A super-solar metallicity for the progenitor of Kepler's supernova

We have performed deep X-ray observations of the remnant of Kepler's supernova (SN 1604) as a Key Project of the Suzaku Observatory. Our main goal is to detect secondary Fe-peak elements in the supernova (SN) ejecta to gain insights into the Type Ia SN explosion mechanism and the nature of the progenitor. Here, we report our initial results. We made a conclusive detection of X-ray emission lines from highly ionized Mn, Cr, and Ni as well as Fe. The observed Mn-to-Cr line flux ratio is ∼0.60, ∼30% larger than that measured in Tycho's remnant. We estimate an Mn-to-Cr mass ratio of ∼0.77, which is strongly suggestive of a large neutron excess in the progenitor star before the onset of the thermonuclear runaway. The observed Ni-to-Fe line flux ratio (∼0.03) corresponds to a mass ratio of ∼0.06, which is generally consistent with the products of the explosive Si-burning regime in Type Ia explosion models, and rules out contamination from the products of neutron-rich nuclear statistical equilibrium in the shocked ejecta. Together with the previously suggested luminous nature of the explosion, these mass ratios provide strong evidence for a super-solar metallicity in the SN progenitor (∼3 Z_·). Kepler's SN was likely the thermonuclear explosion of a white dwarf formed in the recent past that must have exploded through a relatively prompt channel.