GRB 221009A: 10.4m GTC spectroscopic redshift confirmation

Following the detection of the extraordinarily bright GRB 221009A detected by Swift, Fermi, MAXI/GSC, INTEGRAL (SPI-ACS), Konus-Wind, the IPN, AGILE/MCAL, SolO/STIX, SRG/ART-XC, CALET and GRBAlpha (Dichiara et al. GCNC 32632, Veres et al. GCNC 32636, Bissaldi et al. GCNC 32637, Svinkin et al. GCNC 32641, Negoro et al. ATEL 15651, Ursi et al. GCNC 32650, Xiao et al GCNC 32661, Lapshov et al. GCNC 32663, Cannady et al. GCNC 32674, Ripa et al. GCNC 32685), we triggered the 10.4m Gran Telescopio de Canarias (GTC) equipped with Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) in La Palma (Spain), starting on Oct 9, 22:19 UT (~9 hrs after the GBM trigger). Spectroscopy was obtained with both the R1000B (2x900s) and R1000R (2x300s) grisms, covering the 363-1000 nm spectral range. A red continuum is noticeable, in agreement with earlier reports by Perley (GCNC 32638) and Xu et al. (GCNC 32647). The GTC spectrum clearly shows the Ca II H & K absorption doublet implying a redshift of z=0.1505, consistent the value derived from X-shooter/VLT (de Ugarte Postigo et al. GCNC 32648). With this redshift of z= 0.1505, a time-averaged peak energy of 2.52 MeV and a total fluence of 2.6e-2 erg cm^-2, we found that the main emission episode (between 174 and 700 s post trigger) of GRB 221009A is consistent with the Type II (collapsar origin; Zhang et al, 2007, 2009) bursts in the Ep-Eiso diagram (Amati et al. 2002). We thank the staff at GTC for their excellent support.