2006CBET..461....1B

S. Blondin, M. Modjaz, R. Kirshner, and P. Challis, Harvard-Smithsonian Center for Astrophysics; and T. Matheson, National Optical Astronomy Observatory, report that a spectrum (range 350-740 nm) of SN 2006bb (cf. CBET 444, IAUC 8693), obtained on Apr. 2.21 UT by V. Antoniou with the F. L. Whipple Observatory 1.5-m telescope (+ FAST), shows this supernova to be a type-Ia supernova, with a spectral-feature age (Riess et al. 1997, A.J. 114, 722) of roughly two weeks past maximum light. SN 2006bb was classified as a type-Ic supernova around maximum light in CBET 454, on the basis of a "W-shaped" feature around 500 nm. This feature is due to unresolved blends of Fe II lines and is common to both type-Ia and type-Ic post-maximum spectra – hence the possible confusion. However, cross-correlation with a library of supernova spectra shows that SN 2006bb is similar to the type-Ia SN 1992A at 11 days past maximum, at an approximate redshift of z = 0.016 ± 0.010 (cz = 4800 ± 3000 km/s – slightly lower than the recession velocity of its host galaxy UGC 4468, cz = 7563 km/s, as found by Falco et al. 1999, PASP 111, 438). The best-match type-Ic spectrum is that of SN 2004fe at 10 days past maximum, which does not reproduce the spectroscopic features of SN 2006bb redward of 500 nm. In particuliar, the Si II 635.5-nm feature is not a good match. A comparison plot showing the spectrum of SN 2006bb along those of the type-Ia SN 1992A and the type-Ic SN 2004fe can be seen at http://cfa-www.harvard.edu/oir/Research/supernova/spectra/sn2006bb_comp.gif Adopting 7563 km/s for the recession velocity of the supernova, the maximum absorption in the Si II 635.5-nm line is blueshifted by roughly 12500 km/s.