Characteristics of Solar Flare Doppler Shift Oscillations Observed with the Bragg Crystal Spectrometer on Yohkoh

This paper reports the results of a survey of Doppler shift oscillations measured during solar flares in emission lines of S XV and Ca XIX with the Bragg Crystal Spectrometer (BCS) on Yohkoh. Data from 20 flares that show oscillatory behavior in the measured Doppler shifts have been fitted to determine the properties of the oscillations. Results from both BCS channels show average oscillation periods of 5.5 +/- 2.7 minutes, decay times of 5.0 +/-2.5 minutes, amplitudes of 17.1 +/- 17.0 km/s, and inferred displacements of 1070 +/- 1710 km, where the listed errors are the standard deviations of the sample means. For some of the flares, intensity fluctuations are also observed. These lag the Doppler shift oscillations by 1/4 period, strongly suggesting that the oscillations are standing slow mode waves. The relationship between the oscillation period and the decay time is consistent with conductive damping of the oscillations.Comment: Accepted for publication in Ap

X-Ray Photoabsorption in KLL Resonances of O VI And Abundance Analysis

It is shown that photoabsorption via autoionizing resonances may be appreciable and used for abundance analysis. Analogous to spectral lines, the `resonance oscillator strength' f_r may be defined and evaluated in terms of the differential oscillator strength df/d(epsilon) that relates bound and continuum absorption. X-ray photoabsorption in KLL (1s2s2p) resonances of O VI is investigated using highly resolved relativistic photoionization cross sections with fine structure. It is found that f_r is comparable to that for UV dipole transition in O VI (2s - 2p) and the X-ray (1s^2 ^1S_0 - 1s2p ^1P^o_1) transition in O VII. The dominant O VI(KLL) components lie at 22.05 and 21.87 Angstroms. These predicted absorption features should be detectable by the Chandra X-Ray Observatory (CXO) and the X-Ray Multi-Mirror Mission (XMM). The combined UV/X-ray spectra of O VI/O VII should yield valuable information on the ionization structure and abundances in sources such as the `warm absorber' region of active galactic nuclei and the hot intergalactic medium. Some general implications of resonant photoabsorption are addressed.Comment: Astrophys. J. Letters (in press), 9 pages, 3 figure

UV and X-ray Spectral Lines of FeXXIII Ion for Plasma Diagnostics

We have calculated X-ray and UV spectra of Be-like Fe (FeXXIII) ion in collisional-radiative model including all fine-structure transitions among the 2s^2, 2s2p, 2p^2, 2snl, and 2pnl levels where n=3 and 4, adopting data for the collision strengths by Zhang & Sampson (1992) and by Sampson, Goett, & Clark (1984). Some line intensity ratios can be used for the temperature diagnostics. We show 5 ratios in UV region and 9 ratios in X-ray region as a function of electron temperature and density at 0.3keV < T_e < 10keV and $n_e = 1 - 10^{25} cm^{-3}$. The effect of cascade in these line ratios and in the level population densities are discussed.Comment: LaTeX, 18 pages, 10 Postscript figures. To appear in Physica Script

Chandra High Resolution X-ray Spectroscopy of AM Her

We present the results of high resolution spectroscopy of the prototype polar AM Herculis observed with Chandra High Energy Transmission Grating. The X-ray spectrum contains hydrogen-like and helium-like lines of Fe, S, Si, Mg, Ne and O with several Fe L-shell emission lines. The forbidden lines in the spectrum are generally weak whereas the hydrogen-like lines are stronger suggesting that emission from a multi-temperature, collisionally ionized plasma dominates. The helium-like line flux ratios yield a plasma temperature of 2 MK and a plasma density 1 - 9 x10^12 cm^-3, whereas the line flux ratio of Fe XXVI to Fe XXV gives an ionization temperature of 12.4 +1.1 -1.4 keV. We present the differential emission measure distribution of AM Her whose shape is consistent with the volume emission measure obtained by multi-temperature APEC model. The multi-temperature plasma model fit to the average X-ray spectrum indicates the mass of the white dwarf to be ~1.15 M_sun. From phase resolved spectroscopy, we find the line centers of Mg XII, S XVI, resonance line of Fe XXV, and Fe XXVI emission modulated by a few hundred to 1000 km/s from the theoretically expected values indicating bulk motion of ionized matter in the accretion column of AM Her. The observed velocities of Fe XXVI ions are close to the expected shock velocity for a 0.6 M_sun white dwarf. The observed velocity modulation is consistent with that expected from a single pole accreting binary system.Comment: 6 figures, AASTEX style, accepted for publication in Ap

Enhanced dielectronic recombination of lithium-like Ti19+ ions in external ExB fields

Dielectronic recombination(DR) of lithium-like Ti19+(1s2 2s) ions via 2s->2p core excitations has been measured at the Heidelberg heavy ion storage ring TSR. We find that not only external electric fields (0 <= Ey <= 280 V/cm) but also crossed magnetic fields (30 mT <= Bz <= 80 mT) influence the DR via high-n (2p_j nl)-Rydberg resonances. This result confirms our previous finding for isoelectronic Cl14+ ions [Bartsch T et al, PRL 82, 3779 (1999)] that experimentally established the sensitivity of DR to ExB fields. In the present investigation the larger 2p_{1/2}-2p_{3/2} fine structure splitting of Ti19+ allowed us to study separately the influence of external fields via the two series of Rydberg DR resonances attached to the 2s -> 2p_{1/2} and 2s -> 2p_{3/2} excitations of the Li-like core, extracting initial slopes and saturation fields of the enhancement. We find that for Ey > 80 V/cm the field induced enhancement is about 1.8 times stronger for the 2p_{3/2} series than for the 2p_{1/2} series.Comment: 10 pages, 3 figures, to be published in Journal of Physics B, see also http://www.strz.uni-giessen.de/~k

Excitation of the 3.071mm Hyperfine Line in Li-Like 57-Fe in Astrophysical Plasmas

As noted first by Sunyaev & Churazov (1984), the 3.071 mm hyperfine line from $^{57}Fe^{+23}$ might be observable in astrophysical plasmas. We assess the atomic processes which might contribute to the excitation of this line. We determine the intensity of the hyperfine line from an isothermal, coronal plasma in collisional ionization equilibrium and for a coronal plasma cooling isobarically due to its own radiation. Comparisons of the hyperfine line to other lines emitted by the same ion, Fe$^{+23}$, are shown to be useful for deriving the isotopic fraction of $^{57}$Fe. We calculate the ratios of the hyperfine line to the 2s--2p EUV lines at 192 \AA and 255 \AA, and the 2s--3p X-ray doublet at 10.6 \AA.Comment: 28 pages text+figures, Accepted to ApJ in Jan 98, also at http://www.astro.virginia.edu/~nld2n/research.htm

Structural transformations and adsorption properties of PtNi nanoalloy thin film electrocatalysts prepared by magnetron co-sputtering

This is the final peer-reviewed manuscript accepted for publication in Electrochimica Acta Citation of the published version is: Electrochimica Acta 251, 427–441 (2017

K-shell dielectronic resonances in photoabsorption: differential oscillator strengths for Li-like C IV, O VI, and Fe XXIV

Recently X-ray photoabsorption in KLL resonances of O VI was predicted [Pradhan, Astrophys.J. Lett. 545, L165 (2000)], and detected by the Chandra X-ray Observatory [Lee et al, Astrophys. J. {\it Lett.}, submitted]. The required resonance oscillator strengths f_r, are evaluated in terms of the differential oscillator strength df/de that relates bound and continuum absorption. We present the f_r values from radiatively damped and undamped photoionization cross sections for Li-like C,O, and Fe calculated using relativistic close coupling Breit-Pauli R-matrix method. The KLL resonances of interest here are: 1s2p (^3P^o) 2s [^4P^o_{1/2,3/2}, ^2P^o_{1/2,3/2}] and 1s2p (^1P^o) 2s [^2P^o_{1/2,3/2}]. The KLL photoabsorption resonances in Fe XXIV are fully resolved up to natural autoionization profiles for the first time. It is demonstrated that the undamped f_r independently yield the resonance radiative decay rates, and thereby provide a precise check on the resolution of photoionization calculations in general. The predicted photoabsorption features should be detectable by the X-ray space observatories and enable column densities in highly ionized astrophysical plasmas to be determined from the calculated f_r. The dielectronic satellites may appear as redward broadening of resonances lines in emission and absorption.Comment: 9 pages, 2 figurs, Phys. Rev. A, Rapid Communication (submitted