850 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
Detailed Opacity Calculations for Stellar Models
Radiative opacity is an important quantity in the modeling of stellar
structure and evolution. In the present work we recall the role of opacity in
the interpretation of pulsations of different kinds of stars. The detailed
opacity code SCO-RCG for local-thermodynamic-equilibrium (LTE) plasmas is
described, as well as the OPAMCDF project dedicated to the spectroscopy of LTE
and non-LTE plasmas. Interpretations, with the latter codes, of several laser
and Z pinch experiments in conditions relevant to astrophysical applications
are also presented and our work in progress as concerns the internal solar
conditions is illustrated.Comment: submitted to ASP Conf. Se
Plasma-screening effects in the atrophysically relevant He-like and Li-like Mg and Fe ions
The effect of plasma environment on the atomic energy levels of He-like and
Li-like Mg and Fe ions have been studied using Debye model. The
equation-of-motion coupled-cluster (EOMCC) and Fock-space coupled-cluster
(FSCC) formalisms in the relativistic frame work have been adopted to describe
the atomic states and the energy levels of the above plasma embedded ions.
Salient features of these methods have been described to account the two
electron screening effects through the Debye potentials. The two-body screening
potential has been derived in the multipole expansion form to evaluate the
reduced matrix elements in solving the equation of motion. Using this extended
model, we have also predicted that quasi-degeneracy among the energy states
having same principal quantum number () but different angular momentum ()
is slacken, whereas fine structure splitting is unaffected with increasing
plasma strength. These knowledge are useful in estimatingradiative opacity,
photoionization cross sections, line intensities, etc of the aforementioned
astrophysical plasmas.Comment: 10 pages, 5 tables and 4 figures. arXiv admin note: text overlap with
arXiv:1101.3184 by other author
Photoionization of the Be Isoelectronic Sequence: Relativistic and Nonrelativistic R-Matrix Calculations
The photoionization of the beryllium-like isoelectronic series has been studied. The bound state wave functions of the target ions were built with CIV3 program. The relativistic Breit-Pauli R-matrix method was used to calculate the cross sections in the photon energy range between the ionization threshold and 1s24f7/2 threshold for each ion. For the total cross sections of Be, B+, C+2, N+3, and O+4, our results match experiment well. The comparison between the present work and other theoretical works are also discussed. We show the comparison with our LS results as it indicates the importance of relativistic effects on different ions. In the analysis, the resonances converging to 1s22lj and 1s23lj were identified and characterized with quantum defects, energies and widths using the eigenphase sum methodology. We summarize the general appearance of resonances along the resonance series and along the isoelectronic sequence. Partial cross sections are also reported systematically along the sequence. All calculations were performed on the NERSC system
Photoionization of Fullerens and Atoms Confined in Fullerenes
The photoionization cross sections of small fullerenes (C28, C32, C40, C44, and C50), and the outer and near-outer shells of atoms (noble gases, alkaline earth) confined endohedrally inside a C60 molecule are calculated employing a time-dependent local density approximation formulation. Plasmon and confinement resonances are found to be a general feature of these cross sections, and dramatic interchannel coupling effects, significantly increasing the atomic cross sections, are exhibited in all cases in the vicinity of the C60 plasmons. Hybridization effects, the mixing of the atomic and cage bound state wave functions, are also found, but no systematics of the hybridization present themselves. Also, in the case of Ar@C60, Inter-atomic Columbic decay (ICD) has been found and studied
Photoionization of the Potassium Isoelectronic Sequence: Ca+ and Transition Metal Ions
Photoionization cross section calculations are performed for the ground ([Ne]3s23p63d 2D ) and the first two excited ([Ne]3s23p63d 2D and [Ne]3s23p64s 2S ) states of potassium-like transition metal ions (Sc+2, Ti+3, V+4, Cr+5, Mn+6, Fe+7), along with photoionization calculations for K-like Ca+ ions in the ground ([Ne]3s23p64s 2S ) state and the first two excited ([Ne]3s23p63d 2D and [Ne]3s23p63d 2D ) states. The discrete N-electron final state ion system orbitals are generated using the computer program AUTOSTRUCTURE; 24 configurations are included in the configuration-interaction (CI) calculation for transition metal ions, and 30 configurations for the case of Ca+ ions. The initial and final (N+1)-electron wavefunctions are generated using R-matrix along with photoionization cross sections. In addition to the non-relativistic (LS-coupling) R-matrix, we have used the relativistic (Breit-Pauli) R -matrix method to carry out these calculations to focus on relativistic effects. Relativistic and non-relativistic results are compared to demonstrate the influence of relativistic effects. The prominent 3p → 3d giant resonances are analyzed and identified, and our calculated positions and widths are compared with experimental results for K-like ions such as Ca+, Sc+2 and Ti+3. In the case of lower Z (22 Z 20) ions (Ca+, Sc+2 and Ti+3), the photoionization cross section spectra are dominated by the giant (3p 3d excitation) resonances, while in cases of higher Z (26 Z 23) ions (V+4, Cr+5, Mn+6 and Fe+7), the 3p 3d resonances lie below the ionization threshold, and the cross sections are dominated by 3p53d nd and 3p53d n’s Rydberg series of resonances. Comparison of the Ca+, Sc+2 and Ti3+ results with available theoretical and experimental data shows good agreement
Two-photon vibrational transitions in as probes of variation of the proton-to-electron mass ratio
Vibrational overtones in deeply bound molecules are sensitive probes for
variation of the proton-to-electron mass ratio . In nonpolar molecules,
these overtones may be driven as two-photon transitions. Here, we present
procedures for experiments with , including state-preparation
through photoionization, a two-photon probe, and detection. We calculate
transition dipole moments between all X\,^2\Pi_g vibrational levels and those
of the A\,^2\Pi_u excited electronic state. Using these dipole moments, we
calculate two-photon transition rates and AC-Stark-shift systematics for the
overtones. We estimate other systematic effects and statistical precision.
Two-photon vibrational transitions in provide multiple routes to
improved searches for variation.Comment: 19 pages, 3 figures, supplementary material (v2 fixes an
ancillary-file upload issue
- …