TITLE:   GCN CIRCULAR
NUMBER:  11317
SUBJECT: GRB 100728B: X-shooter redshift
DATE:    10/10/07 12:41:13 GMT
FROM:    Hector Flores at Obs.de Paris,Meudon  <hector.flores@obspm.fr>

H. Flores (GEPI/Obs. de Paris), J. P. U. Fynbo, A. de Ugarte
Postigo, B. Milvang-Jensen, D. Malesani (DARK/NBI), P. Goldoni
(APC/Univ. Paris 7 and SAp/CEA), C. C.  Thoene (IAA-CSIC Granada), S.
Piranomonte (INAF/Roma), S. D. Vergani (GEPI/Obs. de Paris),
F. Onori (Univ. Roma 3)
report on behalf of the X-Shooter GRB collaboration:

We have observed the afterglow of the GRB 100728B (Morris et al., GCN
11009; Perley et al., GCN 11010) with the ESO VLT equipped with the
X-shooter spectrograph. The observation started on  2010-07-29 at 08:15
UT (22 hr after the GRB). A total exposure of 4x600 s was obtained,
covering the spectral range from 300 to 2500 nm. Due to technical
problems with the atmospheric dispersion corrector, the sensitivity of
the instrument was below the nominal value during this observation so
the signal-to-noise ratio of the spectrum is very low.

In the spectra we detect a broad absorption line consistent with a
damped Lyman-alpha line and absorption features consistent with the MgII
doublet at a common redshift of z=2.106. Given the low S/N of the
spectrum, this redshift measurement should be considered tentative.

We acknowledge the Paranal staff, in particular Alain Smette, for
valuable support with the observations, and Daniel Perley (Berkeley) for
providing us a finding chart with the afterglow.

[GCN OPS NOTE(07oct10): Per author's request, Onori was added to the
author list.]