ShellSNRs

Shell-type supernova remnants, or shell SNRs, are remnants in which the observable emission is dominated by a roughly spherical shell tracing the outward-moving blast wave. The interior is comparatively faint, though some remnants may show interior filaments or a central source if a pulsar wind nebula is present in a composite system.

The shell is typically limb-brightened, forming a circular or elliptical ring seen across radio, optical, and X-ray bands. The appearance is strongly influenced by the density structure of the surrounding interstellar medium and by the remnant’s age; older remnants can develop radiative cooling shells along their perimeters, while younger ones may show sharper, brighter rims.

The dominant radiation mechanism in most shell SNRs is nonthermal synchrotron emission from relativistic electrons accelerated at the blast wave by diffusive shock acceleration. Radio spectra are characteristically negative, often around -0.5 to -0.7. X-ray emission can be nonthermal in young shells or thermal from shock-heated ambient gas in older shells; some remnants also display optical and infrared filaments from radiative shocks. In certain cases, gamma-ray emission is detected, arising from hadronic interactions or inverse-Compton processes.

Notable examples include SN 1006 and Tycho’s SNR in the Milky Way, RCW 86, and the Cygnus Loop. Shell SNRs provide important laboratories for studying cosmic-ray acceleration and the impact of supernova shocks on the interstellar medium, with observations spanning radio, optical, X-ray, and gamma-ray wavelengths.

In the broader context of supernova remnants, shell SNRs are one of the standard classifications, alongside plerions powered by central pulsar wind nebulae and composite remnants that combine a shell with a central nebula.