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We report, for the first time, the detection of the Mn-K$\alpha$ line in theType IIb supernova (SN IIb) remnant, Cassiopeia A. Manganese ($^{55}$Mn afterdecay of $^{55}$Co), a neutron-rich element, together with chromium ($^{52}$Crafter decay of $^{52}$Fe), is mainly synthesized at the explosive incomplete Siburning regime. Therefore, the Mn/Cr mass ratio with its neutron excessreflects the neutronization at the relevant burning layer during the explosion.Chandra's archival X-ray data of Cassiopeia A indicate a low Mn/Cr mass ratiowith values in the range 0.10--0.66, which, when compared to one-dimensional SNexplosion models, requires that the electron fraction be 0.4990 $\lesssimY_{\rm e} \lesssim$ 0.5 at the incomplete Si burning layer. An explosion modelassuming a solar-metallicity progenitor with a typical explosion energy ($1\times 10^{51}$ erg) fails to reproduce such a high electron fraction. In suchmodels, the explosive Si-burning regime extends only to the Si/O layerestablished during the progenitor's hydrostatic evolution; the $Y_e$ in theSi/O layer is lower than the value required by our observational constraints.We can satisfy the observed Mn/Cr mass ratio if the explosive Si-burning regimewere to extend into the O/Ne hydrostatic layer, which has a higher $Y_{\rm e}$.This would require an energetic ($> 2 \times 10^{51}$ erg) and/or asymmetricexplosion of a sub-solar metallicity progenitor ($Z \lesssim 0.5Z_{\odot}$) forCassiopeia A. The low initial metallicity can be used to rule out a single-starprogenitor, leaving the possibility of a binary progenitor with a compactcompanion (white dwarf, neutron star or black hole). We discuss thedetectability of X-rays from Bondi accretion onto such a compact companionaround the explosion site. We also discuss other possible mass-loss scenariosfor the progenitor system of Cassiopeia A.

A Sub-Solar Metallicity Progenitor for Cassiopeia A, the remnant of a Type IIb Supernova