Astrophysical plasmas in the surrounding of compact objects and subject
to intense gravitational and electromagnetic fields are believed to give
rise to relativistic regimes. Theoretical and observational evidences
suggest that magnetized plasmas of this type are collisionless and can
persist for long times (e.g., with respect to a distant observer,
coordinate, time), while exhibiting geometrical structures characterized
by the absence of well-defined spatial symmetries. In this paper, the
problem is posed whether such configurations can correspond to some kind
of kinetic equilibrium. The issue is addressed from a theoretical
perspective in the framework of a covariant Vlasov statistical
description, which relies on the method of invariants. For this purpose,
a systematic covariant variational formulation of gyrokinetic theory is
developed, which holds without requiring any symmetry condition on the
background fields. As a result, an asymptotic representation of the
relativistic particle magnetic moment is obtained from its formal exact
solution, in terms of a suitably defined invariant series expansion
parameter (perturbative representation). On such a basis, it is shown
that spatially non-symmetric kinetic equilibria can actually be
determined, an example being provided by Gaussian-like distributions. As
an application, the physical mechanisms related to the occurrence of a
non-vanishing equilibrium fluid 4-flow are investigated.
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Based on the Newman-Janis algorithm, the Ayón-Beato-García
spacetime metric [Phys. Rev. Lett. 80, 5056 (1998)] of the regular
spherically symmetric, static, and charged black hole has been converted
into rotational form. It is shown that the derived solution for rotating
a regular black hole is regular and the critical value of the electric
charge for which two horizons merge into one sufficiently decreases in
the presence of the nonvanishing rotation parameter a of the black hole.
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We demonstrate that ultra-high-energy collisions of particles falling
freely from rest at infinity can occur in the field of near-extreme
Kehagias-Sfetsos naked singularities related to the Hořava
gravity. However, the efficiency of escaping of created
ultrarelativistic particles and the energy efficiency of the collisional
process relative to distant observers are significantly lowered due to
large gravitational redshift, which is substantially lower in comparison
to those related to the collisions occurring close to the equatorial
plane of near-extreme Kerr naked-singularity spacetimes. In the
Kehagias-Sfetsos naked-singularity spacetimes, the energy efficiency
relative to distant observers corresponds to the covariant energy of the
colliding particles only. Finally, we demonstrate how the
ultra-high-energy collisions are modified for charged particles if the
Kehagias-Sfetsos naked singularities are immersed in a uniform magnetic
field.
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The problem of the transition from gas to plasma in gravitating axisymmetric structures is addressed under the assumption of having initial and final states realized by kinetic Maxwellian-like equilibria. In astrophysics, the theory applies to accretion-disc scenarios around compact objects. A formulation based on non-relativistic kinetic theory for collisionless systems is adopted. Equilibrium solutions for the kinetic distribution functions describing the initial neutral matter and the resulting plasma state are constructed in terms of single-particle invariants and expressed by generalized Maxwellian distributions. The final plasma configuration is related to the initial gas distribution by the introduction of appropriate functional constraints. Qualitative aspects of the solution are investigated and physical properties of the system are pointed out. In particular, the admitted functional dependences of the fluid fields carried by the corresponding equilibrium distributions are determined. Then, the plasma is proved to violate the condition of quasi-neutrality, implying a net charge separation between ions and electrons. This result is shown to be independent of the precise realization of the plasma distribution function, while a physical mechanism able to support a non-neutral equilibrium state is proposed.
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The problem of the transition from gas to plasma in gravitating
axisymmetric structures is addressed under the assumption of having
initial and final states realized by kinetic Maxwellian-like equilibria.
In astrophysics, the theory applies to accretion-disc scenarios around
compact objects. A formulation based on non-relativistic kinetic theory
for collisionless systems is adopted. Equilibrium solutions for the
kinetic distribution functions describing the initial neutral matter and
the resulting plasma state are constructed in terms of single-particle
invariants and expressed by generalized Maxwellian distributions. The
final plasma configuration is related to the initial gas distribution by
the introduction of appropriate functional constraints. Qualitative
aspects of the solution are investigated and physical properties of the
system are pointed out. In particular, the admitted functional
dependences of the fluid fields carried by the corresponding equilibrium
distributions are determined. Then, the plasma is proved to violate the
condition of quasi-neutrality, implying a net charge separation between
ions and electrons. This result is shown to be independent of the
precise realization of the plasma distribution function, while a
physical mechanism able to support a non-neutral equilibrium state is
proposed.
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Resonant Switch (RS) model has recently been proposed as an alternative to the standard models of twin-peak high-frequency quasi-periodic oscillations (HF QPOs) observed in low-mass X-ray binaries containing a neutron star. The model assumes switch of twin oscillations at a resonant point, where frequencies of the upper and lower oscillations νU and νL become commensurable and one pair of the oscillating modes (corresponding to a specific model of HF QPOs) changes to some other pair due to non-linear resonant phenomena. We test the RS model for the atoll source 4U 1636-53, where we assume two resonant points observed at frequency ratios νU:νL=3:2, 5:4, by fitting the pairs of the oscillatory modes to the observed data in the regions related to the resonant points. Among acceptable variants of the RS model the most promising are those combining the relativistic precession (RP) and the total precession (TP) frequency relations or their modifications. The precision of the fits is shown to be strongly increased in comparison to fits realized by individual pairs along the whole data range. We demonstrate that the χ2 test is significantly improved. Fitting of the HF QPO data in the source 4U 1636-53 by the RP1-RP variant of the RS model gives the best results and implies that the neutron star mass and dimensionless spin are M≈2.2 Msun and a≈0.27.
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The kinetic description of relativistic plasmas in the presence of time-varying and spatially non-uniform electromagnetic (EM) fields is a fundamental theoretical issue both in astrophysics and plasma physics. This refers, in particular, to the treatment of collisionless and strongly-magnetized plasmas in the presence of intense radiation sources. In this paper, the problem is investigated in the framework of a covariant gyrokinetic treatment for Vlasov-Maxwell equilibria. The existence of a new class of kinetic equilibria is pointed out, which occur for spatially-symmetric systems. These equilibria are shown to exist in the presence of non-uniform background EM fields and curved space-time. In the non-relativistic limit, this feature permits the determination of kinetic equilibria even for plasmas in which particle energy is not conserved due to the occurrence of explicitly time-dependent EM fields. Finally, absolute stability criteria are established which apply in the case of infinitesimal symmetric perturbations that can be either externally or internally produced.
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Small oscillations of current-carrying string loops around stable equilibrium positions corresponding to minima of the effective potential in the equatorial plane of the Kerr black holes are studied using the perturbation method. In the lowest approximation, two uncoupled harmonic oscillators are obtained that govern the radial and vertical oscillations; the higher-order terms determine nonlinear phenomena and transition to chaotic motion through quasiperiodic stages of the oscillatory motion. The radial profiles of frequencies of the radial and vertical harmonic oscillations that are also relevant in the quasiperiodic stages of the oscillatory motion are given, and their properties, independent of the spin of the black holes and the angular momentum and tension of the string loops, are determined. It is shown that the radial profiles differ substantially from those corresponding to the radial and vertical frequencies of the geodetical epicyclic motion; however, they have the same mass scaling and their magnitude is of the same order. Therefore, we are able to demonstrate that, assuming the relevance of resonant phenomena of radial and vertical string-loop oscillations at their frequency ratio 3∶2, the oscillatory frequencies of string loops can be related to the frequencies of the twin high-frequency quasiperiodic oscillations (HF QPOs) observed in the microquasars GRS 1915+105, XTE 1550-564, GRO 1655-40. We can conclude that oscillating current-carrying string loops have to be considered as one of the possible explanations of the HF QPOs occurring in the field of compact objects.
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Resonant Switch (RS) model has recently been proposed as an alternative
to the standard models of twin-peak high-frequency quasi-periodic
oscillations (HF QPOs) observed in low-mass X-ray binaries containing a
neutron star. The model assumes switch of twin oscillations at a
resonant point, where frequencies of the upper and lower oscillations
νU and νL become commensurable and one pair
of the oscillating modes (corresponding to a specific model of HF QPOs)
changes to some other pair due to non-linear resonant phenomena. We test
the RS model for the atoll source 4U 1636-53, where we assume two
resonant points observed at frequency ratios
νU:νL=3:2, 5:4, by fitting the pairs of the
oscillatory modes to the observed data in the regions related to the
resonant points. Among acceptable variants of the RS model the most
promising are those combining the relativistic precession (RP) and the
total precession (TP) frequency relations or their modifications. The
precision of the fits is shown to be strongly increased in comparison to
fits realized by individual pairs along the whole data range. We
demonstrate that the χ2 test is significantly improved.
Fitting of the HF QPO data in the source 4U 1636-53 by the RP1-RP
variant of the RS model gives the best results and implies that the
neutron star mass and dimensionless spin are M≍2.2 Msun
and a≍0.27.
Read More
The kinetic description of relativistic plasmas in the presence of
time-varying and spatially non-uniform electromagnetic (EM) fields is a
fundamental theoretical issue both in astrophysics and plasma physics.
This refers, in particular, to the treatment of collisionless and
strongly-magnetized plasmas in the presence of intense radiation
sources. In this paper, the problem is investigated in the framework of
a covariant gyrokinetic treatment for Vlasov-Maxwell equilibria. The
existence of a new class of kinetic equilibria is pointed out, which
occur for spatially-symmetric systems. These equilibria are shown to
exist in the presence of non-uniform background EM fields and curved
space-time. In the non-relativistic limit, this feature permits the
determination of kinetic equilibria even for plasmas in which particle
energy is not conserved due to the occurrence of explicitly
time-dependent EM fields. Finally, absolute stability criteria are
established which apply in the case of infinitesimal symmetric
perturbations that can be either externally or internally produced.
Read More
