The Deepest Radio Study of the Pulsar Wind Nebula G21.5-0.9: Still No Evidence for the Supernova Shell

TL;DR

This study used sensitive 1.4-GHz VLA radio observations to search for a supernova shell around the pulsar wind nebula G21.5-0.9 but found no evidence, while identifying a new supernova remnant and an HII region in the vicinity.

Contribution

The paper provides the deepest radio search for the supernova shell around G21.5-0.9 and reports the discovery of a new supernova remnant and an HII region.

Findings

No radio emission detected from the supernova shell around G21.5-0.9.

Established a low surface brightness upper limit for the shell.

Identified a new shell-type supernova remnant G21.64-0.84 and an HII region G21.45-0.59.

Abstract

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.