TITLE:   GCN CIRCULAR
NUMBER:  11997
SUBJECT: GRB 110503A: redshift confirmation from TNG
DATE:    11/05/04 00:28:12 GMT
FROM:    Paolo D'Avanzo at INAF-OAB  <paolo.davanzo@brera.inaf.it>

P. D'Avanzo (INAF/OAB), V. D'Elia (ASI/ASDC), L. Di Fabrizio, A. Gurtu
(INAF/TNG), report on behalf of a larger collaboration:

We observed the optical afterglow of GRB 110503A (Stamatikos et al. GCN
11991) with the Italian TNG located in La Palma, equipped with the DOLORES
camera in spectroscopic mode.
The optical afterglow (Leloudas et al. GCN 11994; Kann et al. GCN 11996)
is clearly detected in the unfiltered acquisition image, with a magnitude
R~19.5 at at t-t0=4.85 hours (using the calibration of Kann et al. GCN
11996).

A low-resolution spectrum was taken at a mean time t-t0=5.25 hours. The
adopted grism was LR-B, covering the wavelength range 3900-8200 AA with a
resolution of R~600.
Using a preliminary wavelength calibration we clearly detect several
absorption features, including CIV, SiII, AlIII, AlII, MgII and MgI, all
at a common redshift z=1.61, in agreement with the value reported by de
Ugarte Postigo et al. (GCN 11993).
At the same redshift we also detect the fine structure line SiII*1533.
This absorption feature is likely produced in the GRB host, further
supporting the statement that z=1.61 is the afterglow redshift.