[vsnet-grb-info 15680] GRB 141026A: GTC spectroscopy

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16968
SUBJECT: GRB 141026A: GTC spectroscopy
DATE:    14/10/28 16:59:55 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte at iaa.es>

A. de Ugarte Postigo (IAA-CISC, DARK/NBI), C.C. Thoene (IAA-CSIC),
N.R. Tanvir (U. Leicester), J. Gorosabel (UPV, IAA-CSIC), 
J. Fynbo (DARK/NBI), G. Lombardi (GTC), D. Reverte-Paya (GTC), 
and D. Perez (GTC) report:

We observed the afterglow of GRB 141026A (Hagen et al. GCN 16950,
Perley GCN 16951, Littlejohns et al. GCN 16952, Varela et al. GCN 16953,
Gorosabel et al. GCN 16954, Perley et al. GCN 16955, Beardmore et al.
GCN 16956, Butler et al. GCN 16962,Moskvitin et al. GCN 16966) with
OSIRIS at the 10.4m Gran Telescopio Canarias. Observations consisted of
z-band imaging, and spectroscopy using the R1000R grism (covering from
5100 to 10000 A with a resolution of ~1000). The spectroscopic observations
were delayed due to bad weather and had a mean epoch of 6:12 UT (3.59 
hours after the burst), and consisted of 2x900s exposures. Conditions were 
not optimal, as the airmass at that time was ~1.8 and most of the observation
was performed during twilight.

The spectrum shows a low S/N continuum, which is however, visible
between 5100 and 9500 A. Some absorption features seem to be present,
and in particular we identify a broad feature at 5300 A, with an observed
equivalent width of ~30 A. This could correspond to a weak Ly-alpha
(log(N_H/cm^-2) ~< 20) absorption at z=3.35. However, no other features
corresponding to that same redshift are seen, which is not unexpected for
a low density line of sight that would correspond to such a Ly-alpha feature
and a low S/N spectrum. Further absorptions could correspond to intervening
MgII absorbers.

This redshift solution is supported by the photometric drop seen by GROND
in the g-band (Varela et al. GCN 16953), which would correspond to a similar
photometric redshift.