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Supermassive black holes at the centres of galaxies can cycle through periodsof activity and quiescence. Characterising the duty cycle of active galacticnuclei is crucial for understanding the impact of the energy they release onthe host galaxy. For radio AGN, this can be done by identifying dying (remnant)and restarted radio galaxies from their radio spectral properties. Using thecombination of images at 1400 MHz produced by Apertif, the new phased-arrayfeed receiver installed on the Westerbork Synthesis Radio Telescope, and imagesat 150 MHz provided by LOFAR, we have derived resolved spectral index images(at a resolution of ~15 arcsec) for all the sources within ~6 deg^2 area of theLockman Hole region. We were able to select 15 extended radio sources withemission (partly or entirely) characterised by extremely steep spectral indices(steeper than 1.2). These objects represent radio sources in the remnant or therestarted phases of their life cycle. Our findings suggest this cycle to berelatively fast. They also show a variety of properties relevant for modellingthe evolution of radio galaxies. For example, the restarted activity can occurwhile the remnant structure from a previous phase of activity is still visible.This provides constraints on the duration of the 'off' (dying) phase. Inextended remnants with ultra-steep spectra at low frequencies, the activitylikely stopped a few hundred megayears ago, and they correspond to the oldertail of the age distribution of radio galaxies, in agreement with simulationsof radio source evolution. We find remnant radio sources with a variety ofstructures (from double-lobed to amorphous), suggesting different types ofprogenitors. The present work sets the stage for exploiting low-frequencyspectral index studies of extended sources by taking advantage of the largeareas common to the LOFAR and the Apertif surveys.

The best of both worlds: Combining LOFAR and Apertif to derive resolved radio spectral index images