K-shell Transitions Research Articles

ChandraObservations of Diffuse X‐Rays from the Sagittarius B2 Cloud

We present the first Chandra results of the giant molecular cloud Sagittarius B2 (Sgr B2), located about 100 pc away from the Galactic center. Diffuse X-rays are clearly separated from one-and-a-half-dozen resolved point sources. The X-ray spectrum exhibits pronounced iron K-shell transition lines at 6.40 keV (Kα) and 7.06 keV (Kβ), deep iron K-edge at 7.11 keV, and large photoelectric absorption at low energy. The absorption-corrected X-ray luminosity is ~1 × 1035 ergs s-1, 2 orders of magnitude larger than the integrated luminosity of all the resolved point sources. The diffuse X-rays come mainly from the southwest half of the cloud with a concave shape pointing to the Galactic center direction. These results strongly support the ASCA model that Sgr B2 is irradiated by an X-ray source at the Galactic center side.

The Reflection Grating Spectrometer on board XMM-Newton

The ESA X-ray Multi Mirror mission, XMM-Newton, carries two identical Reflection Grating Spectrometers (RGS) behind two of its three nested sets of Wolter I type mirrors. The instrument allows high-resolution (E/(Delta)E = 100 to 500) measurements in the soft X-ray range (6 to 38 A or 2.1 to 0.3 keV) with a maximum effective area of about 140 sq cm at 15 A. Its design is optimized for the detection of the K-shell transitions of carbon, nitrogen, oxygen, neon, magnesium, and silicon. as well as the L shell transitions of iron. The present paper gives a full description of the design of the RGS and its operational modes. We also review details of the calibrations and in-orbit performance including the line spread function, the wavelength calibration, the effective area, and the instrumental background.

Diffuse X-ray emission around the Galactic Center

We represent ASCA/GIS results of the diffuse X-ray emission around the Galactic Center (hereafter GC) region of angular size of about 10 degree. From the observed fields, we found diffuse emissions with K-shell transition lines from highly ionized silicon, sulphur, argon, calcium and/or iron. These give firm evidence for the presence of a thin thermal plasma. With the flux distribution of each line, we conclude that the diffuse X-ray plasma exhibits lower temperature at larger Galactic latitude than that at smaller latitude.

Radiative heating of B, Al and Ni thin foils at 15–25 eV temperatures

Several thin foils have been heated by X-ray radiation emitted by a small gold cavity irradiated with the LULI facility Nd glass laser. The foils were heated to moderate temperatures (10–25 eV) in conditions near local thermodynamic equilibrium. Measurements of the absorption of K α transitions of aluminum, of the K-shell transitions of boron, and of the n=2–3 and 4 transitions of nickel are reported. A 5000 l/mm transmission grating spectrograph recorded the boron data, and a flat thallium hydrogen phthalate (TlAP) crystal was used for aluminum and nickel absorption measurements. The nickel and aluminum spectra indicate a higher temperature (22–25 eV) than the boron case (14 eV), explained by a difference in the absorption spectra of these elements. In the Ni case the measured 2p-3d spin–orbit–split absorption structures compared well with theoretical spectra obtained with the SCO superconfiguration code using the spin orbit split transition arrays formalism (SOSA). The heating of the foils as calculated with radiative hydrodynamic simulations is very similar to the experimental data.

Grazing-incidence measurements of L-shell line emission from highly charged Fe in the soft x-ray region

Line emission from highly charged ions in the soft x-ray region represents an important diagnostic for high-temperature plasmas. The wavelength region of 10–30 Å is occupied by lines from the K-shell transitions in N through Ne and L-shell transitions in Ti through Ni. Crystal spectrometers have previously been used to investigate Fe line radiation under well controlled conditions using the LLNL electron beam ion trap facility in this wavelength region. To complement this work we have instituted a grazing-incidence spectrometer with a variable line-spaced, concave grating and a flat field of focus which is suited for a multichannel detector. Simultaneous observations of the Ne-like Fe16+ n=3→2 transitions were made with this instrument and a flat-crystal spectrometer in order to establish the linear polarization of the Fe XVII lines. We present a description of the grating spectrometer and discuss its performance in comparison with Rowland circle based grazing incidence instruments.

K-SHELL AUGER AND RADIATIVE TRANSITIONS IN THE CARBON ISOELECTRONIC SEQUENCE, 6 ≤Z≤ 54

K-shell energies and transition rates for the Auger and radiative decays in carbon-like ions with 1s2sm2pn(m + n= 5) configurations have been calculated using the multiconfiguration Dirac–Fock method including Breit interaction and quantum electrodynamic corrections. Seventeen ions with atomic numbers 6 ≤Z≤ 54 were considered. Transition rates were evaluated in intermediate coupling with configuration interaction within the same complex. Line fluorescence yields were also computed.

ASCA observation of the Galactic Center

X-ray imaging and spectroscopic observations near the Galactic Center in the 0.5–10 keV energy band were carried out with the X-ray satellite ASCA. Several X-ray sources were detected. Luminosities of the sources are estimated to be of order 10 35–10 36 erg s −1. The largely extended emission with K-shell transition lines from highly ionized atoms were confirmed. Moreover, an extended emission of the 6.4 keV line from the low ionized iron atom was discovered. The 6.4 keV line intensity is elongated to the north-east from the Galactic Center along the galactic plane and well correlates with the distribution of the dense molecular clouds observed in the radio bands. We interpret that the 6.4 keV line originates from the florescent from the molecular cloud irradiated by bright X-ray source.

Overview of the current spectroscopy effort on the Livermore electron beam ion traps

An overview is given of the current spectroscopic effort on the Livermore electron beam ion trap facilities. The effort focuses on four aspects: spectral line position, line intensity, temporal evolution, and line shape. Examples of line position measurements include studies of the K-shell transitions in heliumlike Kr34+ and the 2s-2p intrashell transitions in lithiumlike Th87+ and U89+, which provide benchmark values for testing the theory of relativistic and quantum electrodynamical contributions in high-Z ions. Examples of line intensity measurements are provided by measurements of the electron-impact excitation and dielectronic recombination cross sections of the heliumlike transition-metal ions Ti20+ through Co25+. A discussion of radiative lifetime measurements of metastable levels in heliumlike ions is given to illustrate our time-resolved spectroscopy techniques in the microsecond range. We also present a measurement of the spectral lineshape that illustrates the very low ion temperatures that can be achieved in an EBIT.

The Hot Gas in the Galactic Bulge

X-ray imaging spectroscopic observations near the Galactic center region were carried out with the ASCA satellite. We found two bright spots very close to the Galactic center (Sgr A∗); one is extended and has a soft spectrum associated with strong emission lines from highly ionized irons, while the other is a point-like object with a harder spectrum and a larger absorption. We also found extended emission with K-shell transition lines from highly ionized Si, S, Ar, Ca and Fe. Remarkable feature found with ASCA is an extended emission of 6.4 keV lines of low ionization irons. The 6.4 keV line fluxes are found to be well correlated to the region of cool clouds. We interpret that the 6.4 keV line is due to florescence from the cool clouds irradiated by strong (and obscured from our line of sight) X-ray beams.

Ionic Fragmentation of CF3Cl Photoexcited in the Cl L-Shell, C K-Shell, and F K-Shell Transition Regions

Abstract Fragment ions produced from CF3Cl have been measured in the soft X-ray region using monochromatized synchrotron radiation and a time-of-flight mass spectrometer. The yield ratios for ion production were determined from the TOF spectra over the energy range of 44 through 1200 eV, and the partial yield spectra for the individual ions were observed around the Cl L2,3-edges, the C K-edge, and the F K-edge. The atomic chlorine ion, Cl+, has the greatest intensity between 100 and 700 eV, and exhibits a steep increase at the Cl L2,3-edges. The yield ratios of F+ and C+ decrease sharply at the Cl L2,3-edges and greatly increase at the C K-edge and F K-edge. Some fragment ions, e. g. CF+, have distinctive yield changes upon excitation of the inner-shell electron to unoccupied molecular orbitals.

Studies of He-like krypton for use in determining electron and ion temperatures in very-high-temperature plasmas

High-resolution measurements of the He-like Kr K-shell radiation are presented showing the direct excitation lines as well as Li- and Be-like dielectronic recombination lines appearing in the KLL resonance. The measurements were performed on the Electron Beam Ion Trap (EBIT) facility and motivated by the need for accurate atomic data of high-Z He-like K-shell transitions for diagnostic applications in determining the central ion and electron temperature of very-high-temperature plasmas produced in next-generation tokamaks, such as the International Thermonuclear Experimental Reactor (ITER).

Measurements of 3d-5f and 3d-6f transition energies in nickel-like ions for resonant photopumping by helium-like ions

The 3d-5f and 3d-6f transition energies of several Ni-like ions were measured to study their suitability for use in an X-ray laser scheme that uses He-like K-shell transitions for resonant photopumping. Specifically, 19 transitions in Ag19+, Sn22+, Pr31+, Gd36+, Yb42+, Ta45+, Ir49+, Th62+ and U64+ were measured and compared to 2p–1s transitions in F7+, Ne8+, Si12+, S14+, Cl15+, Ar16+, Ti20+ and V21+. The best resonance found was for the F He-like 21P1–11S0 transition pumping the Ag Ni-like 3d3/2–6f5/2 transition at 737.6 eV. The energy difference of 0.14 (24) eV corresponds to a 190-ppm resonance. An energy shift between the theoretical and measured energies of 3d5/2–6f7/2 transitions is confirmed by the present measurements; a similar effect is also observed in the 3d3/2–5f5/2, 3d5/2–5f7/2 and 3d3/2–6f5/2 transitions.

Diagnostics of the Coronal Plasma

AbstractIn this review I intend to discuss selected aspects of the X and EUV diagnostics of the corona. Especially, I would like to stress on the publications which indicate that the assumptions commonly adopted in the interpretation of solar soft X-ray (SXR) spectra are not necessarily adequate.Multitude of prominent features from excited stages of many abundant ions of N, O, Ne, Na, Mg, Al, Si (K-shell transitions), Ar, Ca, Fe and Ni (K- and L-shell transitions) are contained in the 1-25 Å wavelength band. All these features are observed in emission on top of the continuum.A list of important space experiments performed by 1985 has been prepared by Doschek (R1985). Substantial experimental activity has been observed since that time. Japanese-UK-USAYohkohpayload is delivering thousands of SXR images and spectra each week. Theoretical considerations, reinterpretation of previous measurements and interpretation of incoming data stream indicate that number of assumptions characteristic for the ”classical approach“ have to be at least questioned.

Experimental investigation of non LTE population redistribution in radiatively pumped plasmas

Experiments are described in which simultaneous absorption and fluorescence spectra of K-shell transitions were observed from a well characterized x-ray driven aluminium plasma. Fluorescence was caused by photopumping with the M-band x-rays from a high-Z laser produced plasma. Pump and drive spectra, ionization and density histories and fluorescence spectra were all carefully measured. Fluorescence was observed also from transitions which were not directly photopumped. This is attributed to collisional-radiative redistribution in ions with a 1 s vacancy. The fluorescent intensity suggests a redistribution rate much faster than the total rate at which the 1 s vacancy is filled, contrary to expectations based on simple estimates of the rates.

Optically thick X-ray transfer - The shell game

This paper investigates the radiative transfer of X-rays through a shell that is optically thick to Compton scattering, surrounding a point source of continuum X-rays. The emission and absorption of X-rays due to K-shell transitions of iron are included. The calculations are done in two entirely independent ways: by Monte Carlo simulation and by solving a Fokker-Planck diffusion equation. The emergent spectra agree very well for Thomson depths of at least about 2. The validity is confirmed of the modification to the Fokker-Planck equation of Kompaneets (1957) that is required when the photon energy is large compared with the average thermal energy of the electrons. A procedure is also developed for treating models of compact X-ray sources consisting of incomplete shells.