The radio rings of Hercules A

Abstract

Hercules A is a powerful, double radio source, about 0.5 Mpc across. Although its two lobes differ, they share a family resemblance in overall form. Along the axis of the somewhat weaker west lobe, however, a unique structure is found: a regular sequence of broken circular rings of enhanced radio intensity. The five rings fit a simple kinematic model in which the rings are projected spherical shells of higher radio brightness. They grew as spherical waves with a uniform speed of expansion, all from one common place of origin, roughly where the narrow input jet widens sharply. The shells drift one after another outward at constant speed with the radiating medium of the lobe. Here we propose a model of inelastic proton–proton collisions within dense filamentary condensations of the input jet for the five initiating particle bursts. Source development contrasts sharply with that inferred for the prototype radio double, Cygnus A, where the radio lobe is nearly stationary and the electrons are newly energized at the outermost end of the lobe. The two sources point to quite distinct mechanisms for generating large radio-emitting plasmas.