Rejuvenated radio galaxies J0041+3224 and J1835+6204: how long can the
  quiescent phase of nuclear activity last?
  We present radio observations of two well-known double-double radio galaxies,
J0041+3224 and J1835+6204, at frequencies ranging from 150 to 8460 MHz, using
both the Giant Metrewave Radio Telescope and the Very Large Array. These
observations, over a large radio frequency range, enable us to determine the
spectra of the inner and outer lobes. Our detailed spectral ageing analysis of
their inner and outer lobes demonstrates that the outer doubles of
double-double radio galaxies are created by the previous cycle of activity,
while the inner doubles are due to the present cycle of activity. The (core
subtracted) spectra of the inner doubles of both sources are power laws over a
large frequency range. We found that the duration of the quiescent phase of
J0041+3224 is between 4 and 28 per cent of the active phase of the previous
activity. The outer north-western lobe of J1835+6204 has a compact hotspot and
the regions of both the outer hotspots have close to power-law (rather than
curved) spectra, which indicates that the outer lobes are still fed by jet
material ejected in the previous episode just before the central engine stopped
powering the jet. We estimate that the duration of the quiescent phase of
J1835+6204 is $\lapp$5 per cent of the duration of the active phase of the
previous activity. Therefore, we conclude that the duration of the quiescent
phase can be as short as a few per cent of the active phase in radio galaxies
of this type.
