GRB 221009A: spectroscopic confirmation of SN in LBT spectra

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.