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The prominent radio source Hercules A features complex structures in itsradio lobes. Although it is one of the most comprehensively studied sources inthe radio sky, the origin of the ring structures in the Hercules A radio lobesremains an open question. We present the first sub-arcsecond angular resolutionimages at low frequencies (<300 MHz) of Hercules A, made with the InternationalLOFAR Telescope. With the addition of data from the Karl G. Jansky Very LargeArray, we mapped the structure of the lobes from 144 MHz to 7 GHz. We explorethe origin of the rings within the lobes of Hercules A, and test whether theirproperties are best described by a shock model, where shock waves are producedby the jet propagating in the radio lobe, or by an inner-lobe model, where therings are formed by decelerated jetted plasma. From spectral index mapping ourlarge frequency coverage reveals that the curvature of the different ringspectra increases with distance away from the central active galactic nucleus.We demonstrate that the spectral shape of the rings is consistent withsynchrotron aging, which speaks in favor of an inner-lobe model where the ringsare formed from the deposition of material from past periods of intermittentcore activity.

Origin of the ring structures in Hercules A -- Sub-arcsecond 144 MHz to 7 GHz observations