Fe XXIV Research Articles

Laboratory Measurements of Resonant Contributions to Fe XXIV Line Emission

Using an electron beam ion trap, we have measured the relative cross sections for Fe XXIV line emission at electron energies between 0.7 and 3.0 keV. Good agreements with distorted wave and R-matrix calculations are found at energies above 1.5 keV. At lower energies, the contributions of resonant excitation are observed and agree with R-matrix calculations. Below the excitation thresholds, the intensities of dielectronic recombination satellites for capture into n ≥ 5 levels are measured.

The Active Corona of HD 35850 (F8 V)

We present Extreme Ultraviolet Explorer (EUVE) spectroscopy and photometry of the nearby F8 V star HD 35850 (HR 1817). The EUVE short-wavelength 75-175 A and medium-wavelength 160-365 A spectra reveal 28 emission lines from Fe IX and Fe XV to Fe XXIV. The Fe XXI λλ102, 129 ratio yields an upper limit for the coronal electron density, logne<11.6 cm−3. The EUVE short-wavelength spectrum shows a small but clearly detectable continuum. The 75-150 A line-to-continuum ratio indicates approximately solar Fe abundances, with 0.8<Z<1.6 (90% confidence interval). Upper limits have been derived for a dozen high-emissivity Fe X through Fe XIV lines. The resulting emission measure distribution is characterized by two broad temperature components at logT of 6.8 and 7.4. Over the course of the 1 week observation, large-amplitude, long-duration flares were not seen in the EUVE Deep Survey light curve, although the light curve does show signs of persistent, low-level flaring and possible rotational modulation. The EUVE spectra have been compared with nonsimultaneous ASCA SIS spectra of HD 35850 obtained in 1995. The SPEX DEM analysis of the SIS spectrum indicates the same temperature distribution as the EUVE DEM analysis. However, the SIS spectra suggest subsolar abundances, 0.34<Z<0.81. Although some of the discrepancy may be the result of incomplete X-ray line lists, we cannot explain the disagreement between the EUVE line-to-continuum ratio and the ASCA-derived Fe abundance. The X-ray surface flux on HD 35850 is comparable to that of cooler dwarfs of comparable age and rotation like EK Draconis (G0 V) and AB Doradus (K1 V). Given its youth (t≈100 Myr), its rapid rotation (vsini≈50 km s−1), and its high X-ray activity (LX≈1.5×1030 ergs s−1), HD 35850 may represent an activity extremum for single, main-sequence F-type stars. The variability and emission measure distribution can be reconstructed using the continuous flaring model of Gudel provided that the flare distribution has a power-law index α≈1.8. Similar results obtained for other young solar analogs suggest that continuous flaring is a viable coronal heating mechanism on rapidly rotating, late-type, main-sequence stars.

Calibration of the SERTS-95 Spectrograph from Iron Line Intensity Ratios

Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph was flown on 1995 May 15 (SERTS-95), carrying a multilayer-coated toroidal diffraction grating which enhanced the instrumental sensitivity within its second-order wave band (170-225 A). Spectra and spectroheliograms of NOAA Active Region 7870 (N09/W22) were obtained in this wave band with a spectral resolution (instrumental FWHM) ~30 mA. We developed and applied a technique for deriving the relative radiometric calibration independent of laboratory calibration measurements by employing a method proposed by Neupert & Kastner for monitoring variations in the sensitivities of orbiting EUV spectrometers by means of density- and temperature-insensitive line intensity ratios. Numerous ratios of emission lines from Fe X-XIV are mutually consistent and yield an instrumental response curve that matches the design characteristics of the multilayer coating. This supports the accuracy of the atomic physics parameters and demonstrates the power of the technique. Many of the ratios employed here can be used to carry out a similar calibration exercise on spectra from the Coronal Diagnostic Spectrometer's Grazing Incidence Spectrograph (CDS/GIS) aboard the Solar and Heliospheric Observatory. Because they are relatively free from blending with nearby strong lines, the following density-sensitive ratios are particularly well suited for analysis with the GIS: Fe X λ175.265/λ174.526, Fe X λ175.265/λ184.534, Fe XII λ186.867/λ195.117, Fe XIII λ203.820/λ202.042, Fe XIII λ200.017/λ202.042, and Fe XIV λ219.121/λ211.317. Densities derived from Fe X, XIII, and XIV yield log ne~9.4±0.2.

Atomic data from the IRON Project

Collision strengths for electron induced tran- sitions in Li-like Fe xxiv are calculated using a Breit- Pauli R-matrix technique. The target has 15 ne struc- ture states corresponding to n 4. Autoionizing reso- nances are found to aect 18 of the 39 collision strengths, namely those for transitions 2lj!n 0 l 0 j 0 where n 0 =2 ; 3 : The present calculation is carried out only for electrons incident on the ground state with energies not exceeding 265 Ry. Beyond this we use the relativistic distorted wave collision strengths of Zhang et al. in order to complete our results. Thermally averaged collision strengths are com- puted and tabulated as functions of logT for transitions out of the 2s1=2 ,2 p 1 = 2 and 2p3=2 states. The temperature range considered is 6:2 log T 8:0, Fe xxiv having its maximum coronal abundance when logT =7 :2.

The calculation of photoionization of highly-ionized iron using R-matrix theory

Results of recent R-matrix calculations on the photoionization cross-sections for Fe XXIII and Fe XXIV are presented. These, unlike previous calculations, predict large resonance structure.

Kα emission in equilibrium and non-equilibrium plasmas

A detailed spectral model has been under development for the computer simulation of 2 p → ls Kα x-ray emissions from highly-charged Fe ions in plasmas. Particular interest has been directed at the Ka emissions produced by the ions from Fe XVIII to Fe XXIV, which occur in the spectral range from 1.84 to 1.94 Å. Account has been taken of the fundamental radiative emission processes associated with radiationless electron capture or dielectronic recombination, inner-shell-electron collisional excitation, and inner-shell-electron collisional ionization. In high-temperature plasmas, small or moderate departures from steady-state corona-model charge-state distributions may occur due to the effects of ion transport processes. However, the assumption of equilibrium (Maxwellian) electron energy distributions is usually accepted to be valid. Particular emphasis has now been directed at the identification of spectral features that can serve as diagnostics of extreme non-equilibrium or transient-ionization conditions, which can occur in both low-density and high-density plasmas as well as in electron-ion beam interactions. In the development of the general theoretical description of the emission spectra, it has been necessary to allow for an arbitrary (non-Maxwellian) electron energy distribution. In order to provide a microscopic investigation of the fundamental Ka line-formation processes that can play a dominant role under extreme non-equilibrium conditions, the observed x-ray emission spectra from Fe ions in the Electron Beam Ion Trap (EBIT) facility at the Lawrence Livermore National Laboratory have been simulated. For the precise interpretation of the high-resolution x-ray observations, which may involve the analysis of blended spectral features composed of many lines, it has been necessary to take into account the multitude of fine-structure components of the Ka radiative transitions in the ions from Fe XVIII to Fe XXIV. A complex situation can be encountered for densities that are higher than the validity range of the simple corona-model approximation. With increasing density, collisionally-induced transitions among the low-lying fine-structure states can play an important role. We have developed a hierarchy of simplified statistical-population models for the distribution of the initial ions among the different low-lying fine-structure states. The inadequacies of this simple approximation can be fundamentally remedied only by the application of a detailed (and possibly time-dependent) collisional-radiative-model description of the fine-structure excitation, de-excitation, and ionization processes. It has been found that inner-shell-electron collisional excitation and ionization processes involving the complex intermediate ions from Fe XVIII to Fe XXI produce spectral features, in the wavelength range from 1.89 to 1.94 Å, which are particularly sensitive to density variations and transient-ionization conditions. For a precise analysis of the x-ray observations produced by anisotropic (directed) electron collisional excitations, it will be necessary in a future extension of this investigation to take into account the angular distribution and polarization of the emitted radiation.

Observations of Thermal Plasma in a Solar Limb Flare

All but a small portion of the top of the 1991 October 21 flare was occulted by the solar limb. Thus, both the Bragg crystal spectrometer (BCS) and the soft X-ray telescope (SXT) on Yohkoh observed only a small region of flaring plasma. Using observations from those instruments, we have determined the temporal behavior of the emission measure, temperature, and nonthermal broadening in the loop-top source. We find that for a portion of the decay phase of this flare, the temperatures determined using the Fe XXV, Ca XIX, and S XV BCS channels and SXT filter ratios all agree. Assuming that the plasma is isothermal, we determine the abundances of Fe XXII, Fe XXIII, and Fe XXIV relative to Fe XXV. Comparing the results with recent ionization equilibrium calculations, we find that there are discrepancies between the Fe XXII-to-Fe XXV ratio, the Fe XXIII-to-Fe XXV ratio, and the Fe XXIV-to-Fe XXV ratio and ionization equilibrium calculations, which we attribute to uncertainties in the ionization balance calculations.

Support for X‐Ray Deduced Abundances in Galaxies and Clusters: The Consistency of Iron Abundances from L and K Emission

Significantly subsolar metallicities have been reported in X-rays for a variety of sources, often in disagreement with theoretical expectations or with measurements at other wavelengths. As a test of the reliability of the X-ray abundances in systems dominated by thermal emission from hot gas in collisional ionization equilibrium, we compare Fe abundances determined independently from the Fe L and Fe K emission regions in simple one- and two-temperature thermal fits to ASCA X-ray spectra of the elliptical galaxy M87 and five galaxy clusters. We find that the Fe L and Fe K abundances are consistent within or near their 90% confidence errors, which would not necessarily be the case if there were severe problems with the atomic physics or with the instrument calibrations. Our data cover a range of plasma temperatures kT = 2-4 keV, where the Fe L emission arises primarily from Fe XXIII and XXIV and the Fe K emission arises primarily from Fe XXV. There is also a contribution from less ionized gas at lower temperatures in M87. For M87, the abundances from Fe K are systematically lower by 30%-50% than those from Fe L, and the differences are attributable to the complicated temperature structure in the central cooling flow. Allowing for the possibility of excess absorption of the emission from the cooling flow increases the systematic offset of the Fe L and Fe K abundances to 40%-80% in the simple model we consider. The cluster spectra, on the other hand, are for nearly isothermal regions outside any central cooling flow and the abundances have no systematic offset. The consistency between the Fe K and Fe L abundances, which is also seen in other clusters and in the corona of Ar Lac, indicates that whatever atomic physics uncertainties exist in the current codes are not likely to render Fe abundance measurements unreliable within the ~30%-50% agreement we find for the systems we examined. The results using various thermal emission codes differ by 15%-20%, showing the level of scatter due just to different assumptions about the atomic physics. The atomic rates for H- and He-like ions, which are relevant for the determination of abundances in elements other than Fe, are more accurately known than those for less ionized species that dominate the complicated Fe L emission. In light of the support our results give to the abundances derived from ASCA data for clusters and bright elliptical galaxies, we consider briefly the plausibility of subsolar abundances in elliptical galaxies.

Laboratory Measurements of Fe [CSC]xxiv[/CSC] [ITAL]L[/ITAL]-Shell Line Emission

Recent ASCA spectra exhibit discrepancies with the relative line intensities of various Fe XXIII and XXIV L-shell emission lines predicted by standard plasma emission codes. To address this issue, we have carried out a series of high-resolution, broadband measurements of Fe XXIV line emission using an electron beam ion trap facility. X-ray lines produced in the trap are detected and resolved using Bragg crystal spectrometers. We report measurements of 3 → 2 and 4 → 2 transitions, which result primarily from electron impact excitation. Overall, good agreement is found with distorted wave calculations.

Cross sections and rate coefficients for inner-shell excitation of Li-like ions with 6 < Z < 42

Excitation cross sections and rate coefficients by electron impact were calculated for the 1s22s–1s2s2p, 1s22s–1s2s2 and 1s22s–1s2p2 transitions of the Li-like ions (C IV, N V, O VI, Ne VIII, Mg X, Al XI, Si XII, S XIV, Ar XVI, Ca XVIII, Ti XX, Fe XXIV, Ni XXVI, Zn XXVIII, Ge XXX, Se XXXII, Kr XXXIV and Mo XXXX) in the Coulomb-Born approximation with exchange including relativistic effects and configuration interaction. Level energies, mixing coefficients and transition wavelengths and probabilities were also computed.Calculations performed by the 1/Z perturbation theory and Coulomb-Born approximation are compared with the R-matrix method and the distorted-wave approximation were Z is the nuclear charge.Formulae obtained for the angular factors of n-electron atomic system allow one to generalize this method to an arbitrary system of highly charged ions.

Atomic spectroscopy at NIST - 1995

The current program at NIST to observe spectral lines of atoms and atomic ions and determine atomic energy levels is reviewed. Results for the doubly ionized atoms Zr III, Hg III and Dy III, which are astrophysically important, are summarized in some detail, as are recent results for the Li-like atoms Li I and Fe XXIV. Progress on installation of a very high resolution Fourier transform spectrometer at NIST is reported.

Relationships between Temperature and Emission Measure in Solar Flares Determined from Highly Ionized Iron Spectra and from Broadband X-Ray Detectors

view Abstract Citations (35) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Relationships between Temperature and Emission Measure in Solar Flares Determined from Highly Ionized Iron Spectra and from Broadband X-Ray Detectors Feldman, U. ; Doschek, G. A. ; Mariska, J. T. ; Brown, C. M. Abstract We compare the electron temperature and emission measure of flares at the time of maximum soft X-ray intensity derived using two different techniques: (1) from the ratio of a dielectronic Fe XXIV line to the resonance line of Fe XXV, combined with the absolute intensity of the Fe XXV line, and (2) from the ratio of the 0.5-4.0 Å and 1-8 Å broadband X-ray fluxes, combined with the absolute flux in one of the broadband spectral regions. The high-resolution Fe spectra are obtained with the Bragg Crystal Spectrometer experiment flown on the Japanese Yohkoh spacecraft. The broadband fluxes are obtained from Geostationary Operational Environmental Satellites (GOES). A data set of 540 X-ray magnitude C2 or brighter flares, observed by both spacecraft, is used for the analysis. Both techniques assume an isothermal plasma. The broadband temperatures are substantially lower than the Fe xxv temperatures. We find that the maximum temperature of flares brighter than MS exceeds 2 × 107 K and that the maximum temperature of flares fainter than C4 is substantially lower than 2 × 107 K. We find that the Fe XXV emission measure is linearly proportional to the GOES flux in the 0.5-4.0 Å detector. Publication: The Astrophysical Journal Pub Date: September 1995 DOI: 10.1086/176154 Bibcode: 1995ApJ...450..441F Keywords: SUN: FLARES; SUN: X-RAYS full text sources ADS |

Evidence for elemental variation in the ejecta of the TYCHO supernova remnant

We present an X-ray study of the Tycho supernova remnant utilizing archival data from the high-resolution imagers (HRIs) on Einstein and ROSAT, the low-energy imaging telescopes (LEITs) on EXOSAT, and spectral data from the Broad Band X-ray Telescope (BBXRT). We have made use of the differing HRI bandpasses to construct images of Tycho in two spectral bands, o.7-1.8 keV and 1.8-4.5 keV. We find that the two images differ, with the harder image showing enhanced emission along much of the south, west, and north periphery. There appears to be enhanced soft emission in the interior and in one particular knot of emission in the southeast. Besides continuum (which we model here as thermal bremsstrahlung emission), we believe the hard image shows primarily the distribution of high-ionization Si and S K-shell lines which lie in the 1.8-2.6 keV band, while the softer image has contributions from Si as well as Fe XVII to Fe XXIV L-shell lines in the 0.7-1.4 keV band. Guided by the results of nonequilibrium ionization modeling of the BBXRT spectral data, we interpret the observed contrast in hard and soft X-ray emission in terms of variations in abundance, ionization timescale, and temperature. The most likely explanation for the spectral differences are spatial variations of the relative abundances of Si, S, and Fe.

The iron-to-calcium abundance ratio in the 20 X 10 to the 6th K plasma of medium and large solar flares

view Abstract Citations (19) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Iron-to-Calcium Abundance Ratio in the 20 X 10 6 K Plasma of Medium and Large Solar Flares Phillips, K. J. H. ; Feldman, U. Abstract The iron-to-calcium abundance ratio during several flares observed by instruments on the P78-1 and Solar Maximum Mission spacecraft are determined using the intensity ratio of a dielectronic satellite line of Fe XXIV and the resonance line of Ca XIX. Values of A(Fe)/A(Ca) equal to 6.8 +/- 1.0 and 6.0 +/- 1.5 were derived from these data sets, respectively, the small difference being attributable to errors in the calculated instrument sensitivities. These values fall between two other determinations from X-ray spectra, but are less than the photospheric value. They are fairly constant from flare to flare, contrasting with other recent work which indicates strong flare-to-flare variations. Publication: The Astrophysical Journal Pub Date: September 1991 DOI: 10.1086/170515 Bibcode: 1991ApJ...379..401P Keywords: Calcium; High Temperature Plasmas; Iron; Metallicity; Solar Flares; Solar X-Rays; Abundance; Metal Ions; Resonance Lines; Solar Corona; Solar Maximum Mission; Solar Physics; SUN: ABUNDANCES; SUN: FLARES; SUN: X-RAYS full text sources ADS |

Magnetic energy release in interacting loops observed in a solar flare from Sylab

We studied the energy release morphology in the 21 January 1974 flare observed in the XUV wavelength from Skylab . The XUV images of the flare show that the magnetic configuration consisted of a large cool loop (10 4 – 10 5 K) and a small low-lying high temperature (> 10 7 K) loop. The interaction between the loops, indicated by a hard X-ray burst, triggered the release of stored energy in the two loops. In the large cooler loop, the energy release was manifested in a possible kink instability, which released the stored energy as kinetic energy of motion. In the low lying loop, the energy release was manifested in the heating of the plasma, thus producing the high temperature emissions as observed in Fe XXIV. The observation illustrates clearly the involvement of a multiloop system in the flare energy release and shows that individual loops may undergo different evolutions due to the different conditions in them. This picture is consistent with the flare energy release scenario deduced from the most recent SMM observations /1/.

Effects of initial-state population variations on the 2p-->1s K alpha dielectronic satellite spectra of highly ionized iron ions in high-temperature astrophysical and laboratory plasmas.

Theoretical predictions are presented for the iron K\ensuremath{\alpha} x-ray emission spectra from high-temperature plasmas, assuming steady-state optically thin excitation conditions. Account has been taken of all fine-structure components of the 2p\ensuremath{\rightarrow}1s inner-shell-electron radiative transitions in the iron ions from Fe xviii to Fe xxiv. The K\ensuremath{\alpha} emission spectra are assumed to be produced by means of dielectronic recombination and inner-shell-electron collisional excitation processes that involve intermediate autoionizing states belonging to electronic configurations of the type 1${s}^{1}$2${s}^{r}$2${p}^{s}$. In addition to the electron-temperature variation, which is attributable to the temperature dependences of the radiationless electron capture and inner-shell-electron collisional excitation rate coefficients and to the temperature dependence of the charge-state distribution, the K\ensuremath{\alpha} emission spectra exhibit an electron-density sensitivity. This electron-density sensitivity is a result of the density-dependent distribution of populations among the different fine-structure levels of the initial ions in the dielec- tronic recombination and inner-shell electron collisional excitation processes.

Observation of nonthermal energy distributions during the impulsive phase of solar flares

The Fe XXV resonance line and dielectronic satellite intensities have been measured as functions of time for several flares recorded by the Naval Research Laboratory crystal spectrometer (SOLFLEX) flown on the US Air Force P78-I spacecraft. The intensity ratios of the Fe XXV resonance line, the Fe XXIV n = 2 satellite line j, and the Fe XXIV n = 3 satellite line d13 indicate that nonthermal electron energy distributions occur during the impulsive phase of the flares. For the electron energies at which the j and d13 satellites are formed (4.7 and 5.8 keV, respectively), the electron energy distributions during the impulsive phase are observed to have a bump or to be nearly flat. For all of the flares that were studied, hard X-ray bursts occurred near the time of the nonthermal distributions observed in the SOLFLEX data.

Design of a High Resolution Spectrometer for Ultra-high Temperature Plasma Diagnosis

A designing procedure of a high resolution VUV spectrometer with fixed entrance and exit slits have been studied for estimating the ion temperature of highly energetic plasmas by measuring Doppler broadening of the resonance emission line of Fe XXIV ions (central wavelength; λ0 = 255.29 Å) included as impurity in the plasma.A newly developed cylindrical grating having straight grooves, with separations which decrease exponentially with increasing number of the groove is used. Wavelength scanning is achived by appropriate translation of the grating.Less than 0.1 Å instrumental broadening of the line spectrum will be realized.Deteriorations of resolution of the spectrometer caused by inaccuracies of the surface and groove direction of the grating are discussed, as well as the methods with which the resultant resolution can be corrected.The present spectrometer with small size and simple structure is expected, under easy operation, to have a resolution high enough to estimate the ion temperature of a plasma down to about 2 keV.Simultaneous measurement with several such spectrometers will be a powerful aid for finding the best operation of, for instance, tokamak-type plasma generators.

Comparison of computed fluxes for Fex and Fexiv lines with observed values at 1980 eclipse

Fluxes have been computed for Fex (6374 A) and Fe xiv (5303 A) lines as a function of solar radii and at various coronal tempratures. The electron density derived from the white light corona during the total solar eclipse of 1980 were used in the computations. Fluxes in adjacent continua have also been computed. The computed ratios of line flux to the square of continuum flux at a coronal temperature of 1.6 × 106 K show a good fit with the observed values for Fex line. Further, radiative excitation seems to dominate over collisional excitation beyond 1.3 solar radius.

X-ray spectrum of the 2A 0335 + 096 cluster of galaxies

We present the X-ray spectrum of a poor cluster of galaxies 2A 0335 + 096 obtained with the medium energy and the gas scintillation proportional counter experiments on board EXOSAT. The observed spectrum is best explained by a thermal bremsstrahlung emission process from a hot intracluster gas with a temperature of ~3.4×10 7 K, and a line at 6.5 keV arising from transitions in highly ionized iron, i.e., Fe XXIV, Fe XXV, and Fe XXVI, which is redshifted to the observed peak energy. An iron abundance value of 0.4 times the solar value is derived for the hot gas in the cluster. The 99 percent confidence range derived for the iron abundance is 0.2-0.55 times the solar value, making it one of the most precise measurements of an iron line in X-ray clusters of galaxies. The iron abundance is remarkably similar to the values found for rich clusters and, therefore, is indicative of a universal value of iron abundance in all the X-ray clusters irrespective of their evolutionary and dynamical states.