69 research outputs found
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
Outflow Velocities in Polar Coronal Holes
Observations have been carried out using the oxygen VI multiplet ratio 1032/1038 Â from SUMER on SOHO. Analysis based on the Doppler dimming method shows that the outflow velocity in polar plumes is higher than that in the interplume region, contrary to many published suggestions. The addition of UVCS data for the interplume region, leads to a conclusion that the effective oxygen ion "temperature" in the radial direction has to rise to around 14 MK over the height range 1.5 to 1.8 R⊙
X-ray Line Diagnostics of Hot Accretion Flows around Black Holes
We compute X-ray emission lines from thermal plasma in hot accretion flows.
We show that line profiles are strong probes of the gas dynamics, and we
present line-ratio diagnostics which are sensitive to the distribution of mass
with temperature in the flow. We show how these can be used to constrain the
run of density with radius, and the size of the hot region. We also present
diagnostics which are primarily sensitive to the importance of recombination
versus collisional ionization, and which could help discriminate ADAFs from
photoionization-dominated accretion disk coronae. We apply our results to the
Galactic center source Sagittarius A* and to the nucleus of M87. We find that
the brightest predicted lines are within the detection capability of current
-ray instruments.Comment: 16 pages, 1 table, 9 figures, accepted to 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
Impulsive phase transport
The transport of nonthermal electrons is explored. The thick-target electron beam model, in which electrons are presumed to be accelerated in the corona and typically thermalized primarily in the chromosphere and photosphere, is supported by observations throughout the electromagnetic spectrum. At the highest energies, the anisotropy of gamma-ray emission above 10 MeV clearly indicates that these photons are emitted by anisotropically-directed particles. The timing of this high-energy gamma-radiation with respect to lower-energy hard X-radiation implies that the energetic particles have short life-times. For collisional energy loss, this means that they are stopped in the chromosphere or below. Stereoscopic (two-spacecraft) observations at hard X-ray energies (up to 350 keV) imply that these lower-energy (but certainly nonthermal) electrons are also stopped deep in the chromosphere. Hard X-ray images show that, in spatially resolved flares whose radiation consists of impulsive bursts, the impulsive phase starts with X-radiation that comes mostly from the foot-points of coronal loops whose coronal component is outlined by microwaves
Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. V. Relativistic calculations for Fe XXIV and Fe XXV for X-ray modeling
Photoionization and recombination cross sections and rate coefficients are
calculated for Li-like Fe XXIV and He-like Fe XXV using the Breit-Pauli
R-matrix (BPRM) method. A complete set of total and level-specific parameters
is obtained to enable X-ray photoionization and spectral modeling. The ab
initio calculations for the unified (e + ion) recombination rate coefficients
include both the non-resonant and the resonant recombination (radiative and
di-electronic recombination, RR and DR, respectively) for (e + Fe XXV) -> Fe
XXIV and (e + Fe XXVI) -> Fe XXV. The level specific rates are computed for all
fine structure levels up to n = 10, enabling accurate computation of
recombination-cascade matrices and effective rates for the X-ray lines. The
total recombination rate coefficients for both Fe XXIV and Fe XXV differ
considerably, by several factors, from the sum of RR and DR rates currently
used to compute ionization fractions in astrophysical models. As the
photoionization/recombination calculations are carried out using an identical
eigenfunction expansion, the cross sections for both processes are
theoretically self-consistent; the overall uncertainty is estimated to be about
10-20%. All data for Fe XXIV and Fe XXV (and also for H-like Fe XXVI, included
for completeness) are available electronically.Comment: 31 pages, 10fug
Recombination Rate Coefficients for KLL Di-electronic Satellite Lines of Fe XXV and Ni XXVII
The unified method for total electron-ion recombination is extended to study
the dielectronic satellite (DES) lines. These lines, formed from radiative
decay of autoionizing states, are highly sensitive temperature diagnostics of
astrophysical and laboratory plasma sources. The computation of the unified
recombination rates is based on the relativistic Breit-Pauli R-matrix method
and close coupling approximation. Extending the theoretical formulation
developed earlier we present recombination rate coefficients for the 22
satellite lines of KLL complexes of helium-like Fe XXV and Ni XXVII. The
isolated resonance approximation, commonly used throughout plasma modeling,
treats these resonances essentially as bound features except for dielectronic
capture into, and autoionization out of, these levels. A line profile or cross
section shape is often assumed. On the other hand, by including the coupling
between the autoionizing and continuum channels, the unified method gives the
intrinsic spectrum of DES lines which includes not only the energies and
strengths, but also the natural line or cross section shapes. A formulation is
presented to derive autoionization rates from unified resonance strengths and
enable correspondence with the isolated resonance approximation. While the
rates compare very well with existing rates for the strong lines to <20%, the
differences for weaker DES lines are larger. We also illustrate the application
of the present results to the analysis of K ALPHA complexes observed in
high-temperature X-ray emission spectra of Fe XXV and Ni XXVII. There are
considerable differences with previous results in the total KLL intensity for
Fe XXV at temperatures below the temperature of maximum abundance in coronal
equilibrium. (Abbreviated Abstract)Comment: 21 pages, 5 figures, to appear in Physica Script
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
APPLICATION DES MÉTHODES UTILISÉES EN THÉORIE DES COLLISIONS AU CALCUL DES SECTIONS EFFICACES DE PHOTOIONISATION
Parmi les nombreuses méthodes utilisées en théorie des collisions électron-atome (ou ion), la méthode des équations couplées a été choisie car elle donne les meilleurs résultats comparés à l'expérience. Cette méthode étant assez coûteuse, il est extrêmement intéressant de pouvoir la coupler à une autre méthode qui utilise les propriétés d'analycité des résultats de la première méthode. (Méthode du défaut quantique). Pour illustrer nos propos, nous avons choisi 3 cas particuliers : les photoionisations des couches externes du C2+ et du Be et la photoionisation de la couche interne du Be.Many methods are used in the theory of electron-atom (or ion) collisions. The approach of coupled-equations was chosen because it gives the best agreement with experiment. Since this method is rather costly in terms of computer time, it is of interest to extend the results it gives by making use of their analytical properties (quantum defect method). Sample results are given for three spécifie cases : outer-shell photoionization of C2+ and Be, inner-shell photoionization of Be
Ni XXVII THEORETICAL SPECTRUM
Des spectres de haute résolution du Nickel viennent d'être obtenus au J.E.T. dans le domaine de longueur d'onde 1.584 - 1.615 Å à partir d'un spectromètre à cristal courbe. On présente ici les premières données atomiques nécessaires à leur interprétation, elles correspondent aux raies de résonance du Ni XXVII et aux raies satellites des Ni XXVI et Ni XXV qu'elles soient diélectroniques ou d'excitation d'électrons de couches internes.High resolution spectra of Nickel in the wavelength interval from 1.584 to 1.615 Å have been obtained recently at J.E.T. using a bent crystal spectrometer. Atomic parameters for resonance lines (Ni XXVII) and dielectronic and innershell satellite lines (Ni XXVI and Ni XXV) have been calculated for their interpretation
- …