Sn 2006aj Associated with Xrf 060218 at Late Phases: Nucleosynthesis-signature of a Neutron Star-driven Explosion

Optical spectroscopy and photometry of SN 2006aj have been performed with the Subaru telescope at t > 200 days after GRB060218, the X-ray Flash with which it was associated. Strong nebular emission-lines with an expansion velocity of v ∼ 7, 300 km s were detected. The peaked but relatively broad [OI]λλ6300, 6363 suggests the existence of ∼ 2M⊙ of materials in which ∼ 1.3M⊙ is oxygen. The core might be produced by a mildly asymmetric explosion. The spectra are unique among SNe Ic in (1) the absence of [CaII]λλ7291, 7324 emission, and (2) a strong emission feature at ∼ 7400Å, which requires ∼ 0.05M⊙ of newly-synthesized Ni. Such a large amount of stable neutron-rich Ni strongly indicates the formation of a neutron star. The progenitor and the explosion energy are constrained to 18M⊙ ∼ < Mms ∼ < 22M⊙ and E ∼ (1− 3)× 10 51 erg, respectively. Subject headings: gamma-rays: bursts – supernovae: general – supernovae: individual (SN 2006aj) – nuclear reactions, nucleosynthesis, abundances