[vsnet-grb-info 15714] GRB 141031A: VLT/X-shooter optical observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17002
SUBJECT: GRB 141031A: VLT/X-shooter optical observations
DATE:    14/10/31 18:46:44 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani at dark-cosmology.dk>

D. Xu (DARK/NBI), A. de Ugarte Postigo (IAA/CSIC and DARK/NBI), S. D. 
Vergani (CNRS/GEPI), A. J. Levan (Univ. Warwick), J. P. U. Fynbo 
(DARK/NBI), N. R. Tanvir (Univ. Leicester), B. Milvang-Jensen 
(DARK/NBI), D. Malesani (DARK/NBI), K. Wiersema (Univ. Leicester), 
report on behalf of a larger collaboration:


We observed the field of GRB 141031A (Gompertz et al., GCN 16995) using 
the VLT X-shooter spectrograph, using the robotic rapid response mode (RRM).

In a 15-s acquisition image, taken at 7:41:19 UT (22.8 min after the 
GRB1), we identify three objects consistent with, or in the close 
vicinity of, the currently available XRT error circle (1.4" error 
radius; Evans et al., GCN 16997 and http://www.swift.ac.uk/xrt_positions/).

Their coordinates are (all J2000):

A)
RA = 08:34:26.11
Dec = -59:10:04.2

B)
RA = 08:34:25.73
Dec = -59:10:04.8

C)
RA = 08:34:26.05
Dec = -59:10:04.9

Sources A and C are partially blended. Source B is the one detected by 
GROND (GCN 16996), looks extended in our data (under 0.6" seeing), and 
is now formally outside the latest XRT position (2.7"+-1.4" away). The 
limiting magnitude of the image is R ~ 21.7. A finding chart is 
available at http://www.astro.ku.dk/~malesani/GRB/141031A/finder_XS.jpg

A later image (30 s exposure) was taken 104.5 min after the GRB. Image 
subtraction reveals no obvious variation of any of the sources, despite 
the significant time difference between the two epochs (factor of 4.5 in 
time since GRB).

A spectrum was secured of source A, covering the wavelength range 
3000-25,000 AA. Several features are observed in absorption, including 
the Balmer series from Halpha to H15, and Ca H and K, at z = 0.0013. 
Such redshift is consistent with the Milky Way or the Local Group, 
though the corresponding velocity (390 km/s) is substantial for an 
origin in the MW disk.

Our slit did not cover, unfortunately, objects B and C. Further analysis 
is in progress, and we invite further monitoring of these object to 
single out variability.

We acknowldge excellent support from the observing staff in Paranal, in 
particular Juan Carlos Munoz-Mateos, Francisco Caceres, Rodrigo Romero, 
and Steffen Mieske.