), J.P.U. Fynbo (DAWN/NBI), 
N.R. Tanvir (Univ. Leicester), D.B. Malesani (Radboud Univ. 
and DAWN/NBI), A. Saccardi (GEPI, Observatoire de Paris),  
D. A. Kann (Goethe Univ.), K. Wiersema (Lancaster Univ.), 
B. P. Gompertz (Birmingham), C. C. Thoene (ASU-CAS) and 
A. J. Levan (Radboud Univ.) report on behalf of the Stargate 
collaboration:

We observed the afterglow of the extremely bright GRB221009A 
(Dichiara et al., GCN 32632; Kennea & Williams, GCN 32635; 
Bissaldi et al., GCN 32637; Veres et al., GCN 32636; Lesage et 
al., GCN 32642) with X-shooter at ESO's UT3 of the Very Large 
Telescope (Paranal, Chile). The observations started at 
00:49:58.9 UT  (11.55 hr after the GBM trigger and 10.66 hr 
after the BAT trigger). The observation consisted of 4x600s with 
a spectral coverage between 3000 and 21000 AA.

We detect a very red continuum with absorption features that 
correspond to CaII, CaI and NaID at a redshift of z = 0.151. We 
also detect multiple features due to the Milky Way’s interstellar 
medium, due to the large Galactic column density of material 
along this line of sight. At this redshift the event has an isotropic 
equivalent energy of Eiso=2x10^54 erg (using the GBM fluence 
reported in GCN 32642), barring saturation effects in the 
Fermi/GBM fluence. This is within the upper end of GRB 
energetics. Further follow-up is strongly encouraged.

We acknowledge the excellent support provided by the Paranal 
staff, in particular A. Escorza and Zahed Wahhaj.