Dong Xu, Johan P.U. Fynbo (DARK/NBI), Nial R. Tanvir (Univ. Leicester),
Jens Hjorth, Giorgos Leloudas, Daniele Malesani (DARK/NBI), Pall
Jakobsson (Univ. Iceland), Paul A. Wilson (NOT and Univ. Oslo), Johannes
Andersen (NBI and NOT), report on behalf of a larger collaboration:
We observed the optical afterglow of GRB 091020 (Racusin et al., GCN
10048; Gorbovskoy et al., GCN 10052) with the Nordic Optical Telescope
equipped with ALFOSC. We detect the afterglow with a magnitude R ~ 20,
based on a nearby USNO star, with mean time Oct 21.220 UT (0.315 days
after the GRB).
We secured low-resolution spectroscopy (30 min exposure), covering the
wavelength range 3500-9100 AA, starting on Oct. 21.231 UT. Wavelength
calibration was carried out using archival frames.
In a preliminary analysis we detect several absorption features, which
we interpret as FeII 2344, 2374, 2382, Al III 1854, 1862, Mg II 2798 (Mg
II 2803 is blended with the telluric A band), all at the common z =
1.71. There is flux in the spectrum down to ~3800 AA, setting a firm
upper limit z < 2.13 to the GRB redshift.