A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), J.P.U. Fynbo (DARK/NBI),
C. Thoene (IAA-CSIC), N.R. Tanvir (U. Leicester), R. Sanchez-Ramirez
(UPV, IAA-CSIC), J. Gorosabel (UPV-EHU, IAA-CSIC), P. Pessev (ULL, IAC),
C. Alvarez Iglesias (ULL, IAC), M.A. Rivero (GRANTECAN) report on behalf
of a larger collaboration:
We obtained spectroscopy of the afterglow of GRB 150314A (Hagen et al.,
GCN 17573; Zheng & Filippenko, GCN 17574; Axelsson et al., GCN 17576;
Trotter et al., GCN 17577; Xu et al. GCN 17582) using OSIRIS at the 10.4 m
GTC telescope in La Palma (Spain). Observations consisted of 3 x 1200 s
exposure using the R1000B grism, covering the range between 3700 and
7875 A at a resolution of ~700 (1" slit). The mean observing time was 21:37 UT
(16.70 hr after the burst).
The spectrum shows a continuum with an average SNR ~ 7 (per resolution
element) on which several weak absorption features can be identified. Our
best match corresponds to lines of FeII, MgII, MgI as well as AlII, AlIII and CIV
at lower significance, all at a common redshift of 1.758, which we propose as
the redshift of the GRB.
At this redshift, the isotropic equivalent energy is Eiso = 6.74e53 erg
considering the fluence reported by Fermi/GBM (Yu et. al GCN 17579) in the
(10-1000) keV window.