[vsnet-grb-info 21147] GRB 180914B: VLT/X-shooter spectroscopic redshift

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  23246
SUBJECT: GRB 180914B: VLT/X-shooter spectroscopic redshift
DATE:    18/09/17 15:15:17 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani at dark-cosmology.dk>

P. D'Avanzo (INAF/OABr), K. E. Heintz (Univ. Iceland and DAWN/NBI), A. 
de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), A. J. Levan (Univ. 
Warwick), L. Izzo (HETH/IAA-CSIC), D. B. Malesani (DAWN/NBI and 
DARK/NBI), D. A. Kann (HETH/IAA-CSIC), N. R. Tanvir (Univ. Leicester), 
report on behalf of the Stargate collaboration:

We observed the optical counterpart (e.g. Zheng & Filippenko, GCN 23237) 
of GRB 180914B, detected by AGILE (Ursi et al., GCN 23226; Verrecchia et 
al., GCN 23231), Fermi/LAT (Bissaldi & Longo, GCN 23232), and Konus-Wind 
(Frederiks et al., GCN 23240). We used the X-shooter spectrograph on the 
ESO VLT UT2 (Kueyen), for a total exposure of 2x600 s, starting on 2018 
September 17.07 UT (2.30 days after the GRB). The covered wavelength 
range is 3000-21000 AA, and the seeing was relatively poor, around 1.5".

 From a 15-s acquisition image, we measure the magnitude of the 
afterglow as r = 20.37 +- 0.04 (AB), calibrated against the Pan-STARRS 
catalog.

Several absorption features are detected superimposed on the afterglow 
continuum. Among them, we identify Mn II, Al III, Fe II, Mg II, Mg I, Ca 
II, all at the common redshift of z = 1.096. At the same redshift, 
fine-structure lines from Fe II are also identified, thus firmly 
establishing this value as the GRB redshift. A few emission lines from 
the underlying host are also visible. We identify the [O II] doublet and 
[O III] 5008, while other commonly observed features lie at this 
redshift in regions of poor atmospheric transparency or low S/N.

We note that our spectroscopic value is larger than the photometric 
redshift of the putative host galaxy as listed in the SDSS catalog 
(https://skyserver.sdss.org/dr14/en/tools/explore/Summary.aspx?id=1237680331629267013), 
which is however based on low-S/N photometry. At z = 1.096, the 
isotropic-equivalent energy radiated in gamma-rays is 3.6*10^54 erg, 
using the Konus-Wind fluence (1.15*10^-3 erg cm^-2; Frederiks et al., 
GCN 23240). This ranks among the highest measured values for GRBs.

We acknowledge the ESO staff at Paranal for their support, in particular 
Willem-Jan de Wit, Romain Thomas and Rodrigo Palominos.