TITLE:   GCN CIRCULAR
NUMBER:  32850
SUBJECT: GRB 221009A: spectroscopic confirmation of SN in LBT spectra
DATE:    22/10/25 16:50:09 GMT
FROM:    Andrea Rossi at INAF  <andrea.rossi@inaf.it>

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.