1,097 research outputs found
Thermo-Rotational Instability in Plasma Disks Around Compact Objects
Differentially rotating plasma disks, around compact objects, that are
imbedded in a ``seed'' magnetic field are shown to develop vertically localized
ballooning modes that are driven by the combined radial gradient of the
rotation frequency and vertical gradients of the plasma density and
temperature. When the electron mean free path is shorter than the disk height
and the relevant thermal conductivity can be neglected, the vertical particle
flows produced by of these modes have the effect to drive the density and
temperature profiles toward the ``adiabatic condition'' where
. Here is the plasma temperature and
the particle density. The faster growth rates correspond to steeper
temperature profiles such as those produced by an internal
(e.g., viscous) heating process. In the end, ballooning modes excited for
various values of can lead to the evolution of the disk into a
different current carrying configuration such as a sequence of plasma rings
Interpretation of the I-Regime and transport associated with relevant heavy particle modes
The excitation of a novel kind of heavy particle [1, 2] mode at the edge of the plasma column is
considered as the signature of the I-con nement Regime [3{7]. The outward transport of impurities
produced by this mode is in fact consistent with the observed expulsion of them from the main
body of the plasma column (a high degree of plasma purity is a necessary feature for fusion burning
plasmas capable of approaching ignition). Moreover, the theoretically predicted mode phase velocity,
in the direction of the electron diamagnetic velocity, has been con rmed by relevant experimental
analyses [8] of the excited
uctuations (around 200 kHz). The plasma \spontaneous rotation" in the
direction of the ion diamagnetic velocity is also consistent, according to the Accretion Theory [9] of
this phenomenon, with the direction of the mode phase velocity. Another feature of the mode that
predicted by the theory is that the I-Regime exhibits a knee of the ion temperature at the edge of
the plasma column but not one of the particle density as the mode excitation factor is the relative
main ion temperature gradient exceeding the local relative density gradient. The net plasma current
density appearing in the saturation stage of the relevant instability, where the induced particle and
energy
uxes are drastically reduced, is associated with the signi cant amplitudes of the poloidal
magnetic eld
uctuations [6, 7] observed to accompany the density
uctuations. The theoretical
implications of the signi cant electron temperature
uctuations [10] observed are discussed.United States. Dept. of Energ
Active black holes: Relevant plasma structures, regimes and processes involving all phase space
The presented theory is motivated by the growing body of experimental information on the characteristics, connected with relevant spectral, time, and space resolutions, of the radiation emission from objects considered as rotating black holes. In the immediate surroundings of these objects, three plasma regions are identified: an innermost Buffer Region, an intermediate Three-regime Region, and a Structured Peripheral Region. In the last region, a Composite Disk Structure made of a sequence of plasma rings corresponding to the formation of closed magnetic surfaces is considered to be present and to allow intermittent accretion flows along the relevant separatrices. The nonlinear âMaster Equationâ describing composite disk structures is derived and solved in appropriate asymptotic limits. A ring configuration, depending on the state of the plasma at the microscopic level: (i) can be excluded from forming given the strongly nonthermal nature of the electron distribution (in momentum space) within the Three-regime Region allowing the onset of a spiral structure; the observed High Frequency Quasi Periodic Oscillations are associated with these tridimensional structures; (ii) may be allowed to propagate to the outer edge of the Buffer Region where successive rings carrying currents in opposite directions are ejected vertically (in opposite directions) and originate the observed jets; or (iii) penetrates in the Three-regime Region and is dissipated before reaching the outer edge of the Buffer Region. The absence of a coherent composite disk structure guiding accretion in the presence of a significant magnetic field background is suggested to characterize quiescent black holes.United States. Dept. of Energ
Ballooning Modes in Thin Accretion Disks: Limits for their Excitation
The conditions that limit the possible excitation of ideal MHD axisymmetric
ballooning modes in thin accretion disks are discussed. As shown earlier by
Coppi and Coppi (2001), these modes are well-localized in the vertical
direction but have characteristic oscillatory and non-localized profiles in the
radial direction. A necessary condition for their excitation is that the
magnetic energy be considerably lower than the thermal energy. Even when this
is satisfied, there remains the problem of identifying the possible physical
factors which can make the considered modes radially localized. The general
solution of the normal mode equation describing the modes is given, showing
that it is characterized by a discrete spectrum of eigensolutions. The growth
rates are reduced and have a different scaling relative to those of the
"long-cylinder" modes, commonly known as the Magneto Rotational Instability,
that have been previously studied.Comment: 25 pages, 7 figures. Accepted to Ap
Kinetics of electron-positron pair plasmas using an adaptive Monte Carlo method
A new algorithm for implementing the adaptive Monte Carlo method is given. It
is used to solve the relativistic Boltzmann equations that describe the time
evolution of a nonequilibrium electron-positron pair plasma containing
high-energy photons and pairs. The collision kernels for the photons as well as
pairs are constructed for Compton scattering, pair annihilation and creation,
bremsstrahlung, and Bhabha & Moller scattering. For a homogeneous and isotropic
plasma, analytical equilibrium solutions are obtained in terms of the initial
conditions. For two non-equilibrium models, the time evolution of the photon
and pair spectra is determined using the new method. The asymptotic numerical
solutions are found to be in a good agreement with the analytical equilibrium
states. Astrophysical applications of this scheme are discussed.Comment: 43 pages, 7 postscript figures, to appear in the Astrophysical
Journa
The QUEST-La Silla AGN Variability Survey
We present the characterization and initial results from the QUEST-La Silla
AGN variability survey. This is an effort to obtain well sampled optical light
curves in extragalactic fields with unique multi-wavelength observations. We
present photometry obtained from 2010 to 2012 in the XMM-COSMOS field, which
was observed over 150 nights using the QUEST camera on the ESO-Schmidt
telescope. The survey uses a broadband filter, the -band, similar to the
union of the and the filters, achieving an intrinsic photometric
dispersion of mag, and a systematic error of mag in the
zero-point. Since some detectors of the camera show significant non-linearity,
we use a linear correlation to fit the zero-points as a function of the
instrumental magnitudes, thus obtaining a good correction to the non-linear
behavior of these detectors. We obtain good photometry to an equivalent
limiting magnitude of . Studying the optical variability of X-ray
detected sources in the XMM-COSMOS field, we find that the survey is
% complete to magnitudes , and % complete to a
magnitude . The determination and parameterization of the structure
function () of the variable sources shows
that most BL AGN are characterized by and . It is
further shown that variable NL AGN and GAL sources occupying the same parameter
space in and are very likely to correspond to obscured or low
luminosity AGN. Our samples are, however, small, and we expect to revisit these
results using larger samples with longer light curves obtained as part of our
ongoing survey.Comment: Accepted for publication in Ap
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