2007ApJ...658L...5M

Optical spectroscopy and photometry of SN 2006aj have been performed with the Subaru telescope at t>200 days after GRB 060218, the X-ray flash with which it was associated. Strong nebular emission lines with an expansion velocity of v∼7300 km/s were detected. The peaked but relatively broad [O I] λλ6300, 6363 suggests the existence of ∼2 M_☉of materials in which ∼1.3 M_☉is oxygen. The core might be produced by a mildly asymmetric explosion. The spectra are unique among SNe Ic in (1) the absence of [Ca II] λλ7291, 7324 emission and (2) a strong emission feature at ∼7400 Å, which requires ∼0.05 M_☉of newly synthesized ⁵⁸Ni. 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 18 M_☉≲M_ms≲22 M_☉and E~(1-3)x10⁵¹ ergs, respectively.