The Deepest Radio Study of the Pulsar Wind Nebula G21.5-0.9: Still No
  Evidence for the Supernova Shell
  We report on sensitive new 1.4-GHz VLA radio observations of the pulsar wind
nebula G21.5-0.9, powered by PSR J1833-1034, and its environs. Our observations
were targeted at searching for the radio counterpart of the shell-like
structure seen surrounding the pulsar wind nebula in X-rays. Some such radio
emission might be expected as the ejecta from the <~ 1000 yr old supernova
expand and interact with the surrounding medium. We find, however, no radio
emission from the shell, and can place a conservative 3-sigma upper limit on
its 1-GHz surface brightness of 7 x 10^-22 W/m^2/Hz/sr, comparable to the
lowest limits obtained for radio emission from shells around other pulsar-wind
nebulae. Our widefield radio image also shows the presence of two extended
objects of low-surface brightness. We re-examine previous 327-MHz images, on
which both the new objects are visible. We identify the first, G21.64-0.84, as
a new shell-type supernova remnant, with a diameter of ~13' and an unusual
double-shell structure. The second, G21.45-0.59, ~1' in diameter, is likely an
HII region.
