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Register a new userAcceleration of high energy protons in AGN relativistic jets
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Alexey Gunya PhD student
Publication date
2020
fields
Physics
and research's language is
English
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In this paper, we investigate the acceleration in relativistic jets of high-energy proton preaccelerated in the magnetosphere of a supermassive black hole. The proton reaches maximum energy when passing the total potential difference of $U$ between the jet axis and its periphery. This voltage is created by a rotating black hole and transmitted along magnetic field lines into the jet. It is shown that the trajectories of proton in the jet are divided into three groups: untrapped, trapped and not accelerated. Untrapped particles are not kept by poloidal and toroidal magnetic fields inside the jet, so they escape out the jet and their energy is equal to the maximum value, $eU$. Trapped protons are moving along the jet with oscillations in the radial direction. Their energy varies around the value of $0.74 eU$. In a strong magnetic field protons preaccelerated in the magnetosphere are pressed to the jet axis and practically are not accelerated in the jet. The work defines acceleration regimes for a range of the most well-known AGN objects with relativistic jets and for the microquasar SS433.
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Clusters of galaxies are believed to be capable to accelerate protons at accretion shocks to energies exceeding 10^18 eV. At these energies, the losses caused by interactions of cosmic rays with photons of the Cosmic Microwave Background Radiation (CMBR) become effective and determine the maximum energy of protons and the shape of the energy spectrum in the cutoff region. The aim of this work is the study of the formation of the energy spectrum of accelerated protons at accretion shocks of galaxy clusters and of the characteristics of their broad band emission. The proton energy distribution is calculated self-consistently via a time-dependent numerical treatment of the shock acceleration process which takes into account the proton energy losses due to interactions with the CMBR. We calculate the energy distribution of accelerated protons, as well as the flux of broad-band emission produced by secondary electrons and positrons via synchrotron and inverse Compton scattering processes. We find that the downstream and upstream regions contribute almost at the same level to the emission. For the typical parameters characterising galaxy clusters, the synchrotron and IC peaks in the spectral energy distributions appear at comparable flux levels. For an efficient acceleration, the expected emission components in the X-ray and gamma-ray band are close to the detection threshold of current generation instruments, and will be possibly detected with the future generation of detectors.
The centrifugal acceleration is due to the rotating poloidal magnetic field in the magnetosphere creates the electric field which is orthogonal to the magnetic field. Charged particles with finite cyclotron radii can move along the electric field and receive energy. Centrifugal acceleration pushes particles to the periphery, where their azimuthal velocity reaches the light speed. We have calculated particle trajectories by numerical and analytical methods. The maximum obtained energies depend on the parameter of the particle magnetization $ kappa $, which is the ratio of rotation frequency of magnetic field lines in the magnetosphere $ Omega_F $ to non-relativistic cyclotron frequency of particles $ omega_c $, $ kappa = Omega_F /omega_c << 1 $, and from the parameter $ alpha $ which is the ratio of toroidal magnetic field $ B_T $ to the poloidal one $ B_P $, $ alpha = B_T / B_P $. It is shown that for small toroidal fields, $ alpha <kappa^{1/4} $, the maximum Lorentz factor $ gamma_m $ is only the square root of magnetization, $ gamma_m = kappa^{-1/2} $, while for large toroidal fields, $ alpha >kappa^{1/4} $, the energy increases significantly, $ gamma_m = kappa^{-2/3} $. However, the maximum possible acceleration, $ gamma_m = kappa^{-1} $, is not achieved in the magnetosphere. For a number of active galactic nuclei, such as M87, maximum values of Lorentz factor for accelerated protons are found. Also for special case of Sgr. A* estimations of the maximum proton energy and its energy flux are obtained. They are in agreement with experimental data obtained by HESS Cherenkov telescope.
This paper summarizes recent progresses in our theoretical understanding of particle acceleration at relativistic shock waves and it discusses two salient consequences: (1) the maximal energy of accelerated particles; (2) the impact of the shock-generated micro-turbulence on the multi-wavelength light curves of gamma-ray burst afterglows.
The class of Double-Double Radio Galaxies (DDRGs) relates to episodic jet outbursts. How various regions and components add to the total intensity in radio images is less well known. In this paper we synthesize synchrotron images for DDRGs based on special relativistic hydrodynamic simulations, making advanced approximations for the magnetic fields. We study the synchrotron images for: Three different radial jet profiles; Ordered, entangled or mixed magnetic fields; Spectral ageing from synchrotron cooling; The contribution from different jet components; The viewing angle and Doppler (de-)boosting; The various epochs of the evolution of the DDRG. To link our results to observational data, we adopt to J1835+6204 as a reference source. In all cases the synthesized synchrotron images show two clear pairs of hotspots, in the inner and outer lobes. The best resemblance is obtained for the piecewise isochoric jet model, for a viewing angle of approximately $vartheta sim -71^{circ}$, i.e. inclined with the lower jet towards the observer, with predominantly entangled ($gtrsim 70$ per cent of the magnetic pressure) in turbulent, rather than ordered fields. The effects of spectral ageing become significant when the ratio of observation frequencies and cut-off frequency $ u_{rm obs}/ u_{infty,0} gtrsim 10^{-3}$, corresponding to $sim 3 cdot 10^2$ MHz. For viewing angles $vartheta lesssim -30^{circ}$, a DDRG morphology can no longer be recognized. The second jets must be injected within $lesssim$ 4 per cent of the lifetime of the first jets for a DDRG structure to emerge, which is relevant for Active Galactic Nuclei feedback constraints.
The origin of the extended X-ray emission in the large-scale jets of active galactic nuclei (AGNs) poses challenges to conventional models of acceleration and emission. Although the electron synchrotron radiation is considered the most feasible radiation mechanism, the formation of the continuous large-scale X-ray structure remains an open issue. As astrophysical jets are expected to exhibit some turbulence and shearing motion, we here investigate the potential of shearing flows to facilitate an extended acceleration of particles and evaluate its impact on the resultant particle distribution. Our treatment incorporates systematic shear and stochastic second-order Fermi effects. We show that for typical parameters applicable to large-scale AGN jets, stochastic second-order Fermi acceleration, which always accompanies shear particle acceleration, can play an important role in facilitating the whole process of particle energization. We study the time-dependent evolution of the resultant particle distribution in the presence of second-order Fermi acceleration, shear acceleration, and synchrotron losses using a simple Fokker--Planck approach and provide illustrations for the possible emergence of a complex (multicomponent) particle energy distribution with different spectral branches. We present examples for typical parameters applicable to large-scale AGN jets, indicating the relevance of the underlying processes for understanding the extended X-ray emission and the origin of ultrahigh-energy cosmic rays.
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