TITLE:   GCN CIRCULAR
NUMBER:  22591
SUBJECT: GRB 180404A: VLT/X-shooter redshift
DATE:    18/04/04 03:15:44 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani@dark-cosmology.dk>

J. Selsing (DARK/NBI), D. Malesani (DAWN/NBI and DARK/NBI), D.A. Kann 
(HETH/IAA-CSIC), N.R. Tanvir (Univ. Leicester) report on behalf of the 
Stargate consortium:

We observed the optical counterpart of GRB 180404A (Gropp et al., GCN  
22589) using the ESO Very Large Telescope (VLT) UT2 equipped with the 
X-shooter spectrograph. Observations were carried out in robotic "rapid 
response mode" (RRM), initiated automatically following the X-ray 
localization notice.

In the 15-s r-band acquisition image, taken on 2018 Apr 4.0423 UT (15.3  
min after the trigger), the afterglow is clearly detected, at 
coordinates (J2000):

RA = 05:34:11.67
Dec = -37:10:04.1

We measure a magnitude R = 18.64 +- 0.1 (Vega), calibrated against 
nearby USNO stars, where the error is dominated by the calibration scatter.

Several spectra were acquired. We report here on the exposure with mid 
epoch Apr 4.0495 UT (that is, 25.6 min after the GRB), with 600 s 
exposure time, and covering the wavelength range 3000-25000 AA. The 
afterglow continuum trace is clearly detected. Superposed on it, several 
absorption features are visible, which we identify as Al III, Fe II, Mg 
II, and Mg I, all at a common redshift redshift of z = 1.000.

We acknowledge excellent support from the observing staff at Paranal, in 
particular Stephane Brillant, Luca Sbordone, Andrea Mehner, and Jose 
Velasquez.