Duty cycle of the radio galaxy B2 0258+35
  Radio loud Active Galactic Nuclei are episodic in nature, cycling through
periods of activity and quiescence. In this work we investigate the duty cycle
of the radio galaxy B2~0258+35, which was previously suggested to be a
restarted radio galaxy based on its morphology. The radio source consists of a
pair of kpc-scale jets embedded in two large-scale lobes (~240 kpc) with
relaxed shape and very low surface brightness, which resemble remnants of a
past AGN activity. We have combined new LOFAR data at 145 MHz and new SRT data
at 6600 MHz with available WSRT data at 1400 MHz to investigate the spectral
properties of the outer lobes and derive their age. Interestingly, the spectrum
of both the outer Northern and Southern lobes is not ultra-steep as expected
for an old ageing plasma with spectral index values equal to $\rm
\alpha_{1400}^{145}=0.48\pm0.11$ and $\rm \alpha_{6600}^{1400}=0.69\pm0.20$ in
the outer Northern lobe, and $\rm \alpha_{1400}^{145}=0.73\pm0.07$ in the outer
Southern lobe. Moreover, despite the wide frequency coverage available for the
outer Northern lobe (145-6600~MHz), we do not identify a significant spectral
curvature (SPC$\simeq$0.2$\pm0.2$). While mechanisms such as in-situ particle
reacceleration, mixing or compression can temporarily play a role in preventing
the spectrum from steepening, in no case seem the outer lobes to be compatible
with being very old remnants of past activity as previously suggested (with age
$\gtrsim$ 80 Myr). We conclude that either the large-scale lobes are still
fuelled by the nuclear engine or the jets have switched off no more than a few
tens of Myr ago. Our study shows the importance of combining morphological and
spectral properties to reliably classify the evolutionary stage of low surface
brightness, diffuse emission that low frequency observations are revealing
around a growing number of radio sources.
