196 research outputs found
Magnetohydrodynamics of Gamma-Ray Burst Outflows
Using relativistic, axisymmetric, ideal MHD, we examine the outflow from a
disk around a compact object, taking into account the baryonic matter, the
electron-positron/photon fluid, and the large-scale electromagnetic field.
Focussing on the parameter regime appropriate to gamma-ray burst outflows, we
demonstrate, through exact self-similar solutions, that the thermal force
(which dominates the initial acceleration) and the Lorentz force (which
dominates further out and contributes most of the acceleration) can convert up
to ~50% of the initial total energy into asymptotic baryon kinetic energy. We
examine how baryon loading and magnetic collimation affect the structure of the
flow, including the regime where emission due to internal shocks could take
place.Comment: To be published in ApJ Letters. 4 pages, 1 figur
Angular momentum transport in protostellar discs
Angular momentum transport in protostellar discs can take place either
radially, through turbulence induced by the magnetorotational instability
(MRI), or vertically, through the torque exerted by a large-scale magnetic
field that threads the disc. Using semi-analytic and numerical results, we
construct a model of steady-state discs that includes vertical transport by a
centrifugally driven wind as well as MRI-induced turbulence. We present
approximate criteria for the occurrence of either one of these mechanisms in an
ambipolar diffusion-dominated disc. We derive ``strong field'' solutions in
which the angular momentum transport is purely vertical and ``weak field''
solutions that are the stratified-disc analogues of the previously studied MRI
channel modes; the latter are transformed into accretion solutions with
predominantly radial angular-momentum transport when we implement a
turbulent-stress prescription based on published results of numerical
simulations. We also analyze ``intermediate field strength'' solutions in which
both modes of transport operate at the same radial location; we conclude,
however, that significant spatial overlap of these two mechanisms is unlikely
to occur in practice. To further advance this study, we have developed a
general scheme that incorporates also the Hall and Ohm conductivity regimes in
discs with a realistic ionization structure.Comment: 8 pages, 4 figures, 1 table; accepted for publication in MNRA
Magnetic Acceleration and Collimation of Gamma-Ray Burst Jets
Exact semianalytic solutions for GRB outflows were recently derived using the
equations of special-relativistic ideal MHD (see Vlahakis & Konigl in these
Proceedings for a summary). This contribution focuses on the implications of
these results to various modeling and observational issues in GRB sources,
including the baryon loading problem, polarization measurements of the prompt
and reverse-shock emission, and the possible existence of a two-component
outflow.Comment: 5 pages, 1 figure, to appear in proceedings of 2003 Gamma-Ray Burst
Conference (Santa Fe, New Mexico, September 8-12, 2003), to be published by
AI
Are Magnetic Wind-Driving Disks Inherently Unstable?
There have been claims in the literature that accretion disks in which a
centrifugally driven wind is the dominant mode of angular momentum transport
are inherently unstable. This issue is considered here by applying an
equilibrium-curve analysis to the wind-driving, ambipolar diffusion-dominated,
magnetic disk model of Wardle & Konigl (1993). The equilibrium solution curves
for this class of models typically exhibit two distinct branches. It is argued
that only one of these branches represents unstable equilibria and that a real
disk/wind system likely corresponds to a stable solution.Comment: 5 pages, 2 figures, to be published in ApJ, vol. 617 (2004 Dec 20).
Uses emulateapj.cl
Observational Evidence for a Multiphase Outflow in QSO FIRST J1044+3656
Spectral absorption features in active galactic nuclei (AGNs) have
traditionally been attributed to outflowing photoionized gas located at a
distance of order a parsec from the central continuum source. However, recent
observations of QSO FIRST J104459.6+365605 by de Kool and coworkers, when
intepreted in the context of a single-phase gas model, imply that the
absorption occurs much farther (approx 700 pc) from the center. We reinterpret
these observations in terms of a shielded, multiphase gas, which we represent
as a continuous low-density wind with embedded high-density clouds. Our model
satisfies all the observational constraints with an absorbing gas that extends
only out to about 4 pc from the central source. The different density
components in this model coexist in the same region of space and have similar
velocities, which makes it possible to account for the detection in this source
of absorption features that correspond to different ionization parameters but
have a similar velocity structure. This model also implies that only a small
fraction of the gas along the line of sight to the center is outflowing at the
observed speeds and that the clouds are dusty whereas the uniform gas component
is dust free. We suggest that a similar picture may apply to other sources and
discuss additional possible clues to the existence of multiphase outflows in
AGNs.Comment: 6 pages, 2 figures, Accepted for publication in ApJ v569 n2, April
20, 200
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