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):
RA = 08:34:26.11
Dec = -59:10:04.2
RA = 08:34:25.73
Dec = -59:10:04.8
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.