AGN duty cycle estimates for the ultra-steep spectrum radio relic VLSS J1431.8+1331
1 University of Groningen, Kapteyn Astronomical Institute, Landleven 12, 9747 AD Groningen, The Netherlands
e-mail: firstname.lastname@example.org; email@example.com, firstname.lastname@example.org
2 ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
3 IRA-INAF, via P. Gobetti 101, 40129 Bologna, Italy
4 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
5 Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
6 Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
7 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
8 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
9 Department of Physics and Astronomy, The Open University, Milton Keynes MK7 6AA, England
10 RAL Space, The Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, England
11 Toruń Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziacka 5, 87-100 Toruń, Poland
12 Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul Orla 171, 30-244 Kraków, Poland
Received: 9 January 2015
Accepted: 26 August 2015
Context. Steep spectrum radio sources associated with active galactic nuclei (AGN) may contain remnants of past AGN activity episodes. Studying these sources gives us insight into the AGN activity history. 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.
Aims. 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.
Methods. 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.
Results. We confirm the morphology and the spectral index values found in previous studies of this object. 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. We observe that the area around the faint radio core located in the larger source component is the youngest, while the overall age of the smaller source component shows it to be the oldest part of the source.
Conclusions. 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. We discuss the implications this conclusion has for future studies of radio sources having similar characteristics.
Key words: galaxies: active / radio continuum: galaxies / galaxies: individual: VLSS J1431.8+1331
© ESO, 2015