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

TL;DR

This study reports the discovery of stable titanium in the northeastern jet of Cassiopeia A, suggesting the jet was formed by a weak, sub-energetic explosion process rather than the primary supernova engine.

Contribution

The paper provides the first detection of stable titanium in Cassiopeia A's jet, linking its presence to incomplete silicon burning and a weak jet formation mechanism.

Findings

Stable Ti coexists with other elements at the jet tip.

Ti production indicates sub-energetic explosion conditions.

Jet formation likely involved a low-temperature, weak jet process.

Abstract

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.