Evidence of a double-double morphology in B0818+214
  The so-called double-double structure in radio sources is the most
conspicuous signature of their restarted activity. Observations indicate that
in the majority of double-double radio sources (DDRS), the span of the radio
lobes is larger than 0.7 Mpc. This lower limit is also suggested by theory.
However, it seemed likely that the apparent core of B0818+214, a radio galaxy
with an overall linear size of its radio structure below that limit, could
harbour a compact double well aligned with the outer lobes so that the whole
object would fulfil the criteria of a DDRS. Here, we present evidence that the
central component of B0818+214, when magnified through the EVN+MERLIN 18-cm
observations, shows two FR II-like lobes. As the separation of the inner lobes
is not greater than 5.7 kpc, they are immersed in the ISM of the host galaxy.
This circumstance is the likely reason why the inner double has become visible,
despite the predictions of the theory according to which B0818+214 as a whole
is too small for a new double to develop inside the cocoon inflated during the
previous active phase. Moreover, we speculate that its host galaxy is not
active at the moment and so the inner double may be in the coasting phase often
observed in other medium-sized symmetric objects with intermittent activity. It
could be, therefore, that two different mechanisms of accretion disk
instabilities, ionisation and radiation-pressure driven, may be independently
responsible for triggering active phases, manifesting as the outer and the
inner doubles, respectively.
