TITLE:   GCN CIRCULAR
NUMBER:  15330
SUBJECT: iPTF 13dsw/Fermi403206457: X-Shooter redshift
DATE:    13/10/13 10:27:04 GMT
FROM:    Jochen Greiner at MPI  <jcg@mpe.mpg.de>

A. Rau (MPE Garching), T. Kruehler (ESO), J. Greiner (MPE Garching)
report for the GROND team:

We observed the optical afterglow candidate iPTF13dsw of the Fermi-GBM 
trigger 403206457 (Kasliwal et al. 2013, GCN 15324; Xu et al. 2013, 
GCN 15325; Perley et al. 2013, GCN 15327; Sudilovsky et al. 2013, GCN 15328)
using ESO/VLT UT3 equipped with the X-shooter spectrograph. The observations
started on 2013-10-13 at 03:59 UT. A total exposure of 5x900 s was obtained, 
covering the spectral range from ~3000 AA to ~24000 AA.

In the spectrum, we identify several very weak absorption features
which we interpret as absorption from FeII, MgII and MgI at a common
redshift of z = 1.874. This redshift is consistent with flux being
detected down to ~3200 AA, and a broad absorption feature around
3400 AA corresponding to Lyman\alpha. The combined evidence of fading, 
powerlaw slope SED and redshift suggests iPTF13dsw to be the optical 
afterglow of GRB 131011A.

We thank the Paranal staff for expert support, in particular Andrea
Mehner,  Christophe Martayan, Julio Navarrete, Felipe Gaete, L.
Schmidtobreick, and A. Smette.