Overabundance of Calcium in the young SNR RX J0852-4622: evidence of over-production of ⁴⁴Ti

Recently, COMPTEL has detected $\gamma$-rays of 1157 keV from $^{44}$Ti in the direction of the SNR RX J0852$-$4622 (Iyudin et al. 1998). Since $^{44}$Ti is a product of explosive nucleosynthesis and its half lifetime $\tau\sb{1/2}$ is about 60 yrs, RX J0852$-$4622 must be a young supernova remnant and radiation is dominated by the ejecta rather than by interstellar matter. We have detected an X-ray emission line at $4.1\pm 0.2$ keV which is thought to come from highly ionized Ca. The emission line is so far only seen in the north-west shell region of RX J0852$-$4622. The X-ray spectrum can be well fitted with that of thin hot plasma of cosmic abundances except that of Ca, which is overabundant by a factor of $8 \pm 5$. Assuming that most of Ca is $^{44}$Ca, which originates from $^{44}$Ti by radioactive decay, we estimate a total Ca mass of about $1.1\times 10^{-3}M_\odot$. Combining the amount of $^{44}$Ca and the observed flux of the $^{44}$Ti $\gamma$-ray line, the age of RX J0852$-$4622 is around 1000 yrs.