357 research outputs found
Modeling of Photoionized Plasmas
In this paper I review the motivation and current status of modeling of
plasmas exposed to strong radiation fields, as it applies to the study of
cosmic X-ray sources. This includes some of the astrophysical issues which can
be addressed, the ingredients for the models, the current computational tools,
the limitations imposed by currently available atomic data, and the validity of
some of the standard assumptions. I will also discuss ideas for the future:
challenges associated with future missions, opportunities presented by improved
computers, and goals for atomic data collection.Comment: 17 pages, 8 figures, to appear in the proceedings of Xray2010,
Utrecht, the Netherlands, March 15-17 201
Atomic X-ray Spectroscopy of Accreting Black Holes
Current astrophysical research suggests that the most persistently luminous
objects in the Universe are powered by the flow of matter through accretion
disks onto black holes. Accretion disk systems are observed to emit copious
radiation across the electromagnetic spectrum, each energy band providing
access to rather distinct regimes of physical conditions and geometric scale.
X-ray emission probes the innermost regions of the accretion disk, where
relativistic effects prevail. While this has been known for decades, it also
has been acknowledged that inferring physical conditions in the relativistic
regime from the behavior of the X-ray continuum is problematic and not
satisfactorily constraining. With the discovery in the 1990s of iron X-ray
lines bearing signatures of relativistic distortion came the hope that such
emission would more firmly constrain models of disk accretion near black holes,
as well as provide observational criteria by which to test general relativity
in the strong field limit. Here we provide an introduction to this phenomenon.
While the presentation is intended to be primarily tutorial in nature, we aim
also to acquaint the reader with trends in current research. To achieve these
ends, we present the basic applications of general relativity that pertain to
X-ray spectroscopic observations of black hole accretion disk systems, focusing
on the Schwarzschild and Kerr solutions to the Einstein field equations. To
this we add treatments of the fundamental concepts associated with the
theoretical and modeling aspects of accretion disks, as well as relevant topics
from observational and theoretical X-ray spectroscopy.Comment: 63 pages, 21 figures, Einstein Centennial Review Article, Canadian
Journal of Physics, in pres
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A terminal assessment of stages theory : introducing a dynamic states approach to entrepreneurship
Stages of Growth models were the most frequent theoretical approach to understanding entrepreneurial business growth from 1962 to 2006; they built on the growth imperative and developmental models of that time. An analysis of the universe of such models (N=104) published in the management literature shows no consensus on basic constructs of the approach, nor is there any empirical confirmations of stages theory. However, by changing two propositions of the stages models, a new dynamic states approach is derived. The dynamic states approach has far greater explanatory power than its precursor, and is compatible with leading edge research in entrepreneurship
Accretion and ejection in black-hole X-ray transients
Aims: We summarize the current observational picture of the outbursts of
black-hole X-ray transients (BHTs), based on the evolution traced in a
hardness-luminosity diagram (HLD), and we offer a physical interpretation.
Methods: The basic ingredient in our interpretation is the Poynting-Robertson
Cosmic Battery (PRCB, Contopoulos & Kazanas 1998), which provides locally the
poloidal magnetic field needed for the ejection of the jet. In addition, we
make two assumptions, easily justifiable. The first is that the mass-accretion
rate to the black hole in a BHT outburst has a generic bell-shaped form. This
is guaranteed by the observational fact that all BHTs start their outburst and
end it at the quiescent state. The second assumption is that at low accretion
rates the accretion flow is geometrically thick, ADAF-like, while at high
accretion rates it is geometrically thin.
Results: Both, at the beginning and the end of an outburst, the PRCB
establishes a strong poloidal magnetic field in the ADAF-like part of the
accretion flow, and this explains naturally why a jet is always present in the
right part of the HLD. In the left part of the HLD, the accretion flow is in
the form of a thin disk, and such a disk cannot sustain a strong poloidal
magnetic filed. Thus, no jet is expected in this part of the HLD. The
counterclockwise traversal of the HLD is explained as follows: the poloidal
magnetic field in the ADAF forces the flow to remain ADAF and the source to
move upwards in the HLD rather than to turn left. Thus, the history of the
system determines the counterclockwise traversal of the HLD. As a result, no
BHT is expected to ever traverse the entire HLD curve in the clockwise
direction.
Conclusions: We offer a physical interpretation of accretion and ejection in
BHTs with only one parameter, the mass transfer rate.Comment: Accepted for publication in A&
Magnetic fields in cosmic particle acceleration sources
We review here some magnetic phenomena in astrophysical particle accelerators
associated with collisionless shocks in supernova remnants, radio galaxies and
clusters of galaxies. A specific feature is that the accelerated particles can
play an important role in magnetic field evolution in the objects. We discuss a
number of CR-driven, magnetic field amplification processes that are likely to
operate when diffusive shock acceleration (DSA) becomes efficient and
nonlinear. The turbulent magnetic fields produced by these processes determine
the maximum energies of accelerated particles and result in specific features
in the observed photon radiation of the sources. Equally important, magnetic
field amplification by the CR currents and pressure anisotropies may affect the
shocked gas temperatures and compression, both in the shock precursor and in
the downstream flow, if the shock is an efficient CR accelerator. Strong
fluctuations of the magnetic field on scales above the radiation formation
length in the shock vicinity result in intermittent structures observable in
synchrotron emission images. Resonant and non-resonant CR streaming
instabilities in the shock precursor can generate mesoscale magnetic fields
with scale-sizes comparable to supernova remnants and even superbubbles. This
opens the possibility that magnetic fields in the earliest galaxies were
produced by the first generation Population III supernova remnants and by
clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review
Implications of H.E.S.S. observations of pulsar wind nebulae
In this review paper on pulsar wind nebulae (PWN) we discuss the properties
of such nebulae within the context of containment against cross-field diffusion
(versus normal advection), the effect of reverse shocks on the evolution of
offset ``Vela-like'' PWN, constraints on maximum particle energetics, magnetic
field strength estimates based on spectral and spatial properties, and the
implication of such field estimates on the composition of the wind. A
significant part of the discussion is based on the High Energy Stereoscopic
System ({\it H.E.S.S.} or {\it HESS}) detection of the two evolved pulsar wind
nebulae Vela X (cocoon) and HESS J1825-137. In the case of Vela X (cocoon) we
also review evidence of a hadronic versus a leptonic interpretation, showing
that a leptonic interpretation is favored for the {\it HESS} signal. The
constraints discussed in this review paper sets a general framework for the
interpretation of a number of offset, filled-center nebulae seen by {\it HESS}.
These sources are found along the galactic plane with galactic latitudes
, where significant amounts of molecular gas is found. In these
regions, we find that the interstellar medium is inhomogeneous, which has an
effect on the morphology of supernova shock expansion. One consequence of this
effect is the formation of offset pulsar wind nebulae as observed.Comment: to appear in Springer Lecture Notes on Neutron Stars and Pulsars: 40
years after their discovery, eds. W. Becke
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