21 research outputs found
K-Shell Photoabsorption Studies of the Carbon Isonuclear Sequence
K-shell photoabsorption cross sections for the isonuclear C I - C IV ions
have been computed using the R-matrix method. Above the K-shell threshold, the
present results are in good agreement with the independent-particle results of
Reilman & Manson (1979). Below threshold, we also compute the strong 1s -> np
absorption resonances with the inclusion of important spectator Auger
broadening effects. For the lowest 1s -> 2p, 3p resonances, comparisons to
available C II, C III, and C IV experimental results show good agreement in
general for the resonance strengths and positions, but unexplained
discrepancies exist. Our results also provide detailed information on the C I
K-shell photoabsorption cross section including the strong resonance features,
since very limited laboratory experimental data exist. The resultant R-matrix
cross sections are then used to model the Chandra X-ray absorption spectrum of
the blazar Mkn 421
Radiation Damping in the Photoionization of Fe^{14+}
A theoretical investigation of photoabsorption and photoionization of
Fe^{14+} extending beyond an earlier frame transformation R-matrix
implementation is performed using a fully-correlated, Breit-Pauli R-matrix
formulation including both fine-structure splitting of strongly-bound
resonances and radiation damping. The radiation damping of
resonances gives rise to a resonant photoionization cross section that is
significantly lower than the total photoabsorption cross section. Furthermore,
the radiation-damped photoionization cross section is found to be in good
agreement with recent experimental results once a global shift in energy of
eV is applied. These findings have important implications.
Firstly, the presently available synchrotron experimental data are applicable
only to photoionization processes and not to photoabsorption; the latter is
required in opacity calculations. Secondly, our computed cross section, for
which the L-shell ionization threshold is aligned with the NIST value, shows a
series of Rydberg resonances that are uniformly 3-4 eV
higher in energy than the corresponding experimental profiles, indicating that
the L-shell threshold energy values currently recommended by NIST are likely in
error.Comment: 4 pages, 1 figures, and 2 table
Sulfur X-ray absorption in the local ISM
We present a study S K-edge using high-resolution HETGS {\it Chandra} spectra
of 36 low-mas X-ray binaries. For each source, we have estimated column
densities for {\rm S}~{\sc i}, {\rm S}~{\sc ii}, {\rm S}~{\sc iii}, {\rm
S}~{\sc xiv}, {\rm S}~{\sc xv} and {\rm S}~{\sc xvi} ionic species, which trace
the neutral, warm and hot phases of the Galactic interstellar medium. We also
estimated column densities for a sample of interstellar dust analogs. We
measured their distribution as a function of Galactic latitude, longitude, and
distances to the sources. While the cold-warm column densities tend to decrease
with the Galactic latitude, we found no correlation with distances or Galactic
longitude. This is the first detailed analysis of the sulfur K-edge absorption
due to ISM using high-resolution X-ray spectra.Comment: 8 pages, 6 figure
Carbon X-ray absorption in the local ISM: Fingerprintsin X-ray Novae spectra
We present a study of the C K-edge using high-resolution LETGS Chandraspectra of four novae during their super-soft-source (SSS) phase. We identified absorption lines due to C II Kα, C III Kα, and C III Kβ resonances. We used these astronomical observations to perform a benchmarking of the atomic data, which involves wavelength shifts of the resonances and photoionization cross-sections. We used improved atomic data to estimate the C II and C III column densities. The absence of physical shifts for the absorption lines, the consistence of the column densities between multiple observations, and the high temperature required for the SSS nova atmosphere modelling support our conclusion about an interstellar medium origin of the respective absorption lines. Assuming a collisional ionization equilibrium plasma the maximum temperature derived from the ratio of C II/C III column densities of the absorbers correspond to T_(max) < 3.05 × 10^4 K
A Comprehensive X-ray Absorption Model for Atomic Oxygen
An analytical formula is developed to represent accurately the
photoabsorption cross section of O I for all energies of interest in X-ray
spectral modeling. In the vicinity of the Kedge, a Rydberg series expression is
used to fit R-matrix results, including important orbital relaxation effects,
that accurately predict the absorption oscillator strengths below threshold and
merge consistently and continuously to the above-threshold cross section.
Further minor adjustments are made to the threshold energies in order to
reliably align the atomic Rydberg resonances after consideration of both
experimental and observed line positions. At energies far below or above the
K-edge region, the formulation is based on both outer- and inner-shell direct
photoionization, including significant shake-up and shake-off processes that
result in photoionization-excitation and double photoionization contributions
to the total cross section. The ultimate purpose for developing a definitive
model for oxygen absorption is to resolve standing discrepancies between the
astronomically observed and laboratory measured line positions, and between the
inferred atomic and molecular oxygen abundances in the interstellar medium from
XSTAR and SPEX spectral models
K-Shell photoabsorption in Si11+: Relativistic contributions via Breit-Pauli R-matrix calculations
As a part of the investigation of the entire Si isonuclear sequence for x-ray spectral diagnostics, we focus on the Li-like Si11+ ion, a stable component that features prominently in x-ray astrophysical spectra. We perform R-matrix calculations for the photoabsorption cross section of Si11+ including relativistic contributions via the use of a Breit-Pauli Hamiltonian. Relativistic effects-predominantly spin-orbit splitting of resonances and mass-velocity and one-body Darwin global shifts of energies -are shown to be tractable and of importance for accuracy in x-ray spectral modeling data banks
Correlation study of endohedrally confined alkaline-earth-metal atoms (A@C-60)
The effects of endohedral confinement on the correlation energy of Be, Mg, and Ca atoms have been investigated using modified Hartree-Fock and multiconfiguration Hartree-Fock methods where the endohedral system (A@C-60) is approximated as an atom enclosed in an attractive spherically symmetric potential well of inner radius r similar to 5.8 a.u. and thickness of Delta similar to 1.89 a.u., and correlation energies are studied as a function of the depth of the confining potential (0 <= U-0 <= 1 a.u.) to give some idea as to how the correlation energy behaves in different endohedral environments. In general, we have found that as a function of well depth, starting from the free atom, valence electrons diffuse outward in the presence of the confining potential, which causes the electrons to be further apart, thereby decreasing the correlation energy; however, with further increase of well depth, the valence electrons become trapped in the confining well and, as a result of their being closer together, the correlation energy increases
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Strong LSJ dependence of fluorescence yields : Breakdown of the configuration-average approximation
Using the five-electron K-shell vacancy 1s2s(2)2p(2) configuration as an example, we show that the fluorescence yields of the eight LSJ states of the configuration exhibit a dramatic dependence on LSJ. These results demonstrate that, in general, configuration-average fluorescence data are inappropriate for astrophysical modeling