Speaker
Description
The nearest radio galaxy, Centaurus A, known for its large-scale jet, has been monitored by multiwavelength observations. Analysis of the Fermi-LAT and H.E.S.S. observation data in 2019 showed up an unnatural hardening in very high energy gamma-ray spectrum [1], and its origin is still a matter of controversy. Imaging analysis also reveals that gamma-rays are distributed over the entire jet, requiring ubiquitous particle acceleration source, such as stochastic or shear acceleration.
In this study, we attempt to explain the very high energy gamma-ray spectrum by our refined model of filamentary jet, coupled with diffusive shock acceleration scenario of electrons. Shock waves generated by intermittent mass ejection pass through the jet, to be observed as knot-like features. The shocks are expected to create numerous magnetized current filaments having various transverse sizes, as compatible with observed X-ray substructure of the knots. We construct the spectrum of synchrotron radiation from the accelerated electrons being trapped in the magnetized filaments of the knots, taking inverse cascade of the turbulent magnetic fields into consideration.
Here, we focus on the knots A and B, which are the brightest among the structures resolved by radio and X-rays. The gamma-ray spectrum is constructed by fitting the theoretical synchrotron spectrum to the observed radio and X-ray data and boosting the spectrum up to the higher energy via inverse Compton scattering process {\it in situ}. The point is that the knots A and B are overshadowed by dense dust lane, so that fluxes from infrared to ultraviolet have not been measured. Although this circumstance makes it difficult to fix physical parameters only by the synchrotron spectral fitting, the values of the key parameters could be identified within a narrow range with reference to the gamma-ray spectral analysis. We calculate the upscattered spectrum by taking into account the Klein-Nishina effect and enhanced radiative cooling effect by the filamentation instabilities, i.e., significant increase of emitting surface-to-volume ratio. Provided these effects, we are able to reproduce the unnatural hardening of very high energy gamma-rays by adding up the fluxes of the knots.
[1] Abdalla, H. et al., the H.E.S.S. Collaboration, Nature, vol. 582, 356. (2020)
Authors: Yasuko S. Honda, Mitsuru Honda
