2018A&A...618A..45B

Context. Radio loud active galactic nuclei (AGN) are episodic in nature, cycling through periods of activity and quiescence. The study of this duty cycle has recently gained new relevance because of the importance of AGN feedback for galaxy evolution.
Aims. 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 (∼240kpc) with relaxed shape and very low surface brightness, which resemble remnants of a past AGN activity.
Methods. We have combined new LOFAR data at 145MHz and new Sardinia Radio Telescope data at 6600MHz with available WSRT data at 1400MHz to investigate the spectral properties of the outer lobes and derive their age.
Results. 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 α¹⁴⁵₁₄₀₀=0.48±0.11 and α¹⁴⁰⁰₆₆₀₀=0.69±0.20 in the outer northern lobe, and α¹⁴⁵₁₄₀₀=0.73±0.07 in the outer southern lobe. Moreover, despite the wide frequency coverage available for the outer northern lobe (145-6600MHz), we do not identify a significant spectral curvature (SPC~=0.2±0.2).
Conclusions. 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 ≥80Myr). 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, allowing us to observe both the inner and outer structure simultaneously. 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.