Abstract:
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 ≤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.