Is Vela Jr. a young supernova remnant?

The supernova remnant RX J0852.0-4622 (or Vela Jr.) has been considered to be very young (∼680 yr) based mainly on the possible detection of a radioactive decay line of 44Ti whose lifetime is only ∼90 yr. Here, we report on expansion measurements of the northwestern rim of this remnant, based on XMM-Newton observations separated by 6.5 yr. The expansion rate is derived to be 0.026±0.05±0.09%yr-1 (the first and second-term errors are responsible for statistical and systematic uncertainties, respectively). We estimate the age of this remnant to be ∼∼1000–3000 yr. Therefore, Vela Jr. turns out to be not so young as it was considered previously. The distance to this remnant is also derived to be at least ∼740 pc, assuming a high shock speed of 3000kms-1.