Distributed accelerators in the jet of Centaurus A: the origin of the spectral hardening of very high energy gamma-rays

Abstract

We propose the synchrotron self-Compton (SSC) scenario coupled with filamentary jet model, to reproduce the very high energy $γ$-ray emissions from Cen A. With reference to self-similarity of knot-like features in the jet, we assume nonuniform magnetic field associated with current filaments having various transverse sizes. For energetic electron production, the diffusive shock acceleration at sites distributed over the kiloparsec-scale jet is considered. We show that maximum Lorentz factor of the electron steadily exceeds $10^{8}$ due to suppression of synchrotron loss of the electrons trapped in weak magnetic field of the thin filaments, and inhomogeneous SSC in the inner jet can dominantly contribute to establishment of the pronounced hardening of $γ$-ray flux detected by the H.E.S.S. It is also suggested that the spectral contribution from diffuse regions of the outer jet potentially amounts to the observed Fermi fluxes.

Figures (4)

• Figure 1: The schematic view of the current filamentation and coalescence at a knotty region. When plasmoids ejected intermittently from the central engine catch up with the preceding flow, the bulk breaks up into numerous separate fragments. The filaments are represented by many pipes; the inset illustrates a sample of the realistic one bent or twisted. The filaments soon coalesce due to the attractive force acting on the currents in the same direction. These processes actively repeat, because of multiple shocks. The corresponding spectra of turbulent magnetic fields are depicted on the right side. The filamentation is driven at the scale of plasma skin depth ($c/\omega_{p}$), and the magnetic field energy accumulates at the largest scale ($D$), via the inverse cascade in the $k$-space (leftward arrow).
• Figure 2: A transverse cut of the knotty region that contains, with a filling rate less than unity, filaments having various transverse sizes $\lambda$. Note the relation of $\lambda\leq D<D_{j}$, where $D$ is diameter of the largest filament and $D_{j}$ corresponds to diameter of the jet.
• Figure 3: The maximum Lorentz factor of electrons accelerated in the filaments having various sizes $\lambda$ in a knot (top) and flux density due to contribution from sampled filaments (the labels 'a-d' correspond; bottom). The solid black curve (labeled as 'Total') shows the flux that is the sum of contributions from all filaments in the considered $\lambda$-range. For an explanation, see the text.
• Figure 4: The $\nu F_{\nu}$-spectra of the knots ( top) and diffuse region ( bottom) in the range of radio to $\gamma$-rays, for the parameter values given in Table 1. For the knots, the observed radio fluxes are cited from BFS83, and the X-ray fluxes are obtained by adding up the fluxes of subknots 2002ApJ...569...54K. The calculated spectra and observed fluxes of knot A/B are denoted by red/blue curves and marks (asterisks/open-squares), respectively. The purple solid and dot-dashed curves show the calculated spectra of the IC component (knot A+B) with and without radiative cooling effect of filaments, respectively. The overall spectrum of synchrotron+IC is shown as the black solid curve. The calculated spectra of the diffuse region are denoted by green curves, and the observed fluxes from that region (green crosses) are cited from 2006MNRAS.368L..15H. The cases for which the observed X-ray flux is assumed to originate from synchrotron radiation and IC(+radiative cooling) are respectively shown by the thin dashed and bold curves. The observed $\gamma$-ray fluxes of the H.E.S.S. ( top: purple triangles) and Fermi-LAT ( bottom: brown open-circles) are cited from 2018AA...619A..71H.