488 research outputs found
Using the X-ray Emission Lines of Seyfert 2 AGN to Measure Abundance Ratios
We measure the metal abundance ratios in the X-ray photoionized gas located
near the narrow line region of a sample of Seyfert 2 AGN. The high-resolution
X-ray spectra observed with the Chandra high- and low-energy transmission
grating spectrometers are compared with models of the resonant scattering and
recombination emission from a plasma in thermal balance, and with multiple
temperature zones. The abundance ratios in the sample are close to the Solar
values, with slight over-abundances of N in NGC 1068, and of Ne in NGC 4151.
Our X-ray spectral models use fewer degrees of freedom than previous works.Comment: 2 pages, 1 figure, to appear in the proceedings of the conference
"Multiwavelenth AGN Surveys", held in Cozumel, Mexico, December 200
Theory of Weak Localization in Ferromagnetic (Ga,Mn)As
We study quantum interference corrections to the conductivity in (Ga,Mn)As
ferromagnetic semiconductors using a model with disordered valence band holes
coupled to localized Mn moments through a p-d kinetic-exchange interaction. We
find that at Mn concentrations above 1% quantum interference corrections lead
to negative magnetoresistance, i.e. to weak localization (WL) rather than weak
antilocalization (WAL). Our work highlights key qualitative differences between
(Ga,Mn)As and previously studied toy model systems, and pinpoints the mechanism
by which exchange splitting in the ferromagnetic state converts valence band
WAL into WL. We comment on recent experimental studies and theoretical analyses
of low-temperature magnetoresistance in (Ga,Mn)As which have been variously
interpreted as implying both WL and WAL and as requiring an impurity-band
interpretation of transport in metallic (Ga,Mn)As.Comment: 16 pages, 10 figures; submitted to Phys. Rev.
Kondo Temperature in Multilevel Quantum Dots
We develop a general method to evaluate the Kondo temperature in a multilevel
quantum dot that is weakly coupled to conducting leads. Our theory reveals that
the Kondo temperature is strongly enhanced when the intradot energy-level
spacing is comparable to or smaller than the charging energy. We propose an
experiment to test our result, which consists of measuring the size-dependence
of the Kondo temperature.Comment: 4 pages, 1 figure and supplementary material. Revised and improved
version, to appear in Phys. Rev. Let
Atomic X-Ray Spectra of Accretion Disk Atmospheres in the Kerr Metric
We calculate the atmospheric structure of an accretion disk around a Kerr
black hole and obtain its X-ray spectrum, which exhibits prominent atomic
transitions under certain circumstances. The gravitational and Doppler
(red)shifts of the C V, C VI, O VII, O VIII, and Fe I-XXVI emission lines are
observable in active galaxies. We quantify the line emissivities as a function
of radius, to identify the effects of atmospheric structure, and to determine
the usefulness of these lines for probing the disk energetics. The line
emissivities do not always scale linearly with the incident radiative energy,
as in the case of Fe XXV and Fe XXVI. Our model incorporates photoionization
and thermal balance for the plasma, the hydrostatic approximation perpendicular
to the plane of the disk, and general relativistic tidal forces. We include
radiative recombination rates, fluorescence yields, Compton scattering, and
photoelectric opacities for the most abundant elements.Comment: 4 pages, 1 figure, to appear in the Proc. of the 10th Marcel
Grossmann Meeting on General Relativity, World Scientific, Rio de Janeiro,
July 20-26, 200
Non-Adiabatic Spin Transfer Torque in Real Materials
The motion of simple domain walls and of more complex magnetic textures in
the presence of a transport current is described by the
Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations
depend sensitively on the ratio between the dimensionless material parameter
which characterizes non-adiabatic spin-transfer torques and the Gilbert
damping parameter . This ratio has been variously estimated to be close
to 0, close to 1, and large compared to 1. By identifying as the
influence of a transport current on , we derive a concise, explicit and
relatively simple expression which relates to the band structure and
Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate
that intrinsic spin-orbit interactions lead to intra-band contributions to
which are often dominant and can be (i) estimated with some confidence
and (ii) interpreted using the "breathing Fermi surface" model.Comment: 18 pages, 9 figures; submitted to Phys. Rev.
Resonant Scattering and Recombination in CAL 87
The eclipsing supersoft X-ray binary CAL 87 has been observed with Chandra on
August 13/14, 2001 for nearly 100 ksec, covering two full orbital cycles and
three eclipses. The shape of the eclipse light curve derived from the
zeroth-order photons indicates that the size of the X-ray emission region is
about 1.5 solar radii. The ACIS/LETG spectrum is completely dominated by
emission lines without any noticeable continuum. The brightest emission lines
are significantly redshifted and double-peaked, suggestive of emanating in a
2000 km/s wind. We model the X-ray spectrum by a mixture of recombination and
resonant scattering. This allows us to deduce the temperature and luminosity of
the ionizing source to be kT = 50-100 eV and L_X = 5E37 erg/s.Comment: To appear in Proceedings of IAU Coll. 194 "Compact binaries in the
Galaxy and beyond" (Rev. Mex. A&A Conf. Series), eds. G. Tovmassian and E.
Sio
Reprocessing of Soft X-ray Emission Lines in Black Hole Accretion Disks
By means of a Monte Carlo code that accounts for Compton scattering and
photoabsorption followed by recombination, we have investigated the radiation
transfer of Ly alpha, He alpha, and recombination continua photons of H- and
He-like C, N, O, and Ne produced in the photoionized atmosphere of a
relativistic black hole accretion disk. We find that photoelectric opacity
causes significant attenuation of photons with energies above the O VIII
K-edge; that the conversion efficiencies of these photons into lower-energy
lines and recombination continua are high; and that accounting for this
reprocessing significantly (by factors of 21% to 105%) increases the flux of
the Ly alpha and He alpha emission lines of H- and He-like C and O escaping the
disk atmosphere.Comment: 4 pages including 4 encapsulated postscript figures; LaTeX format,
uses aastex.cls and emulateapj5.sty; accepted on 2004 January 13 for
publication in The Astrophysical Journa
Soliton Solutions with Real Poles in the Alekseev formulation of the Inverse-Scattering method
A new approach to the inverse-scattering technique of Alekseev is presented
which permits real-pole soliton solutions of the Ernst equations to be
considered. This is achieved by adopting distinct real poles in the scattering
matrix and its inverse. For the case in which the electromagnetic field
vanishes, some explicit solutions are given using a Minkowski seed metric. The
relation with the corresponding soliton solutions that can be constructed using
the Belinskii-Zakharov inverse-scattering technique is determined.Comment: 8 pages, LaTe
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