D. Xu, D. Malesani (DARK/NBI), N. Tanvir (U. Leicester), T. Kruehler (ESO),
J. Fynbo (DARK/NBI) report on behalf of the X-shooter GRB
We have obtained spectroscopic observations of the afterglow of GRB 131105A
(Cummings et al., GCN 15446; Xu et al., GCN 15447), beginning at 03:46 UT,
approximately 102 minutes post-trigger. We used the ESO VLT equipped
with the X-shooter spectrograph, covering the wavelength range 3000-20000 AA.
The exposure time was 8x600 s. In the NIR arm, we detect two emission lines,
interpreted as [O III] (5007) and H-alpha at a common redshift z = 1.686.
In the spectrum we also detect absorption lines at the same redshift, albeit at
low signal-to-noise ratio. Furthermore, we measure significant fading of the source
between two sets of acquisition images, confirming that we are seeing
afterglow and host light. Hence, we consider z = 1.686 the likely redshift of
GRB 131105A. The afterglow appears to be red presumably due to dust extinction.
We acknowledge excellent support from the observing staff at Paranal, in
particular Dimitri Gadotti, Roger Wesson, and Claudia Reyes.