TITLE:   GCN CIRCULAR
NUMBER:  19600
SUBJECT: GRB 160625B: VLT/X-shooter redshift
DATE:    16/06/27 09:50:38 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani@dark-cosmology.dk>

D. Xu (NAOC/CAS), D. Malesani (DARK/NBI), J. P. U. Fynbo (DARK/NBI), N. 
R. Tanvir (Univ. Leicester), A. J. Levan (Univ. Warwick), D. A. Perley 
(DARK/NBI), report on behalf of a larger collaboration:

We observed the optical afterglow of the Fermi GRB 160625B (Dirirsa et 
al., GCN 19580; Burns, GCN 19581; Troja et al., GCN 19588) with the ESO 
VLT UT2 (Kueyen) equipped with the X-shooter spectrograph. Observations 
started on 2016 June 27.19 UT (1.25 days after the Fermi trigger) and 
consisted of 4 spectra by 600 s each, covering the wavelength range 
3500-20000 AA.

The afterglow is well detected in the acquisition image with R ~ 19.1 +- 
0.3, where the large error is due to scatter in the USNO magnitudes used 
as reference, and the r versus R filter transformation.

The continuum trace is well detected in our exposures, down to ~3100 AA. 
The lack of any drop sets an upper limit to the redshift z < 1.55. A 
number of superimposed absorption features are visible; among others, we 
detect Mg I, Mg II, Fe II, C IV, Si IV, O I, Si II, Zn II, Al II, and Al 
III, all at a common redshift z = 1.406, which we conclude to be the 
redshift of GRB 160625B. We also detect an intervening Mg II absorption 
system at z = 1.319.

At z = 1.406, the fluence in the 10-1000 keV Fermi band (Burns, GCN 
19587) corresponds to an isotropic-equivalent energy of ~2.5*10^54 erg.

We acknowledge excellent support from the ESO observing staff in 
Paranal, in particular Juan Carlos Munoz, Elizabeth (Liz) Bartlett, 
Dimitri Gadotti and Francisco Caceres.