Abstract. The line ratios R and G of the three main lines of He-like ions (triplet: resonance, intercombination, forbidden lines) are calculated for C v, N vi, O vii, Ne ix, Mg xi, and Si xiii. These ratios can be used to derive electron density ne and temperature Te of hot late-type stellar coronae and O, B stars from high-resolution spectra obtained with Chandra (LETGS, HETGS) and XMM-Newton (RGS). All excitation and radiative processes between the levels and the effect of upper-level cascades from collisional electronic excitation and from dielectronic and radiative recombination have been considered. When possible the best experimental values for radiative transition probabilities are used. For the higher-Z ions (i.e. Ne ix, Mg xi, Si xiii) possible contributions from blended dielectronic satellite lines to each line of the triplets were included in the calculations of the line ratios R and G for four specific spectral resolutions: RGS, LETGS, HETGS-MEG, HETGS-HEG. The influence of an external stellar radiation field on the coupling of the 2 3 S (upper level of the forbidden line) and 2 3 P levels (upper levels of the intercombination lines) is taken into account. This process is mainly important for the lower-Z ions (i.e. C v, N vi, O vii) at moderate radiation temperature (T rad ). These improved calculations were done for plasmas in collisional ionization equilibrium, but will be later extended to photo-ionized plasmas and to transient ionization plasmas. The values for R and G are given in extensive tables, for a large range of parameters, which could be used directly to compare to the observations.

Abstract.We analyze the high-resolution X-ray spectrum of the Seyfert 1 galaxy NGC 5548, for the full 0.1-10 keV band, using improved calibration results of the Chandra-LETGS instrument. The warm absorber consists of at least three ionization components, with a low, medium and high ionization parameter. The X-ray absorbing material, from an outflowing wind, covers the full range of velocity components found from UV absorption lines. The presence of redshifted emission components for the strongest blue-shifted resonance absorption lines indicate that the absorber is located at a distance larger than the edge of the accretion disk. We derive an upper limit to the edge of the accretion disk of 1 light year. Absorption lines from ions of at least ten chemical elements have been detected, and in general there are no strong deviations from solar abundances. The narrow emission lines from the O vii and Ne ix forbidden and intercombination lines probably originate from much larger distances to the black hole. We find evidence for weak relativistically broadened oxygen and nitrogen emission lines from the inner parts of the accretion disk, but at a much smaller flux level than those observed in some other active galactic nuclei. In addition, there is a broad, non-relativistic C vi Lyα emission line that is consistent with emission lines from the inner part of the optical/UV broad line region.

Abstract.We report the analysis of the high-resolution soft X-ray spectrum of the nearby F-type star Procyon in the wavelength range from 5 to 175Å obtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board Chandra and with the Reflection Grating Spectrometers (RGS) and the EPIC-MOS CCD spectrometers on board XMM-Newton. Line fluxes have been measured separately for the RGS and LETGS. Spectra have been fitted globally to obtain self-consistent temperatures, emission measures, and abundances. The total volume emission measure is ∼4.1 × 10 50 cm −3 with a peak between 1 and 3 MK. No indications for a dominant hot component (T ∼ > 4 MK) were found. We present additional evidence for the lack of a solar-type FIP-effect, confirming earlier EUVE results.

Abstract. Electron density diagnostics based on the triplets of helium-like C v, N vi, and O vii are applied to the X-ray spectra of Capella and Procyon measured with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray Observatory. New theoretical models for the calculation of the line ratios between the forbidden (f ), intercombination (i), and the resonance (r) lines of the helium-like triplets are used. The (logarithmic) electron densities (in cgs units) derived from the f/i ratios for Capella are < 9.38 cm −3 for O vii (2σ upper limit) (f/i = 4.0 ± 0.25), 9.86 ± 0.12 cm −3 for N vi (f/i = 1.78 ± 0.25), and 9.42 ± 0.21 cm −3 for C v (f/i = 1.48 ± 0.34), while for Procyon we obtain 9.28for N vi (f/i = 1.33 ± 0.28), and < 8.92 cm −3 for C v (f/i = 0.48 ± 0.12). These densities are quite typical of densities found in the solar active regions, and also pressures and temperatures in Procyon's and Capella's corona at a level of T ∼ 10 6 K are quite similar. We find no evidence for densities as high as measured in solar flares. Comparison of our Capella and Procyon measurements with the Sun shows little difference in the physical properties of the layers producing the C v, N vi, and O vii emission. Assuming the X-ray emitting plasma to be confined in magnetic loops, we obtain typical loop length scales of L Capella ≥ 8 LProcyon from the loop scaling laws, implying that the magnetic structures in Procyon and Capella are quite different. The total mean surface fluxes emitted in the helium-and hydrogen-like ions are quite similar for Capella and Procyon, but exceed typical solar values by one order of magnitude. We thus conclude that Procyon's and Capella's coronal filling factors are larger than corresponding solar values.

Abstract. We present XMM-Newton data of several RS CVn binary systems. High-resolution X-ray spectra obtained with the Reflection Grating Spectrometers have been interpreted simultaneously with the European Photon Imaging Camera spectra. Highly active stars show a depletion of elements with a low first ionization potential (FIP) relative to high-FIP elements, whereas intermediately active binaries show either no FIP bias or a possible solar-like FIP effect. We find that the low-FIP abundance ratios to oxygen vary with the coronal average temperature whereas the ratios for high-FIP elements stay constant. Since we observe that the absolute Fe (low-FIP) abundance increases with decreasing activity, this suggests that the abundances of elements with low FIP vary with the coronal activity level. Compared with laboratory measurements of the intensity ratios of the Fe  λλ 15.01 and 15.26 Å lines, the coronal plasmas are in the optically thin regime.

Abstract.We report an analysis of the X-ray spectrum of Capella from 6 to 175Å obtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board of the X-ray space observatory CHANDRA. Many emission line features appear that can be resolved much better as compared to former instruments (EUVE and ASCA). Coronal electron densities (ne) and temperatures (T ) of brightly emitting regions are constrained by an analysis of ratios of density-and temperature-sensitive lines of helium-like ions and highly ionized iron atoms. Lines emitted by e.g., O VII & VIII, Mg X-XII, Si XII-XIV, Fe IX, X & XV-XXIII are used to derive T . Line ratios in the helium-like triplets of C V, N VI, O VII, Mg XI, and Si XIII yield T in the range 0.5-10 MK, and ne in the range 10 9 -10 13 cm −3 . The Fe IX/X ratio yields T 0.9 MK, while lines from Fe XVIII to XXII give T ∼ 6-10 MK. Flux ratios of Fe XX-XXII lines indicate for the electron densities an upper limit in the range ne ∼ < (2-5) 10 12 cm −3 . From line ratios of Fe XVII and Fe XVIII we derive constraints on the optical depth τ of ∼1-1.5. An emission measure distribution is derived from Fe line intensities. Results for element abundances (relative to Fe) from a 4-T model are: O and Ne/Fe about solar, N, Mg and Si/Fe ∼2× solar. The results for T and ne are described in terms of quasi-static coronal loop models and it is shown that the X-ray emission originates from compact structures much smaller than the stellar radii.

Motivated by recent claims of a compelling ∼3.5 keV emission line from nearby galaxies and galaxy clusters, we investigate a novel plasma model incorporating a charge exchange component obtained from theoretical scattering calculations. Fitting this kind of component with a standard thermal model yields positive residuals around 3.5 keV, produced mostly by S xvi transitions from principal quantum numbers n ≥ 9 to the ground. Such high-n states can only be populated by the charge exchange process. In this scenario, the observed 3.5 keV line flux in clusters can be naturally explained by an interaction in an effective volume of ∼1 kpc 3 between a ∼3 keV temperature plasma and cold dense clouds moving at a few hundred keV −1 . The S xvi lines at ∼3.5 keV also provide a unique diagnostic of the charge exchange phenomenon in hot cosmic plasmas.

Abstract. We present XMM-Newton observations of γ 2 Velorum (WR 11, WC8+O7.5III, P = 78.53 d), a nearby Wolf-Rayet binary system, at its X-ray high and low states. At high state, emission from a hot collisional plasma dominates from about 1 to 8 keV. At low state, photons between 1 and 4 keV are absorbed. The hot plasma is identified with the shock zone between the winds of the primary Wolf-Rayet star and the secondary O giant. The absorption at low state is interpreted as photoelectric absorption in the Wolf-Rayet wind. This absorption allows us to measure the absorbing column density and to derive a mass loss rate−6 M yr −1 for the WC8 star. This mass loss rate, in conjunction with a previous Wolf-Rayet wind model, provides evidence for a clumped WR wind. A clumping factor of 16 is required. The X-ray spectra below 1 keV (12 Å) show no absorption and are essentially similar in both states. There is a rather clear separation in that emission from a plasma hotter than 5 MK is heavily absorbed in low state while the cooler plasma is not. This cool plasma must come from a much more extended region than the hot material. The Neon abundance in the X-ray emitting material is 2.5 times the solar value. The unexpected detection of C  (25.3 Å) and C  (31.6 Å) radiative recombination continua at both phases indicates the presence of a cool (∼40 000 K) recombination region located far out in the binary system.