skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Particle Acceleration in Mildly Relativistic Shearing Flows: The Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets

Abstract

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 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 originmore » of ultrahigh-energy cosmic rays.« less

Authors:
; ;  [1]
  1. Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg (Germany)
Publication Date:
OSTI Identifier:
22663514
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 842; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; ASTROPHYSICS; COSMIC RADIATION; DISTRIBUTION; EMISSION; ENERGY SPECTRA; EVOLUTION; FOKKER-PLANCK EQUATION; GALAXIES; GALAXY NUCLEI; RELATIVISTIC RANGE; STOCHASTIC PROCESSES; SYNCHROTRON RADIATION; SYNCHROTRONS; TIME DEPENDENCE; TURBULENCE; X RADIATION

Citation Formats

Liu, Ruo-Yu, Rieger, F. M., and Aharonian, F. A., E-mail: ruoyu@mpi-hd.mpg.de, E-mail: frank.rieger@mpi-hd.mpg.de, E-mail: aharon@mpi-hd.mpg.de. Particle Acceleration in Mildly Relativistic Shearing Flows: The Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA7410.
Liu, Ruo-Yu, Rieger, F. M., & Aharonian, F. A., E-mail: ruoyu@mpi-hd.mpg.de, E-mail: frank.rieger@mpi-hd.mpg.de, E-mail: aharon@mpi-hd.mpg.de. Particle Acceleration in Mildly Relativistic Shearing Flows: The Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets. United States. doi:10.3847/1538-4357/AA7410.
Liu, Ruo-Yu, Rieger, F. M., and Aharonian, F. A., E-mail: ruoyu@mpi-hd.mpg.de, E-mail: frank.rieger@mpi-hd.mpg.de, E-mail: aharon@mpi-hd.mpg.de. Sat . "Particle Acceleration in Mildly Relativistic Shearing Flows: The Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets". United States. doi:10.3847/1538-4357/AA7410.
@article{osti_22663514,
title = {Particle Acceleration in Mildly Relativistic Shearing Flows: The Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets},
author = {Liu, Ruo-Yu and Rieger, F. M. and Aharonian, F. A., E-mail: ruoyu@mpi-hd.mpg.de, E-mail: frank.rieger@mpi-hd.mpg.de, E-mail: aharon@mpi-hd.mpg.de},
abstractNote = {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 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.},
doi = {10.3847/1538-4357/AA7410},
journal = {Astrophysical Journal},
number = 1,
volume = 842,
place = {United States},
year = {Sat Jun 10 00:00:00 EDT 2017},
month = {Sat Jun 10 00:00:00 EDT 2017}
}
  • The problem of converting a fairly narrow nonrelativistic electron distribution into a broad power-law distribution extending to mildly relativistic energies (500 keV) is considered. When the initial electron distribution is sufficiently unstable to produce a level of electron plasma waves which exceeds the threshold for modulational instability, also known as strong electron plasma waves turbulence, the following sequence occurs. Regions of reduced plasma density and enhanced plasma wave energy density known as cavitons are formed in the plasma. Due to the self-focusing properties of electron plasma waves, these cavitons contract, which reduces the wavelength of the plasma waves trapped inmore » the cavitons and decreases their phase velocities. These waves then interact with and accelerate a large number of electrons from the tail of the main body of the distribution to mildly relativistic energies. The problem of the initial generation of the electron plasma wave turbulence from particle distributions with a streaming electron component or an extended electron tail is considered. The state of current research on the nonlinear state of the modulational instability and its electron acceleration potential are reviewed. Finally, the question of how these results apply to astrophysical situations and how power-law electron distributions are formed is discussed.« less
  • The free energy stored in the stressed magnetic fields in AGN jets could be dissipated via generating turbulent plasma waves. The authors review several key wave-particle resonant interactions and point out the importance of a broad wave spectrum. Under several idealized assumptions, they show that the transit-time damping process can accelerate electrons to TeV energies in an AGN jet environment, and present a preliminary calculation on the evolution of plasma wave, electron, and photon distributions. The authors especially emphasize several open questions on particle acceleration by waves, and argue that a plausible scenario is to energize electrons out of themore » thermal background via transit-time damping and further accelerate them by the parallel propagating right-handed waves.« less
  • We present transonic wind-type solutions of the relativistic quasi--two-dimensional Navier-Stokes fluid equations, which we assume to govern the initial acceleration of the plasma in astrophysical jets emerging from the funnel of an accretion disk orbiting a compact central object. The solutions depend on geometrical parameters characterizing the shape and height of the accretion funnel and on radiation parameters characterizing the luminosity and collimation of the radiation field inside this funnel. The two major results of our study are, first, that rapid expansion of the gas at the exit of the accretion funnel, which interacts synergistically with momentum deposition by radiationmore » pressure, can lead to multiple critical points in the flow and to supersonic speeds very close to the central object; this main feature of our solution is consistent with observations that jets might already be accelerated to relativistic speeds on the sub--0.1 pc distance scale. Second, we show that for suitable values of the parameters characterizing the shape of the accretion funnel and its associated radiation field, multiple transonic solutions for the same initial conditions of the bulk flow speed are obtained, with shock transitions connecting some of these transonic solutions. Because of the sensitivity of the flow to slight variations of the disk and radiation parameters, such discontinuous transitions between distinct transonic flows might be related to the observed phenomenology and variability of active galactic nuclei.« less
  • We investigate the consequences of acceleration of nuclei in jets of active galaxies not far from the surface of an accretion disk. The nuclei can be accelerated in the re-connection regions in the jet and/or at the jet boundary, between the relativistic jet and its cocoon. It is shown that the relativistic nuclei can efficiently fragment onto specific nucleons in collisions with the disk radiation. Neutrons, directed toward the accretion disk, take a significant part of energy from the relativistic nuclei. These neutrons develop a cascade in the dense accretion disk. We calculate the neutrino spectra produced in such amore » hadronic cascade within the accretion disk. We propose that the neutrinos produced in such a scenario, from the whole population of super-massive black holes in active galaxies, can explain the extragalactic neutrino background recently measured by the IceCube neutrino detector, provided that a 5% fraction of galaxies have an active galactic nucleus and a few percent of neutrons reach the accretion disk. We predict that the neutrino signals in the present neutrino detectors, produced in terms of such a model, will not be detectable even from the nearby radio galaxies similar to M87.« less
  • We analyze the model of stochastic re-acceleration of electrons that are emitted by supernova remnants (SNRs) in the Galactic Disk and then propagate into the Galactic Halo in order to explain the origin of nonthermal (radio and gamma-ray) emission from Fermi bubbles (FB). We assume that the energy for re-acceleration in the Halo is supplied by shocks generated by processes of star accretion onto the central black hole. Numerical simulations show that regions with strong turbulence (places for electron re-acceleration) are located high up in the Galactic Halo several kpc above the disk. The energy of the SNR electrons thatmore » reach these regions does not exceed several GeV due to synchrotron and inverse Compton energy losses. At appropriate parameters of re-acceleration these electrons can be re-accelerated up to an energy of 10{sup 12} eV, which explains in this model the origin of the observed radio and gamma-ray emission from the FB. However, although the model gamma-ray spectrum is consistent with the Fermi results, the model radio spectrum is steeper than that observed by WMAP and Planck. If adiabatic losses due to plasma outflows from the Galactic central regions are taken into account, then the re-acceleration model nicely reproduces the Planck data points.« less