Ming Feng Gu

Indirect X-Ray Line-Formation Processes in Iron L-Shell Ions

We present a systematic study of the role of indirect processes in the soft X-ray line formation of iron L-shell ions, using a newly developed, relativistic, multiconfigurational atomic package. These indirect processes involve the neighboring charge states of the target ion, namely, radiative recombination, dielectronic recombination, and resonance excitation. For Fe XVII, the inner-shell collisional ionization (CI) is also relevant. Lines originating from 3s and 3p upper levels of Fe XVII-XX are found to be significantly affected by these processes, with some lines being enhanced by nearly a factor of 2 at the temperature of maximum fractional abundance in CI equilibrium. Such enhancement, although not enough to completely explain the observed line ratios from various astrophysical sources, is a vast improvement over the previous models neglecting these processes. Rate coefficients for individual processes are tabulated, which can be conveniently included in spectral models to correctly account for these effects. As a by-product of this investigation, the total recombination and ionization rates of all Fe L-shell ions are tabulated as a function of temperature.

XMM-Newton reflection grating spectrometer observations of discrete soft-x-ray emission features from NGC 1068

We present the first high-resolution, soft X-ray spectrum of the prototypical Seyfert 2 galaxy, NGC 1068. This spectrum was obtained with the XMM-Newton Reflection Grating Spectrometer (RGS). Emission lines from H-like and He-like low-Z ions (from C to Si) and Fe L-shell ions dominate the spectrum. Strong, nar- row radiative recombination continua (RRCs) for several ions are also present, implying that most of the observed soft X-ray emission arises in low-temperature plasma (kTea few eV). This plasma is photoion- ized by the inferred nuclear continuum (obscured along our line of sight), as expected in the unified model of active galactic nuclei (AGNs). We find excess emission (compared to pure recombination) in all resonance lines (1s!np) up to the photoelectric edge, demonstrating the importance of photoexcitation as well. We introduce a simple model of a cone of plasma irradiated by the nuclear continuum; the line emission we observe along our line of sight perpendicular to the cone is produced through recombination/radiative cas- cade following photoionization and radiative decay following photoexcitation. A remarkably good fit is obtained to the H-like and He-like ionic line series, with inferred radial ionic column densities consistent with recent observations of warm absorbers in Seyfert 1 galaxies. Previous Chandra imaging revealed a large (extending out to � 500 pc) ionization cone containing most of the X-ray flux, implying that the warm absorber in NGC 1068 is a large-scale outflow. To explain the ionic column densities, a broad, flat distribu- tion in the logarithm of the ionization parameter (� ¼ LX=ner 2 ) is necessary, spanning log � ¼ 0-3. This sug- gests either radially stratified ionization zones, the existence of a broad density distribution (spanning a few orders of magnitude) at each radius, or some combination of both. Subject headings: galaxies: individual (NGC 1068) — galaxies: Seyfert — line: formation — X-rays: galaxies

Dielectronic Recombination Rate Coefficients for H-like through Ne-like Isosequences of Mg, Si, S, Ar, Ca, Fe, and Ni

We present distorted-wave calculations and analytic fits to the dielectronic recombination (DR) rate coefficients of H-like to Ne-like isosequences of seven abundant astrophysical elements, including Mg, Si, S, Ar, Ca, Fe, and Ni. Both ΔN = 0 (low-temperature) and ΔN > 0 (high-temperature) DR channels are included for L-shell ions. Values of ΔN > 0 DR for less abundant ions between Mg and Ni are also obtained through interpolation along the isosequences. Interpolation is not performed for ΔN = 0 DR, since such rate coefficients are not smooth along the isosequences in general. Comparison is made with existing theoretical and experimental results, and collisional ionization equilibria resulting from the current DR rate coefficients are presented.

THE SLOW TEMPERATURE EQUILIBRATION BEHIND THE SHOCK FRONT OF SN 1006

We report on the observation of O VII Doppler line broadening in a compact knot at the edge of SN 1006 detected with the reflective grating spectrometer on board XMM-Newton. The observed line width of σ = 3.4 ± 0.5 eV at a line energy of 574 eV indicates an oxygen temperature of kT = 528 ± 150 keV. Combined with the observed electron temperature of ~1.5 keV, the observed broadening is direct evidence for temperature nonequilibration in high Mach number shocks and slow subsequent equilibration. The O VII line emission allows an accurate determination of the ionization state of the plasma, which is characterized by a relatively high forbidden line contribution, indicating log net 9.2.

Radiative Recombination Rate Coefficients for Bare through F-like Isosequences of Mg, Si, S, Ar, Ca, Fe, and Ni

We present distorted-wave calculations and analytic fits to the radiative recombination rate coefficients of bare to F-like isosequences of seven abundant astrophysical elements, including Mg, Si, S, Ar, Ca, Fe, and Ni. The fits are valid over a wide range of temperature, from 10-4 to 104 eV. Extensive comparisons with existing data are made.

High-Resolution Chandra Spectroscopy Of Gamma Cassiopeiae (B0.5e)

γ Cas is the prototypical classical B0.5e star and is now known to be the primary in a wide binary system. It has long been famous for its unique hard X-ray characteristics, among which are variations that correlate with changes in a number of optical light and UV line and continuum properties. These peculiarities have led to a picture in which processes on or near the Be star produce the observed X-ray emission. In this paper we report on a 53 ks Chandra High Energy Transmission Grating Spectrometer observation of this target. An inspection of our spectrum shows that it is quite atypical for a massive star. The emission lines appear weak because of a strong short-wavelength continuum that arises from a hot plasma with kT = 11-12 keV. The spectrum exhibits many lines, the strongest of which are Lyα features of H-like species from Fe through the even-Z intermediate elements (S, Si, Mg, and Ne), down to O and N. Line ratios of the rif triplet for a variety of He-like ions and of Fe XVII are consistent with the dominance of collisional atomic processes. However, the presence of Fe and Si fluorescence K features indicates that photoionization also occurs in nearby cold gas. The line profiles indicate a mean velocity at rest with an rms line broadening of 500 km s-1 and little or no asymmetry. An empirical global-fitting analysis of the line and continuum spectrum suggests that there are actually three or four plasma emission components. The first is the dominant hot (12 keV) component, of which some fraction (10%-30%) is heavily absorbed, while the remainder is affected by a much lower column density of only 3 × 1021 cm-2. The hot component has a Fe abundance of only 0.22 ± 0.05 solar. The other two or three major emission components are warm and are responsible for most other emission lines. These components are dominated by plasma having temperatures near 0.1, 0.4, and 3 keV. Altogether, the warm components have an emission measure of about 14% of the hot component, a low column density, and a more nearly solar composition. The 100 eV component is consistent with X-ray temperatures associated with a wind in a typical early B star. Nonetheless, its emission measure is a few times higher than would be expected from this explanation. The strength of the fluorescence features and the dual-column absorption model for the hot plasma component suggest the presence near the hot sites of a cold gas structure with a column density of ~1023 cm-2. Because this is also the value determined by Millar and Marlborough for the vertical column of the Be disk of γ Cas, these attributes suggest that the X-ray-emitting sources could be close to the disk and hence to the Be star. Finally, we discuss the probably related issues of the origin of the warm emission components, as well as the puzzling deficient Fe abundance in the hot component. It is possible that the latter anomaly is related to the FIP (abundance fractionation) effect found in certain coronal structures on the Sun and RS CVn stars. This would be yet another indication that the X-rays are produced in the immediate vicinity of the Be star.

Laboratory Measurements of the Relative Intensity of the 3s → 2p and 3d → 2p Transitions in Fe XVII

The intensity ratios of the 3s → 2p and 3d → 2p lines in Fe XVII were measured on the Livermore electron beam ion trap employing a complementary set of spectrometers, including a high-resolution crystal spectrometer and the Goddard 32 pixel calorimeter. The resulting laboratory data are in agreement with satellite measurements of the Sun and astrophysical sources in collisional equilibrium such as Capella, Procyon, and NGC 4636. The results disagree with earlier laboratory measurements and assertions that processes not accounted for in laboratory measurements must play a role in the formation of the Fe XVII spectra in solar and astrophysical plasmas.

Laboratory Measurements of Iron L-Shell Emission: 3→2 Transitions of Fe XXI-XXIV between 10.5 and 12.5 Å

Using the electron beam ion trap EBIT-II facility at Lawrence Livermore National Laboratory, we have measured the iron L-shell spectrum between 10.5 and 12.5 A for Fe XXI-XXIV with a spectral resolution of ~30 mA. The relative line intensities of strong 3 → 2 transitions for each charge state are measured as functions of electron energy and compared to distorted wave (DW) calculations. The contributions of resonant processes, namely, resonant excitation (RE) and spectroscopically unresolved dielectronic recombination (DR) satellites, are investigated. The RE contributions are shown to be less than the experimental and theoretical uncertainties for plasma in collisional ionization equilibrium. The unresolved DR satellites, however, enhance the emissivities of almost all lines, some by as large as 15%, consistent with our earlier measurements for Fe XXIV. The DW results agree with our measurements to better than 20% under the condition of collisional ionization equilibrium. The line emissivities in the widely used spectral synthesis model, MEKAL, are also compared to our measurements and are found to be discrepant at a greater than 20% level for some lines.

ΔN = 0 Dielectronic Recombination as a Line Formation Process in Photoionized Plasmas

In photoionized plasmas, X-ray line emission is dominated by radiative cascades following recombination and photoexcitation, with a negligible contribution from collisional excitation due to the relatively low electron temperatures. Most existing photoionized plasma emission codes include only radiative recombination (RR) when calculating the level populations. Such models have been shown to be inadequate to explain the X-ray emission from H-like and He-like ions in the spectra of some Seyfert 2 active galactic nuclei and X-ray binaries obtained by the grating spectrometers on board Chandra and XMM-Newton. It has been shown that photoexcitation must be included to obtain theoretical models consistent with observations. In this Letter we show that for X-ray emission from iron L-shell ions, one must also include dielectronic recombination (DR) as a level population mechanism. As an example, the well-known 3 → 2 lines of Fe XVII have been analyzed in detail. The 3F line at 16.78 A is most affected by DR of Fe XVIII at low temperatures, while it is negligibly weak in purely RR models.

Recombination X-Ray Line Formation of Iron L-Shell Ions in Low-Temperature Plasmas

In low-temperature plasmas, X-ray line emission is dominated by radiative cascades following recombination and photoexcitation, with a negligible contribution from collisional excitation. In this paper, we present theoretical line formation rate coefficients for all n → 2 (3 ≤ n ≤ 5) transitions of Fe XVII-XXIV in the temperature range between 1 and 500 eV. Both radiative recombination (RR) and ΔN = 0 dielectronic recombination (DR) are taken into account. In addition, the rate coefficients for radiative recombination continua onto n = 2 and 3 states of Fe XVII-XXIV are tabulated. The total RR rate coefficients of Fe XVIII-XXV and total ΔN = 0 DR rate coefficients of Fe XVIII-XXIV are presented as well, and compared with existing experimental measurements and theoretical calculations.