2006CBET..410....1F

D. Fugazza and P. D'Avanzo, Istituto Nazionale di Astrofisica (INAF) and Osservatorio Astronomico di Brera (OAB); D. Malesani, International School for Advanced Studies; M. Della Valle, INAF and Osservatorio Astrofisico di Arcetri; E. Pian, INAF and Osservatorio Astrononomico di Trieste; G. Chincarini; Universita degli Studi di Milano-Bicocca; L. Stella, INAF and Osservatorio Astronomico di Roma; G. Tagliaferri, INAF and OAB; J. Deng, Chinese Academy of Sciences; and K. Maeda and K. Nomoto, University of Tokyo, report that they acquired a spectrogram (resolution 1.0 nm) with the 3.6-m Telescopio Nazionale Galileo on Feb. 20.93 UT of the optical counterpart of GRB 060218 (cf. Cusumano et al., GCN 4775; IAUC 8674). Inspection of the spectrum reveals a smooth continuum below several nebular emission lines. Moreover, broad deviations from the power-law shape are clearly visible, suggestive of bumps similar to those of broad-lined type-Ib/c supernovae. In particular, a bump at about 500 nm is consistent with the feature defined by the absorption troughs due to Fe II and Si II, as observed in SN 1998bw (Patat et al. 2001, Ap.J. 555, 900). The spectrum is well matched by the superposition of a power law plus the spectrum of SN 1998bw obtained at about one week before maximum. The strength of the features implies that the supernova was contributing about half of the received light, the remaining part being due to a power-law continuum.