|Radio images of the pulsar-powered supernova remnant G21.5−0.9,
from 2006 and 1991, visually showing the expansion over the
15-year period. The two images are on the same scale, and the
larger and smoother of the two is that from 2006, with the
smoother appearance being due to the higher signal-to-noise.
Between 1991 and 2006, we find that the average expansion rate
of the remnant is 0.11 +/− 0.02 % year-1, corresponding, for a
distance of 5 kpc, to a speed of 910 +/− 160 km s-1 wrt. the
centre of the nebula. Assuming undecelerated expansion, this
expansion speed implies that the age of G21.5−0.9 is 870 (+200,
−150) yr, PSR J1833−1034 one of the youngest, if not the
youngest, known pulsars in the Galaxy. See
http://lanl.arxiv.org/abs/0802.015 for more details.