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Steep spectrum radio sources associated with active galactic nuclei (AGN) maycontain remnants of past AGN activity episodes. Novel instruments like the LOwFrequency ARray (LOFAR) are enabling studies of these fascinating structures tobe made at tens to hundreds of MHz with sufficient resolution to analyse theircomplex morphology. Our goal is to characterize the integrated and resolvedspectral properties of VLSS J1431+1331 and estimate source ages based onsynchrotron radio emission models, thus putting constraints on the AGN dutycycle. Using a broad spectral coverage, we have derived spectral and curvaturemaps, and used synchrotron ageing models to determine the time elapsed from thelast time the source plasma was energized. We used LOFAR, Giant Metrewave RadioTelescope (GMRT) and Jansky Very Large Array (VLA) data. Based on our ageinganalysis, we infer that the AGN that created this source currently has very lowlevels of activity or that it is switched off. The derived ages for the largersource component range from around 60 to 130 Myr, hinting that the AGN activitydecreased or stopped around 60 Myr ago. Our analysis suggests that VLSSJ1431.8+1331 is an intriguing, two-component source. The larger component seemsto host a faint radio core, suggesting that the source may be an AGN radiorelic. The spectral index we observe from the smaller component is distinctlyflatter at lower frequencies than the spectral index of the larger component,suggesting the possibility that the smaller component may be a shocked plasmabubble. From the integrated source spectrum, we deduce that its shape and slopecan be used as tracers of the activity history of this type of steep spectrumradio source.

AGN duty cycle estimates for the ultra-steep spectrum radio relic VLSS J1431.8+1331