|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.