GCN Circular 19600
Subject
GRB 160625B: VLT/X-shooter redshift
Date
2016-06-27T09:50:38Z (8 years ago)
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.