  The origin of the jet-like structures observed in Cassiopeia A is still
unclear, although it seems to be related to its explosion mechanism. X-ray
observations of the characteristic structures could provide us useful
information on the explosive nucleosynthesis via the observation of elements,
which is a unique approach to understand its origin. We here report the
discovery of shocked stable Ti, which is produced only at the inner region of
exploding stars, in the northeast jet of Cassiopeia A using the 1-Ms deep
observation with the Chandra X-ray observatory. The observed Ti coexists with
other intermediate-mass elements (e.g. Si, S, Ar, Ca) and Fe at the tip of the
X-ray jet structure. We found that its elemental composition is well explained
with the production by the incomplete Si burning regime, indicating that the
formation process of the jet structure was sub-energetic at the explosion (the
peak temperature during the nuclear burning must be $\lesssim$
5$\times$10$^{9}$ K at most). Thus, we conclude that the energy source that
formed the jet structure was not the primary engine for the supernova
explosion. Our results are useful to limit the power of the jet-structure
formation process, and a weak jet mechanism with low temperature may be needed
to explain it.
