173 research outputs found
Electron Heating by the Ion Cyclotron Instability in Collisionless Accretion Flows. II. Electron Heating Efficiency as a Function of Flow Conditions
In the innermost regions of low-luminosity accretion flows, including Sgr A*
at the center of our Galaxy, the frequency of Coulomb collisions is so low that
the plasma is two-temperature, with the ions substantially hotter than the
electrons. This paradigm assumes that Coulomb collisions are the only channel
for transferring the ion energy to the electrons. In this work, the second of a
series, we assess the efficiency of electron heating by ion velocity-space
instabilities in collisionless accretion flows. The instabilities are seeded by
the pressure anisotropy induced by magnetic field amplification, coupled to the
adiabatic invariance of the particle magnetic moments. Using two-dimensional
(2D) particle-in-cell (PIC) simulations, we showed in Paper I that if the
electron-to-ion temperature ratio is < 0.2, the ion cyclotron instability is
the dominant mode for values of ion beta_i ~ 5-30 (here, beta_i is the ratio of
ion thermal pressure to magnetic pressure), as appropriate for the midplane of
low-luminosity accretion flows. In this work, we employ analytical theory and
1D PIC simulations (with the box aligned with the fastest growing wavevector of
the ion cyclotron mode) to fully characterize how the electron heating
efficiency during the growth of the ion cyclotron instability depends on the
electron-to-proton temperature ratio, the plasma beta, the Alfven speed, the
amplification rate of the mean field (in units of the ion Larmor frequency) and
the proton-to-electron mass ratio. Our findings can be incorporated as a
physically-grounded sub-grid model into global fluid simulations of
low-luminosity accretion flows, thus helping to assess the validity of the
two-temperature assumption.Comment: 18 pages, 6 figures, 6 tables, 2 appendices, submitted to ApJ. Paper
I appeared on Monday November 24t
Production of magnetic energy by macroscopic turbulence in GRB afterglows
Afterglows of gamma-ray bursts are believed to require magnetic fields much
stronger than that of the compressed pre-shock medium. As an alternative to
microscopic plasma instabilities, we propose amplification of the field by
macroscopic turbulence excited by the interaction of the shock with a clumpy
pre-shock medium, for example a stellar wind. Using a recently developed
formalism for localized perturbations to an ultra-relativistic shock, we derive
constraints on the lengthscale, amplitude, and volume filling factor of density
clumps required to produce a given magnetic energy fraction within the
expansion time of the shock, assuming that the energy in the field achieves
equipartion with the turbulence. Stronger and smaller-scale inhomogeneities are
required for larger shock Lorentz factors. Hence it is likely that the magnetic
energy fraction evolves as the shock slows. This could be detected by
monitoring the synchrotron cooling frequency if the radial density profile
ahead of the shock, smoothed over clumps, is known.Comment: 24 pages, 3 figure
Relativistic Reconnection: an Efficient Source of Non-Thermal Particles
In magnetized astrophysical outflows, the dissipation of field energy into
particle energy via magnetic reconnection is often invoked to explain the
observed non-thermal signatures. By means of two- and three-dimensional
particle-in-cell simulations, we investigate anti-parallel reconnection in
magnetically-dominated electron-positron plasmas. Our simulations extend to
unprecedentedly long temporal and spatial scales, so we can capture the
asymptotic state of the system beyond the initial transients, and without any
artificial limitation by the boundary conditions. At late times, the
reconnection layer is organized into a chain of large magnetic islands
connected by thin X-lines. The plasmoid instability further fragments each
X-line into a series of smaller islands, separated by X-points. At the
X-points, the particles become unmagnetized and they get accelerated along the
reconnection electric field. We provide definitive evidence that the late-time
particle spectrum integrated over the whole reconnection region is a power-law,
whose slope is harder than -2 for magnetizations sigma>10. Efficient particle
acceleration to non-thermal energies is a generic by-product of the long-term
evolution of relativistic reconnection in both two and three dimensions. In
three dimensions, the drift-kink mode corrugates the reconnection layer at
early times, but the long-term evolution is controlled by the plasmoid
instability, that facilitates efficient particle acceleration, in analogy to
the two-dimensional physics. Our findings have important implications for the
generation of hard photon spectra in pulsar winds and relativistic
astrophysical jets.Comment: 6 pages, 5 figures, ApJL accepted, movies available at
https://www.cfa.harvard.edu/~lsironi/Site/sigma10.no.guide.field
Relativistic Shocks: Particle Acceleration and Magnetization
We review the physics of relativistic shocks, which are often invoked as the
sources of non-thermal particles in pulsar wind nebulae (PWNe), gamma-ray
bursts (GRBs), and active galactic nuclei (AGN) jets, and as possible sources
of ultra-high energy cosmic-rays. We focus on particle acceleration and
magnetic field generation, and describe the recent progress in the field driven
by theory advances and by the rapid development of particle-in-cell (PIC)
simulations. In weakly magnetized or quasi parallel-shocks (where the magnetic
field is nearly aligned with the flow), particle acceleration is efficient. The
accelerated particles stream ahead of the shock, where they generate strong
magnetic waves which in turn scatter the particles back and forth across the
shock, mediating their acceleration. In contrast, in strongly magnetized
quasi-perpendicular shocks, the efficiencies of both particle acceleration and
magnetic field generation are suppressed. Particle acceleration, when
efficient, modifies the turbulence around the shock on a long time scale, and
the accelerated particles have a characteristic energy spectral index of ~ 2.2
in the ultra-relativistic limit. We discuss how this novel understanding of
particle acceleration and magnetic field generation in relativistic shocks can
be applied to high-energy astrophysical phenomena, with an emphasis on PWNe and
GRB afterglows.Comment: 32 pages; 9 figures; invited topical review, comments welcome;
submitted for publication in "The Strongest Magnetic Fields in the Universe"
(Space Sciences Series of ISSI, Springer), Space Science Review
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