D. Malesani (DARK/NBI), J. P. U. Fynbo (NBI), K. E. Heintz (Univ.
Iceland and DARK/NBI), M. Stone (Turku Univ.), K. Karhunen (Turku
Univ.), report on behalf of a larger collaboration:
Following our imaging sequence (Malesani et al., GCN 22029), we secured
spectra of the optical afterglow of GRB 171020A (Page et al., GCN
22028), using the AlFOSC spectrograph at the Nordic Optical Telescope. A
total of 4x1200 s exposure was secured using grism #4, covering the
wavelength range ~3600-9400 AA. Spectroscopy started on 2017 Oct 21.005
UT (1 hr after the trigger).
A faint trace is detected down to at least ~4000 A. Several weak
absorption features are detected, which we interpret as due to Mg II, Fe
II, and C IV all at a common redshift z = 1.87.
We thank the NOT staff for support, in particular Amanda Djupvik and