16 research outputs found
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
Theory of magnetically powered jets
The magnetic theory for the production of jets by accreting objects is
reviewed with emphasis on outstanding problem areas. An effort is made to show
the connections behind the occasionally diverging nomenclature in the
literature, to contrast the different points of view about basic mechanisms,
and to highlight concepts for interpreting the results of numerical
simulations. The role of dissipation of magnetic energy in accelerating the
flow is discussed, and its importance for explaining high Lorentz factors. The
collimation of jets to the observed narrow angles is discussed, including a
critical discussion of the role of `hoop stress'. The transition between disk
and outflow is one of the least understood parts of the magnetic theory; its
role in setting the mass flux in the wind, in possible modulations of the mass
flux, and the uncertainties in treating it realistically are discussed. Current
views on most of these problems are still strongly influenced by the
restriction to 2 dimensions (axisymmetry) in previous analytical and numerical
work; 3-D effects likely to be important are suggested. An interesting problem
area is the nature and origin of the strong, preferably highly ordered magnetic
fields known to work best for jet production. The observational evidence for
such fields and their behavior in numerical simulations is discussed. I argue
that the presence or absence of such fields may well be the `second parameter'
governing not only the presence of jets but also the X-ray spectra and timing
behavior of X-ray binaries.Comment: 29 pages. Publication delays offered the opportunity for further
corrections, an expansion of sect 4.2, and one more Fig. To appear in
Belloni, T. (ed.): The Jet Paradigm - From Microquasars to Quasars, Lect.
Notes Phys. 794 (2009