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GCN Circular 18540

Subject
GRB 151031A: VLT/X-shooter optical counterpart and redshift
Date
2015-10-31T07:23:44Z (8 years ago)
From
Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst <malesani@dark-cosmology.dk>
D. Malesani (DARK/NBI), N. R. Tanvir (Univ. Leicester), T. Kruehler 
(MPE), A. de Ugarte Postigo (IAA-CSIC and DARK/NBI), H. Flores 
(GEPI/Paris obs), D. Xu (NAOC/CAS), J.P.U. Fynbo (DARK/NBI), report on 
behalf of the X-shooter GRB collaboration:

We observed the field of GRB 151031A (Sonbas et al., GCN 18538) with the 
ESO Very Large Telescope UT2 (Kueyen) equipped with the X-shooter 
spectrograph. Observations were carried out using the automatic "rapid 
response mode" (RRM), and started immediately following the reception of 
the XRT position notice.

In the acquisition images, we detect a single source consistent with the 
position of the XRT afterglow, at coordinates (J2000):

RA = 05:32:47.11
Dec = -39:07:19.1

We measure for this object R = 20.4 +- 0.1 (calibrated against three 
nearby USNO stars) in an image taken on Oct 31.2565 UT, that is 18.8 min 
after the GRB.

A sequence of spectra with increasing exposure time (3 to 32 min) was 
secured covering the wavelength range 3,000 - 25,000 AA. From the 
analysis of the first 8 min data, several absorption features can be 
seen on top of the object continuum, which we interpret as due to Ca II, 
Mg I, Mg II and Fe II in absorption at a common redshift of z = 1.167. 
In the visible arm, an emission feature is also seen, corresponding to 
[O ii] at a consistent redshift, which we thus consider to be the 
redshift of GRB 151031A. Analysis of the full data set is ongoing.

We acknowledge excellent support from the ESO observing staff in 
Paranal, in particular Thomas Rivinius, Marcela Espinoza, and Claudia Cid.
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