  Of the 30 or so Galactic magnetars, about 8 are in supernova remnants (SNRs).
One of the most extreme magnetars, 1E 1841-045, is at the center of the SNR Kes
73 (G27.4+0.0), whose age is uncertain. We measure its expansion using three
Chandra observations over 15 yr, obtaining a mean rate of 0.023% +/- 0.002% per
yr. For a distance of 8.5 kpc, we obtain a shell velocity of 1100 km/s and
infer a blast-wave speed of 1400 km/s. For Sedov expansion into a uniform
medium, this gives an age of 1800 yr. Derived emission measures imply an
ambient density of about 2 cm$^{-3}$ and an upper limit on the swept-up mass of
about 70 solar masses, with lower limits of tens of solar masses, confirming
that Kes 73 is in an advanced evolutionary stage. Our spectral analysis shows
no evidence for enhanced abundances as would be expected from a massive
progenitor. Our derived total energy is $1.9 \times 10^{51}$ erg, giving a very
conservative lower limit to the magnetar's initial period of about 3 ms, unless
its energy was lost by non-electromagnetic means. We see no evidence of a
wind-blown bubble as would be produced by a massive progenitor, or any evidence
that the progenitor of Kes 73/1E 1841-045 was anything but a normal red
supergiant producing a Type IIP supernova, though a short-lived
stripped-envelope progenitor cannot be absolutely excluded. Kes 73's magnetar
thus joins SGR 1900+14 as magnetars resulting from relatively low-mass
progenitors.
