Origin of the ring structures in Hercules A -- Sub-arcsecond 144 MHz to
  7 GHz observations
  The prominent radio source Hercules A features complex structures in its
radio lobes. Although it is one of the most comprehensively studied sources in
the radio sky, the origin of the ring structures in the Hercules A radio lobes
remains an open question. We present the first sub-arcsecond angular resolution
images at low frequencies (<300 MHz) of Hercules A, made with the International
LOFAR Telescope. With the addition of data from the Karl G. Jansky Very Large
Array, we mapped the structure of the lobes from 144 MHz to 7 GHz. We explore
the origin of the rings within the lobes of Hercules A, and test whether their
properties are best described by a shock model, where shock waves are produced
by the jet propagating in the radio lobe, or by an inner-lobe model, where the
rings are formed by decelerated jetted plasma. From spectral index mapping our
large frequency coverage reveals that the curvature of the different ring
spectra increases with distance away from the central active galactic nucleus.
We demonstrate that the spectral shape of the rings is consistent with
synchrotron aging, which speaks in favor of an inner-lobe model where the rings
are formed from the deposition of material from past periods of intermittent
core activity.
