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

Discovery of Stable Titanium at the Northeastern Jet of Cassiopeia A: Need for a Weak Jet Mechanism?