N. R. Tanvir (U. Leicester), D. Xu (NAO/CAS), T. Kruehler (MPE),
D. Malesani, J. P. U. Fynbo (DARK/NBI), G. Pugliese (U. Amsterdam),
A. J. Levan (U. Warwick), Z. Cano (U. Iceland) report on behalf
of a larger collaboration:
We observed the optical counterpart of the unusual
GRB 160425A (Krimm et al., GCN 19343) with the
ESO Very Large Telescope UT 2 (Kueyen) equipped
with the X-shooter spectrograph, covering the wavelength
range 3500-20000 AA. Observations started on April 26.26 UT
(approx 6.9 hr after the GRB) and consisted of 4 exposures of
1200 s each.
In our acquisition images, we detect an extended object
consistent with the position of the UVOT counterpart,
whose morphology exhibits two knots separated by ~1.2".
That object is faintly visible in the digitized sky survey,
and is likely the host galaxy of GRB 160425A.
The spectrum reveals two distinct systems at z=0.555, based
on emission lines of Halpha, Hbeta, [O III], [O II] and [S III].
The velocity separation between them is ~180 km/s, and the
spatial separation is consistent with the two knots seen in
the imaging (corresponding to ~8 kpc at this distance),
suggesting an interacting pair of galaxies.
We acknowledge expert support from the ESO observing staff,
particularly Jose Velazquez and Juan Carlos Munoz, in obtaining