The Unique Type Ib Supernova 2005bf at Nebular Phases: A Possible Birth Event of a Strongly Magnetized Neutron Star

Late-phase nebular spectra and photometry of Type Ib Supernova (SN) 2005bf taken by the Subaru telescope at ~270 and ~310 days since the explosion are presented. Emission lines ([O I] λλ6300, 6363; [Ca II] λλ7291, 7324; and [Fe II] λ7155) show a blueshift of ~1500-2000 km s -1 . The [O I] doublet shows a doubly peaked profile. The line luminosities can be interpreted as coming from a blob or jet containing only ~0.1-0.4 M ☉ , in which ~0.02-0.06 M ☉ is 56 Ni synthesized at the explosion. To explain the blueshift, the blob should either be unipolar, moving at the center-of-mass velocity v ~ 2000-5000 km s -1 , or suffer from self-absorption within the ejecta, as seen in SN 1990I. In both interpretations, the low-mass blob component dominates the optical output both at the first peak (~20 days) and at the late phase (~300 days). The low luminosity at the late phase (the absolute R magnitude M R ~ -10.2 mag at ~270 days) sets the upper limit for the mass of 56 Ni ≲ 0.08 M ☉ , which is in contradiction to the value necessary to explain the second, main peak luminosity ( M R ~ -18.3 mag at ~40 days). Encountered by this difficulty in the 56 Ni heating model, we suggest an alternative scenario in which the heating source is a newly born, strongly magnetized neutron star (a magnetar) with the surface magnetic field B mag ~ 10 14 -10 15 G and the initial spin period P 0 ~ 10 ms. Then, SN 2005bf could be a link between normal SNe Ib/c and an X-ray flash associated SN 2006aj, connected in terms of B mag and/or P 0 .