GRB 221009A: Spectroscopic detection of emerging SN features

We have been spectroscopically monitoring the evolution of the extremely bright GRB 221009A (Swift detection: Dichiara et al., GCN 32632; Fermi GBM detection: Veres et al., GCN 32636) using OSIRIS at the 10.4m GTC telescope. In a spectrum combining data taken on 16 and 17 October (at an average time of ~8 days after the burst), covering the spectral range between 3700 and 10000 AA, the emission is still clearly dominated by the afterglow. However, after subtracting a model of the continuum, the spectrum shows undulations characteristic of a GRB-SN. In particular we identify broad features at similar wavelengths (after correcting for the redshift difference) as those present in SN 1998bw, with somewhat larger velocities. This is indicative of the emergence of the associated supernova. Further follow-up observations are planned.