2008ApJ...678L..35K

The detection of the radioactive decay line of ⁴⁴Ti provides unique evidence that the γ-ray source is a young (<1000 yr) supernova remnant because of the short ⁴⁴Ti lifetime of ∼90 yr. Only two Galactic remnants, Cassiopeia A and RX J0852.0-4622, have hitherto been reported as ⁴⁴Ti line emitters, although the detection from the latter has been debated. Here we report on an expansion measurement of the northwestern rim of RX J0852.0-4622 obtained with X-ray observations separated by 6.5 yr. The expansion rate is derived to be 0.023%±0.006%, which is about 5 times lower than those of young historical remnants. Such a slow expansion suggests that RX J0852.0-4622 is not as young a remnant as has been expected. We estimate an age of 1700-4300 yr for this remnant, depending on its evolutionary stage. Assuming a high shock speed of ∼3000 km/s, which is suggested by the detection of nonthermal X-ray radiation, the distance of ∼750 pc to this remnant is also derived.