Mass entrainment and turbulence-driven acceleration of ultra-high energy cosmic rays in Centaurus A

¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL, Nijmegen, The Netherlands
e-mail: sarka@astro.ru.nl
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
³ School of Physics and Astronomy, University of Southampton, University Road, Southampton, Hampshire SO17 1BJ, UK
⁴ School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK
⁵ National Radio Astronomy Observatory, Socorro NM 87801, USA
⁶ Physics Department, New Mexico Tech, Socorro NM 87801, USA
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Institute for Nuclear Physics, Karlsruhe Institute of Technology, PO Box 3640, 76021 Karlsruhe, Germany
⁹ Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA
¹⁰ Department of Physics, University of Alabama at Huntsville, Huntsville, AL 35899, USA
¹¹ School of Chemistry & Physics, University of Adelaide, SA 5005, Australia
¹² CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia
¹³ Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706, USA
¹⁴ Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ Racah Institute of Physics, The Hebrew University, 91904 Jerusalem, Israel

Received: 1 April 2013
Accepted: 1 August 2013

Abstract

Observations of the FR I radio galaxy Centaurus A in radio, X-ray, and gamma-ray bands provide evidence for lepton acceleration up to several TeV and clues about hadron acceleration to tens of EeV. Synthesising the available observational constraints on the physical conditions and particle content in the jets, inner lobes and giant lobes of Centaurus A, we aim to evaluate its feasibility as an ultra-high-energy cosmic-ray source. We apply several methods of determining jet power and affirm the consistency of various power estimates of ~1 × 10⁴³ erg s^-1. Employing scaling relations based on previous results for 3C 31, we estimate particle number densities in the jets, encompassing available radio through X-ray observations. Our model is compatible with the jets ingesting ~3 × 10²¹ g s^-1 of matter via external entrainment from hot gas and ~7 × 10²² g s^-1 via internal entrainment from jet-contained stars. This leads to an imbalance between the internal lobe pressure available from radiating particles and magnetic field, and our derived external pressure. Based on knowledge of the external environments of other FR I sources, we estimate the thermal pressure in the giant lobes as 1.5 × 10^-12 dyn cm^-2, from which we deduce a lower limit to the temperature of ~1.6 × 10⁸ K. Using dynamical and buoyancy arguments, we infer ~440−645 Myr and ~560 Myr as the sound-crossing and buoyancy ages of the giant lobes respectively, inconsistent with their spectral ages. We re-investigate the feasibility of particle acceleration via stochastic processes in the lobes, placing new constraints on the energetics and on turbulent input to the lobes. The same “very hot” temperatures that allow self-consistency between the entrainment calculations and the missing pressure also allow stochastic UHECR acceleration models to work.