Sextet transitions in Fe ii

We have calculated oscillator strengths for the J – J ' levels of the a  ⁶D– z  ⁶P^o, the a  ⁶D– z  ⁶D^o and the a  ⁶D– z  ⁶F^o multiplets in Fe  ii , using configuration interaction wave functions. The importance of the mixing of the ⁶P^o, ⁶D^o and ⁶F^o states is shown, as is the effect of core polarization of both the 3 p and 3 s orbitals. Good agreement with recent experimental results is obtained.     

Emission lines of Fe xv in spectra obtained with the Solar Extreme-Ultraviolet Research Telescope and Spectrograph

Recent R-matrix calculations of electron impact excitation rates in Mg-like Fe  xv are used to derive theoretical emission-line ratios involving transitions in the 243–418 Å  wavelength range. A comparison of these with a data set of solar active region, subflare and off-limb spectra, obtained during rocket flights by the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS), reveals generally very good agreement between theory and observation, indicating that most of the Fe  xv emission lines may be employed with confidence as electron density diagnostics. In particular, the 312.55-Å  line of Fe  xv is not significantly blended with a Co  xvii transition in active region spectra, as suggested previously, although the latter does make a major contribution in the subflare observations. Most of the Fe  xv transitions which are blended have had the species responsible clearly identified, although there remain a few instances where this has not been possible. We briefly address the long-standing discrepancy between theory and experiment for the intensity ratio of the  3s^2 1S–3s3p ³P₁  intercombination line at 417.25 Å  to the  3s^2 1S–3s3p ¹P  resonance transition at 284.16 Å.     

Fe xiixii emission lines in solar active regions observed by the RES-C spectroheliograph on the CORONAS-I mission

Theoretical line intensity ratios involving Fe  xii transitions in the 186–201 Å wavelength range are compared with observational data for five solar active regions, obtained by the RES-C spectroheliograph on the CORONAS-I mission. Generally good agreement is found between theory and observation, hence resolving discrepancies previously found in the comparison of calculations with active region and subflare spectra from the Solar EUV Rocket Telescope and Spectrograph ( SERTS ). However, the Fe  xii 190.06- and 201.12-Å lines are blended with Fe  x 190.04 Å and Fe  xiii 201.13 Å, respectively. In addition, a weak feature at ∼197 Å, tentatively identified as Fe  xii 196.87 Å, does not appear to be due to this ion.     

An investigation of Fe xvi emission lines in solar and stellar extreme-ultraviolet and soft X-ray spectra

New fully relativistic calculations of radiative rates and electron impact excitation cross-sections for Fe  xvi are used to determine theoretical emission-line ratios applicable to the 251–361 and 32–77 Å portions of the extreme-ultraviolet (EUV) and soft X-ray spectral regions, respectively. A comparison of the EUV results with observations from the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS) reveals excellent agreement between theory and experiment. However, for emission lines in the 32–49 Å portion of the soft X-ray spectral region, there are large discrepancies between theory and measurement for both a solar flare spectrum obtained with the X-Ray Spectrometer/Spectrograph Telescope (XSST) and for observations of Capella from the Low-Energy Transmission Grating Spectrometer (LETGS) on the Chandra X-ray Observatory . These are probably due to blending in the solar flare and Capella data from both first-order lines and from shorter wavelength transitions detected in second and third order. By contrast, there is very good agreement between our theoretical results and the XSST and LETGS observations in the 50–77 Å wavelength range, contrary to previous results. In particular, there is no evidence that the Fe  xvi emission from the XSST flare arises from plasma at a much higher temperature than that expected for Fe  xvi in ionization equilibrium, as suggested by earlier work.     

Ultraviolet Fe ii emission in z∼ 2 quasars

We present spectra of six luminous quasars at   z ∼ 2  , covering rest wavelengths 1600−3200 Å. The fluxes of the UV Fe  ii emission lines and Mg  ii λ2798 doublet, the line widths of Mg  ii and the 3000 Å luminosity were obtained from the spectra. These quantities were compared with those of low-redshift quasars at   z = 0.06–0.55  studied by Tsuzuki et al. In a plot of the Fe  ii (UV)/Mg  ii flux ratio as a function of the central black hole mass, Fe  ii (UV)/Mg  ii in our   z ∼ 2  quasars is systematically greater than in the low-redshift quasars. We confirmed that luminosity is not responsible for this excess. It is unclear whether this excess is caused by rich Fe abundance at   z ∼ 2  over low-redshift or by non-abundance effects such as high gas density, strong radiation field and high microturbulent velocity.     

Fe xxv and Fe xxvi lines from low-velocity, photoionized gas in the X-ray spectra of active galactic nuclei

We have calculated the equivalent widths of the absorption lines produced by Fe  xxv and Fe  xxvi in a Compton-thin, low-velocity photoionized material illuminated by the nuclear continuum in active galactic nuclei. The results, plotted against the ionization parameter and the column density of the gas, are a complement to those presented by Bianchi & Matt for the emission lines from the same ionic species. As an extension to the work by Bianchi & Matt, we also present a qualitative discussion on the different contributions to the He-like iron emission line complex in the regimes where recombination or resonant scattering dominates, providing a useful diagnostic tool to measure the column density of the gas. Future high-resolution missions (e.g. Astro-E2 ) will allow us to fully take advantage of these plasma diagnostics. In the meantime, we compare our results with an up-to-date list of Compton-thick and unobscured (at least at the iron line energy) Seyfert galaxies with emission and/or absorption lines from H- and He-like iron observed with Chandra and XMM–Newton .     

Anomalous mixing of the w2Po3/2 and x6Po3/2 levels in Fe ii

The neighbouring lines a ⁶D_5/2– x ⁶P^o_3/2 (1272.617 Å) and a ⁶D_5/2– w ²P^o_3/2 (1272.657 Å) have been observed in the UV spectrum of χ Lupi to be of comparable intensity. The latter, Δ S  = 2, transition would be expected to be very weak. The two upper states should display negligible mixing. We give a detailed, quantitative discussion of how the two upper states are in fact strongly mixed through their mixing with 3d⁶(³D)4p ⁴P^o_3/2, and hence we explain the relative strengths of the two UV lines.     

The origin of [Fe ii] emission in NGC 4151

The centre of NGC 4151 has been observed in the J band with the SMIRFS integral field unit (IFU) on the UK Infrared Telescope. A map of [Fe  ii ] emission is derived, and compared with the distributions of the optical narrow-line region and radio jet. We conclude that, because the [Fe  ii ] emission is associated more closely with the visible narrow-line region than with the radio jet, it arises mainly through photoionization of gas by collimated X-rays from the Seyfert nucleus. The velocity field and strength with respect to Pa β are found to be consistent with this argument. The performance of the IFU is considered briefly, and techniques for observation and data analysis are discussed.     

X-ray absorption in the strong Fe ii narrow-line Seyfert 1 galaxy Markarian 507

We present results from spectral analysis of ASCA data on the strong Fe  ii narrow-line Seyfert 1 galaxy Mrk 507. This galaxy was found to have an exceptionally flat ROSAT spectrum among the narrow-line Seyfert 1 galaxies (NLS1s) studied by Boller, Brandt & Fink. The ASCA spectrum, however, shows a clear absorption feature in the energy band below 2 keV, which partly accounts for the flat spectrum observed with the ROSAT Position Sensitive Proportional Counter (PSPC). Such absorption is rarely observed in other NLS1s. The absorption is mainly the result of cold (neutral or slightly ionized) gas with a column density of (2–3) × 10²¹ cm⁻². A reanalysis of the PSPC data shows that an extrapolation of the best-fitting model for the ASCA spectrum underpredicts the X-ray emission observed with the PSPC below 0.4 keV if the absorber is neutral (which indicates that the absorber is slightly ionized), covers only part of the central source, or there is extra soft thermal emission from an extended region. There is also evidence that the X-ray absorption is complex; an additional edge feature marginally detected at 0.84 keV suggests the presence of an additional high-ionization absorber, which imposes a strong O  viii edge on the spectrum. After correction for the absorption, the photon index of the intrinsic continuum, Γ ≃ 1.8, obtained from the ASCA data is quite similar to that of ordinary Seyfert 1 galaxies. Mrk 507 still has one of the flattest continuum slopes among the NLS1s, but is no longer exceptional. The strong optical Fe  ii emission remains unusual in the light of the correlation between Fe  ii strengths and steepness of soft X-ray slope.     

Extreme ultraviolet transitions of Fe xxi in solar, stellar and laboratory spectra

Recent R -matrix calculations of electron impact excitation rates for transitions among the 2s²2p², 2s2p³ and 2p⁴ levels of Fe  xxi are used to derive theoretical electron density ( N _e) sensitive emission-line ratios involving 2s²2p²–2s2p³ transitions in the ∼98–146Å wavelength range. A comparison of these with observations from the PLT tokamak plasma, for which the electron density has been independently determined, reveals generally very good agreement between theory and experiment, and in some instances removes discrepancies found previously. The observed Fe  xxi ratios for a solar flare, obtained with the OSO–5 satellite, imply electron densities which are consistent, with discrepancies that do not exceed 0.2 dex. In addition, the derived values of N _e are similar to those estimated for the high-temperature regions of other solar flares. The good agreement between theory and observation, in particular for the tokamak spectra, provides experimental support for the accuracy of the present line-ratio calculations, and hence for the atomic data on which they are based.     

Inner-shell photoionization of O iii

A 25-state R -matrix calculation is performed to obtain photoionization cross-sections for transitions from the 1s²2s²2p²³P ground state of the O  iii ion. Results are obtained for a range of photon energies, including those at which K-shell photoionization processes take place. We compare our results with those from previous calculations. Excellent agreement is obtained. We also consider resonances owing to transitions of a 1s electron excited into the 2p orbital and compare with a recent calculation.     

Theoretical calculations of the H i, He i and He ii free–bound continuous emission spectra

We present coefficients for the calculation of the continuous emission spectra of H  i , He  i and He  ii due to electron–ion recombination. Coefficients are given for photon energies from the first ionization threshold for each ion to the   n = 20  threshold of hydrogen  (36.5 μm)  , and for temperatures  100 ≤  T ≤ 10⁵ K  . The emission coefficients for He  i are derived from accurate ab initio photoionization data. The coefficients are scaled in such a way that they may be interpolated by a simple scheme with uncertainties less than 1 per cent in the whole temperature and wavelength domain. The data are suitable for incorporation into photoionization/plasma codes and should aid with the interpretation of spectra from the very cold ionized gas phase inferred to exist in a number of gaseous clouds.