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 ⁵⁶Ni 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 19901. 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 ⁵⁶Ni ≲ 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 ⁵⁶Ni 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 ∼ 10¹⁴-10¹⁵ G and the initial spin period P₀ ∼ 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 P₀.