2011MNRAS.412.1221B

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 ≲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 3σ upper limit on its 1-GHz surface brightness of 7 {x} 10^–22 W/m2/Hz/sr, 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 ∼13 arcmin and an unusual double-shell structure. The second, G21.45-0.59, ∼1 arcmin in diameter, is likely an H II region.