E. Maiorano, E. Palazzi, A. Rossi (INAF-OAS), V. D'Elia (SSDC &
INAF-OAR), and A. Melandri (INAF-OAR) on behalf of the CIBO
collaboration, and F. Cusano (INAF-OAS), L. Izzo (DARK/NBI) report:
We report the results of the spectroscopic follow-up observations of the
afterglow of GRB 221009A (Dichiara et al., GCN 32632; Fermi GBM
detection: Veres et al., GCN 32636). The optical spectra were obtained
with the Multi-Object Double Spectrographs (MODS) instrument mounted on
the 2x8.4-m LBT telescope (Mt. Graham, AZ, USA) at 3 UT on 2022-10-18,
8.56 days after the burst trigger. The spectra cover the wavelength
range 3200-10000 AA, and we obtained a total of 6 exposures of 900 s.
After correction for the foreground Galactic extinction (E(B-V)=1.324,
Schlafly & Finkbeiner 2011), we find that the resulting spectrum is
clearly dominated by the afterglow. We subtracted the afterglow
contribution assuming a spectral slope -0.7 (using the convention F_nu ~
nu^-beta), anchored to the simultaneous NIR flux. This value has been
obtained modeling the NIR light curve of the afterglow (Ferro et al.,
GCN 32804; D'Avanzo et al., GCN 32755). The remaining low S/N spectrum
shows features typical of type Ic-BL supernovae. Therefore, we confirm
the emerging contribution of SN 2022xiw (de Ugarte Postigo et al. et al.
TNSCR, 2022-3047) as reported by de Ugarte Postigo et al. (GCN 32800).
We acknowledge the excellent support from the LBTO and LBT-INAF staff,
particularly A. Becker and D. G. Huerta, in obtaining these observations.