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

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 × 10 43 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 × 10 21 g s −1 of matter via external entrainment from hot gas and ∼7 × 10 22 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 × 10 8 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.

Centaurus A: constraints on the nature of the giant lobe filaments fromXMM-Newtonobservations

“…However, the reported detection could also originate from the Galactic X-ray foreground (Stawarz et al 2013;Wykes et al 2013). Deep INTEGRAL-SPI observations (spatial resolution 2.5…”

“…They interpreted the origin of the vertex and vortex filaments in terms of weak shocks from transonic magnetohydrodynamical (MHD) turbulence or from the last vestiges of the activity of the pre-existing jet, or an interplay of both. In Wykes et al (2013), we proposed that the giant lobes became filled with baryonic material entrained in the current and pre-existing jet, and we predicted a thermal particle content of the lobes of n ∼ 1 × 10 −8 cm −3 . Centaurus A is a strong case of dominant X-ray synchrotron in the jet on kpc scales (e.g.…”

“…The present-day jet represents a power of Pj ∼ 1×10 43 erg s −1 (e.g. Croston et al 2009;Wykes et al 2013). Based on, independently, the energy required to expand the giant (i.e.…”

“…Based on, independently, the energy required to expand the giant (i.e. outer) lobes and on the turbulent dissipation rate in them, Wykes et al (2013) have found a time-averaged power of the preexisting jet of Pj ∼ 1 − 5 × 10 43 erg s −1 . A similar power range has recently been established by Neff et al (2015).…”

“…A likely scenario is additional material from entrainment, heated to temperatures substantially above that of the surrounding intragroup gas Wykes et al 2013;Croston & Hardcastle 2014;Kolokythas et al 2015).…”