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 Very Large Array 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 less than or similar to 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-1 sr-1, comparable to the lowest limits obtained for radio emission from shells around other pulsar wind nebulae. In addition, our wide-field 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 similar to 13 arcmin and an unusual double-shell structure. The second, G21.45-0.59, similar to 1 arcmin in diameter, is likely an H ii region.