TITLE:   GCN CIRCULAR
NUMBER:  32686
SUBJECT: GRB 221009A: 10.4m GTC spectroscopic redshift confirmation
DATE:    22/10/11 13:37:44 GMT
FROM:    Alberto J. Castro-Tirado at IAA-CSIC  <ajct@iaa.es>

A. J. Castro-Tirado, R. Sanchez-Ramirez, Y.-D. Hu, M.D. 
Caballero-Garcia, M. A. Castro Tirado, E. Fernandez-Garcia, I. 
Perez-Garcia (IAA-CSIC), G. Lombardi (GTC, IAC), S. B. Pandey (ARIES), 
J. Yang (NJU) and B.-B. Zhang (NJU) on behalf of a larger collaboration, 
report:

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.