A new class of nonthermal radio sources

Abstract

Current radio catalogues^1,2 list some 145 sources as galactic supernova remnants which are included primarily on the basis of their exhibiting a nonthermal radio spectrum, evidence for extended emission and being located within a few degrees of the galactic plane. Detailed maps including spatially-resolved spectral information and polarimetric data have been obtained for about half of the catalogued objects, primarily those lying within ∼5 kpc of the Sun, and have revealed three classes of remnants: shell-like sources, Crab-like synchrotron nebulae and composite remnants^3,4 containing a central Crab-like component surrounded by a shell. While mapping many of the remnants not well studied to date, we have discovered at least two nonthermal extended objects which defy classification in any of these categories. The two sources exhibit a marked axial symmetry with evidence for a compact component at one extreme of the axis. Their diffuse emission includes bright filamentary structures superimposed on a lower surface brightness component which gradually fades, becoming undetectable at the end of the axis opposite the compact source. The mean spectral indices of these sources are intermediate between the shell- and Crab-like remnants. We propose here that these objects are representative of a new class of nonthermal radio sources; elsewhere in this issue⁵, we suggest that such sources are formed through the emission of relativistic particles produced by high-velocity accreting binary systems.