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 similar to 270 and similar to 310 days since the explosion are presented. Emission lines ([O I] lambda lambda 6300, 6363; [ Ca II] lambda lambda 7291, 7324; and [Fe II] lambda 7155) show a blueshift of similar to 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 similar to 0.1-0.4M(circle dot), in which similar to 0.02-0.06M(circle dot) is Ni-56 synthesized at the explosion. To explain the blueshift, the blob should either be unipolar, moving at the center-of-mass velocity nu similar to 2000-5000 km s(-1), or suffer from self-absorption within the ejecta, as seen in SN 1990I. In bothinterpretations, the low-mass blob component dominates the optical output both at the first peak (similar to 20 days) and at the late phase (similar to 300 days). The low luminosity at the late phase (the absolute R magnitude M-R similar to -10.2 mag at similar to 270 days) sets the upper limit for the mass of 56Ni P 0: 08 M-circle dot, which is in contradiction to the value necessary to explain the second, main peak luminosity (M-R similar to -18.3 mag at similar to 40 days). Encountered by this difficulty in the Ni-56 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 similar to 10(14)-10(15) G and the initial spin period P-0 similar to 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.