He-like Sulfur Research Articles

Absolute measurement of the relativistic magnetic dipole transition in He-like sulfur

We have made an absolute, reference-free measurement of the $1s2s\phantom{\rule{0.28em}{0ex}}^{3}S_{1}\ensuremath{\rightarrow}1{s}^{2}\phantom{\rule{0.28em}{0ex}}^{1}S_{0}$ relativistic magnetic dipole transition in He-like sulfur. The highly charged sulfur ions were provided by an electron-cyclotron resonance ion source, and the x rays were analyzed with a high-precision double-crystal spectrometer. A transition energy of 2430.3685(97) eV was obtained and is compared to most advanced bound-state quantum electrodynamics calculations, providing an important test of two-electron QED effects and precision atomic structure methods in medium-$Z$ species. Thanks to the extremely narrow natural linewidth of this transition and to the large dispersion of the spectrometer at this energy, a complementary study is also performed to evaluate the impact of different silicon crystal atomic form-factor models in the transition-energy analysis. We find no significant dependence on the model used to determine the transition energy.

Diffuse X-ray sky in the Galactic center

Abstract The Galactic diffuse X-ray emission (GDXE) in the Milky Way Galaxy is spatially and spectrally decomposed into the Galactic center X-ray emission (GCXE), the Galactic ridge X-ray emission (GRXE), and the Galactic bulge X-ray emission (GBXE). The X-ray spectra of the GDXE are characterized by the strong K-shell lines of the highly ionized atoms, and the brightest lines are the K-shell transition (principal quantum number transition of n = 2 → 1) of neutral iron (Fe i-Kα), He-like iron (Fe xxv-Heα), and He-like sulfur (S xv-Heα). Accordingly, the GDXE is composed of a high-temperature plasma of ∼7 keV (HTP) and a low-temperature plasma of ∼1 keV, which emit the Fe xxv-Heα and S xv-Heα lines, respectively. The Fe i-Kα line is emitted from nearly neutral irons, and hence the third component of the GDXE is a cool gas (CG). The Fe i-Kα distribution in the GCXE region is clumpy (Fe i-Kα clump), associated with giant molecular cloud (MC) complexes (Sagittarius A, B, C, D, and E) in the central molecular zone. The origin of the Fe i-Kα clumps is the fluorescence and Thomson scattering from the MCs irradiated by past big flares of the supermassive black hole Sagittarius A*. The scale heights and equivalent widths of the Fe i-Kα, Fe xxv-Heα, and Fe xxvi-Lyα (n = 2 → 1 transition of H-like iron) lines are different among the GCXE, GBXE, and GRXE. Therefore, their structures and origins are separately examined. This paper gives an overview of the research history and the present understandings of the GDXE, while in particular focusing on the origin of the HTP and CG in the GCXE.

KLL Dielectronic Recombination of Highly Charged Sulfur and Silicon Ions

Dielectronic recombination measurements for highly charged ions were performed at the Stockholm refrigerated electron beam ion trap. We have obtained KLL DR resonance strengths for highly charged H- and He-like sulfur and silicon ions. The experimental results are compared with the theoretical data obtained from GRASP II code. Both the experimental and calculated results agree well within the experimental error bars. Moreover, the dielectronic recombination resonance strengths are used to obtain the new scaling parameters by incorporating our results with the previous measurements and to check the behaviour of scaling formula for H- and He-like isoelectronic sequences.

Electron–ion recombination of H- and He-like sulfur

Electron–ion recombination of sulfur ions with electrons in the energy range of 1.6–3 keV was studied at the Stockholm Refrigerated Electron Beam Ion Trap. We obtained the KLn dielectronic recombination (DR) cross sections up to n = 5 for H-like and He-like sulfur ions by observing the x-rays from the trapped ions. A fully relativistic distorted wave approximation method was used for calculating the DR cross sections, while the resonance energies were obtained with a multiconfiguration Dirac–Fock approach using the GRASP II code. The calculations agree with the experimental results within the experimental error bars. Additionally, the obtained total DR resonance strengths were used to check the behaviour of a scaling formula for low-Z, He-like iso-electronic sequence (Watanabe et al 2001 J. Phys. B: At. Mol. Opt. Phys. 34 5095).

A Super Bubble Candidate in the Galactic Center and a Local Enhancement G359.77$-$0.09

Abstract A 20$^\prime$$\times$ 16$^\prime;$ elliptical ring-like structure has been found near the Galactic center in the narrow energy band corresponding to the K$\alpha$ line from He-like sulfur. In the ring, two diffuse sources are found, a supernova remnant candidate, G359.79$-$0.26, and an unidentified source, G359.77$-$0.09. The X-ray spectrum of G359.77$-$0.09 is similar to that of G359.79$-$0.26, which can be explained by an absorbed thin thermal plasma model with temperatures of 0.7 and 1.0 keV. The absorption column densities of these two sources are large ($N_{\rm H}$$=$ 6.9 $\times$ 10$^{22}$ and 4.5 $\times$ 10$^{22}$cm$^{-2}$) and are consistent with that of the Galactic center distance. The X-ray spectrum extracted from the ring-like structure is also represented by an absorbed thin thermal plasma model ($kT_{\rm e}$$\sim$ 0.9 keV). The thermal energy of the plasma filling the ring-like structure is estimated to be 1.0 $\times$ 10$^{51}$erg. We therefore propose that the two sources comprise a single ring-like object, which is possibly a super bubble with a size of 49 pc $\times$40 pc in the Galactic center region.

Rate coefficients for the electron impact excitation of He-like sulfur ions

Cross section for the electron impact excitation of S XV is calculated with the use of the R-matrix method. Nineteen lowest LS states are included. Rate coefficients are obtained for the transitions from the ground 1s2 1S state to all the states of 1s2l (l = 0, 1) and 1s3l (l = 0, 1, 2) configurations for the temperature range 1 × 106−1 × 108 K. The resulting effective collision strengths are compared to those reported by Keenan et al. [5]. For the excitation of the n = 3 states, some significant differences are found between the two results.

Observation of H- and He-like X-Ray Line Emission in High-Density Tokamak Plasmas

Characteristic x-ray spectra of plasma impurity ions have been measured in the region $\ensuremath{\lambda}=4.3\ensuremath{-}5.1$ \AA{}. The observation of strong principal $n=2$ to $n=1$ x-ray lines for H-like sulfur and for He-like sulfur and chlorine in the density range (1-6) \ifmmode\times\else\texttimes\fi{} ${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ is reported. Dielectric recombination and inner-shell excitation satellite lines are weak. The results, which are of interest for both atomic physics and plasma diagnostic applications, are compared with atomic calculations of line spectra and transition rates.

Energy shifts ofKαx rays from highly stripped sulfur ions traveling in solids

The energies and intensities of the $2^{3}P_{1}\ensuremath{\rightarrow}1^{1}S_{0}$, and $2^{1}P_{1}\ensuremath{\rightarrow}1^{1}S_{0}$ transitions in He-like sulfur ions, and of the $2^{2}P\ensuremath{\rightarrow}1^{2}S$ transition in H-like sulfur ions have been studied as a function of the thickness and electron density of the solid through which the ions travel. The thickness dependence of the x-ray intensities was analyzed in terms of a three-component model description of $K$-shell vacancy production and decay. Cross sections for electron excitation or ionization and capture deduced from this analysis were used to establish the energies of the x-ray peaks for complete emission in vacuum (i.e., outside the target). Energy shifts were obtained by comparing the peak energies for emission in thick targets to those for emission in vacuum. The results show that the energy shifts increase approximately linearly with the square root of the valence electron density of the target and are in good agreement with theoretical expectations.