AGN duty cycle estimates for the ultra-steep spectrum radio relic VLSS
  J1431.8+1331
  Steep spectrum radio sources associated with active galactic nuclei (AGN) may
contain remnants of past AGN activity episodes. Novel instruments like the LOw
Frequency ARray (LOFAR) are enabling studies of these fascinating structures to
be made at tens to hundreds of MHz with sufficient resolution to analyse their
complex morphology. Our goal is to characterize the integrated and resolved
spectral properties of VLSS J1431+1331 and estimate source ages based on
synchrotron radio emission models, thus putting constraints on the AGN duty
cycle. Using a broad spectral coverage, we have derived spectral and curvature
maps, and used synchrotron ageing models to determine the time elapsed from the
last time the source plasma was energized. We used LOFAR, Giant Metrewave Radio
Telescope (GMRT) and Jansky Very Large Array (VLA) data. Based on our ageing
analysis, we infer that the AGN that created this source currently has very low
levels of activity or that it is switched off. The derived ages for the larger
source component range from around 60 to 130 Myr, hinting that the AGN activity
decreased or stopped around 60 Myr ago. Our analysis suggests that VLSS
J1431.8+1331 is an intriguing, two-component source. The larger component seems
to host a faint radio core, suggesting that the source may be an AGN radio
relic. The spectral index we observe from the smaller component is distinctly
flatter at lower frequencies than the spectral index of the larger component,
suggesting the possibility that the smaller component may be a shocked plasma
bubble. From the integrated source spectrum, we deduce that its shape and slope
can be used as tracers of the activity history of this type of steep spectrum
radio source.
