Probing anisotropic particle acceleration and limb-brightening in Centaurus A's jet

Abstract

Relativistic jets are among the most fascinating objects in the Universe, and recent high-resolution Very Long Baseline Interferometric (VLBI) observations, including the Global mm-VLBI Array and the Event Horizon Telescope (EHT), are able to resolve their structure close to their launching site. These observations reveal strongly limb-brightened jet structures for Centaurus A (Cen A), M 87 and 3C 84. Thus, the question arises which physical mechanism can generate the limb-brightened structure, and if this structure is common for jets from low-luminosity active galactic nuclei (LLAGN) seen under large viewing angles. Therefore, as a pilot study, we aim to model the EHT observations of Cen A. We performed a 3D two-temperature general-relativistic magnetohydrodynamic (GRMHD) simulation of an accreting supermassive black hole (SMBH) and jet launching to study the plasma dynamics and computed the connected emission via general relativistic radiative transfer (GRRT) calculations considering possible anisotropies in the distribution of the radiating particles. In order to adjust our simulations to the EHT observations of Cen A, we carried out a Bayesian fitting in the Fourier plane. We find that GRMHD simulations of magnetically arrested disks (MADs) combined with anisotropically emitting particle distributions along the direction of the magnetic field, parametrized by a value $η=0.07$, are able to mimic the recent EHT observations of Cen A. In addition, we extracted a black hole mass of $M_\mathrm{BH} = 6\times10^7 M_\odot$ and a viewing angle of $\vartheta=72°$. Our obtained model can reproduce key features of the EHT and Atacama Large Millimeter/submillimeter Array (ALMA) observations in total and polarized emission. Finally, we predict that the black hole shadow in Cen A will be observable at a frequency of $\sim$ 3 THz.

Figures (14)

• Figure 1: Evolution of the GRMHD simulation. The top and middle panels show the logarithm of the density in the equatorial and the meridional plane, respectively, for six different times focused on the late evolution. The bottom panel shows the temporal evolution of the mass accretion rate $\dot{m}$ and the MAD parameter $\Phi=\phi_{\rm BH}/\sqrt{\dot{m}}$. The blue vertical dashed lines mark the time frames of the density plots.
• Figure 2: Distribution of the kappa parameter, $\kappa$, the width of the kappa-eDF, $w$, and the break frequency, $\nu_{\rm br}$, for $t=55\,\rm kM$. The black contour line corresponds to $\sigma=1$ and the green one to $\kappa=8$. Between these contours we apply the kappa-eDF during the radiative transfer calculations.
• Figure 3: Reconstructed image from the EHT observations of Cen A Janssen2021
• Figure 4: Posterior distribution of the black hole mass, $M_{\rm BH}$, viewing angle, $\vartheta$, eDF anisotropy, $\eta$ and mass unit, $M$. The black dashed lines correspond to mean and $1\sigma$ errors of the distributions, while the orange lines indicate the location of the model with the highest likelihood, i.e., best $\chi_{\rm tot}^2$.
• Figure 5: Result of the MCMC runs. The top panel shows a 230 GHz GRRT image of Cen A computed using the maximum likelihood position of the MCMC run together with a zoom displaying the horizon scale structure. The middle panel reports on the visibility amplitudes of the EHT observation of Cen A Janssen2021 (gray) and our best GRRT image (blue). In the bottom panel, we report the closure phases for the EHT observations of Cen A Janssen2021 (gray) and our best GRRT image (blue).