Gamma-ray emission in radio galaxies under the VLBI scope

I. Parsec-scale jet kinematics and high-energy properties of γ-ray-detected TANAMI radio galaxies^⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: angioni@mpifr-bonn.mpg.de
² Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
³ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ Catholic University of America, Washington, DC 20064, USA
⁵ University of Maryland, Baltimore County, 1000 Hilltop Cir, Baltimore, MD 21250, USA
⁶ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010 6500 GL Nijmegen, The Netherlands
⁷ CSIRO Astronomy and Space Science, PO Box 76 Epping, NSW 1710, Australia
⁸ Wyle Science, Technology and Engineering Group, Greenbelt, MD 20771, USA
⁹ Institute for Radio Astronomy & Space Research, AUT University, 1010 Auckland, New Zealand
¹⁰ Bundesamt für Kartographie und Geodäsie, 93444 Bad Kötzting, Germany
¹¹ CSIRO Astronomy and Space Science, Canberra Deep Space Communications Complex, PO Box 1035 Tuggeranong, ACT 2901, Australia
¹² GRAPPA & Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹³ School of Mathematics & Physics, University of Tasmania, Private Bag 37, Hobart 7001, Tasmania, Australia
¹⁴ Hartebeesthoek Radio Astronomy Observatory, PO Box 443 1740 Krugersdorp, South Africa
¹⁵ ASTRON, The Netherlands Institute for Radio Astronomy, Post-bus 2, 7990 AA Dwingeloo, The Netherlands
¹⁶ Dr. Remeis-Sternwarte & ECAP, Universität Erlangen-Nürnberg, Sternwartstraße 7, 96049 Bamberg, Germany

Received: 15 April 2019
Accepted: 19 June 2019

Abstract

Aims. In the framework of the multi-wavelength and very long baseline interferometry (VLBI) monitoring program TANAMI (Tracking Active Nuclei with Austral Milliarcsecond Interferometry), we study the evolution of the parsec-scale radio emission in radio galaxies in the southern hemisphere and their relationship to the γ-ray properties of the sources. Our study investigates systematically, for the first time, the relationship between the two energy regimes in radio galaxies. In this first paper, we focus on Fermi-LAT-detected sources.

Methods. The TANAMI program monitors a large sample of radio-loud AGN at 8.4 GHz and 22.3 GHz with the Australian long baseline array (LBA) and associated telescopes in Antarctica, Chile, New Zealand and South Africa. We performed a kinematic analysis for five γ-ray detected radio galaxies using multi-epoch 8.4 GHz VLBI images, deriving limits on intrinsic jet parameters such as speed and viewing angle. We analyzed 103 months of Fermi-LAT data in order to study possible connections between the γ-ray properties and the pc-scale jets of Fermi-LAT-detected radio galaxies, both in terms of variability and average properties. We discuss the individual source results and draw preliminary conclusions on sample properties including published VLBI results from the MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) survey, with a total of fifteen sources.

Results. We find that the first γ-ray detection of Pictor A might be associated with the passage of a new VLBI component through the radio core, which appears to be a defining feature of high-energy emitting Fanaroff-Riley type II radio galaxies. We detect subluminal parsec-scale jet motions in the peculiar AGN PKS 0521−36, and we confirm the presence of fast γ-ray variability in the source down to timescales of six hours, which is not accompanied by variations in the VLBI jet. We robustly confirm the presence of significant superluminal motion, up to β_app ∼ 3, in the jet of the TeV radio galaxy PKS 0625−35. Our VLBI results constrain the jet viewing angle to be θ <  53°, allowing for the possibility of a closely aligned jet. Finally, by analyzing the first pc-scale multi-epoch images of the prototypical compact symmetric object (CSO) PKS 1718−649, we place an upper limit on the separation speed between the two mini-lobes. This in turn allows us to derive a lower limit on the age of the source.

Conclusions. We can draw some preliminary conclusions on the relationship between pc-scale jets and γ-ray emission in radio galaxies, based on Fermi-LAT-detected sources with available multi-epoch VLBI measurements. We find that the VLBI core flux density correlates with the γ-ray flux, as seen in blazars. On the other hand, the γ-ray luminosity does not show any dependence on the core brightness temperature and core dominance, which are two common indicators of jet Doppler boosting. This seems to indicate that γ-ray emission in radio galaxies is not driven by orientation-dependent effects, as in blazars, in accordance with the unified model of jetted AGN.