90 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
Absorption spectra of Fe L-lines in Seyfert 1 galaxies
Absorption L-lines of iron ions are observed, in absorption, in spectra of
Seyfert 1 galaxies by the new generation of X-ray satellites: Chandra (NASA)
and XMM-Newton (ESA). Lines associated to Fe23+ to Fe17+ are well resolved.
Whereas, those corresponding to Fe16+ to Fe6+ are unresolved. Forbidden
transitions of the Fe16+ to Fe6+ ions were previously observed, for the same
objects, in the visible and infra-red regions, showing that the plasma had a
low density. To interpret X-ray, visible and infra-red data, astrophysical
models assume an extended absorbing medium of very low density surrounding an
intense X-ray source. We have calculated atomic data (wavelengths, radiative
and autoionization rates) for n=2 to n'=3-4 transitions and used them to
construct refined synthetic spectra of the unresolved part of the L-line
spectra.Comment: 17 pages, 5 figures, Journal of Quantitative Spectroscopy and
Radiative Transfer, in pres
Highly Ionized Potassium Lines in Solar X-ray Spectra and the Abundance of Potassium
The abundance of potassium is derived from X-ray lines observed during flares
by the RESIK instrument on the solar mission CORONAS-F between 3.53 A and 3.57
A. The lines include those emitted by He-like K and Li-like K dielectronic
satellites, which have been synthesized using the CHIANTI atomic code and newly
calculated atomic data. There is good agreement of observed and synthesized
spectra, and the theoretical behavior of the spectra with varying temperature
estimated from the ratio of the two GOES channels is correctly predicted. The
observed fluxes of the He-like K resonance line per unit emission measure gives
log A(K) = 5.86 (on a scale log A(H) = 12), with a total range of a factor 2.9.
This is higher than photospheric abundance estimates by a factor 5.5, a
slightly greater enhancement than for other elements with first ionization
potential (FIP) less than about 10 eV. There is, then, the possibility that
enrichment of low-FIP elements in coronal plasmas depends weakly on the value
of the FIP which for K is extremely low (4.34 eV). Our work also suggests that
fractionation of elements to form the FIP effect occurs in the low chromosphere
rather than higher up, as in some models.Comment: 14 pages, 3 figure
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
RESIK observations of He-like Ar X-ray line emission in solar flares
The Ar XVII X-ray line group principally due to transitions 1s2 - 1s2l (l=s,
p) near 4 Anstroms was observed in numerous flares by the RESIK bent crystal
spectrometer aboard CORONAS-F between 2001 and 2003. The three line features
include the Ar XVII w (resonance line), a blend of x and y (intercombination
lines), and z (forbidden line), all of which are blended with Ar XVI
dielectronic satellites. The ratio G, equal to [I(x+y) + I(z)]/I(w), varies
with electron temperature Te mostly because of unresolved dielectronic
satellites. With temperatures estimated from GOES X-ray emission, the observed
G ratios agree fairly well with those calculated from CHIANTI and other data.
With a two-component emission measure, better agreement is achieved. Some S XV
and S XVI lines blend with the Ar lines, the effect of which occurs at
temperatures greater than 8MK, allowing the S/Ar abundance ratio to be
determined. This is found to agree with coronal values. A nonthermal
contribution is indicated for some spectra in the repeating-pulse flare of 2003
February 6.Comment: Latex file and 3 ps files. Astrophysical Journal Letters (accepted,
June 2008
Highly ionized Fe X-ray lines at energies 7.7-8.6 keV
Fe XXV lines at 1.85 A (6.70 keV) and nearby Fe XXIV satellites have been
widely used for determining the temperature of the hottest parts of solar flare
and tokamak plasmas, though the spectral region is crowded and the lines are
blended during flare impulsive stages. The aim of this work is to show that
similarly excited Fe lines in the 7.7--8.6 keV (1.44--1.61 A) region have the
same diagnostic capability with the advantage of not being so crowded. Spectra
in the 7.7--8.6 keV range are synthesized using the CHIANTI spectral package
for conditions (temperature, turbulent velocities) appropriate to solar flares.
The calculated spectra show that the Fe lines in the 7.7--8.6 keV are well
separated even when turbulent velocities are present, and Fe XXIV/Fe XXV line
ratios should therefore provide valuable tools for diagnosing flares and
tokamak plasmas. It is concluded that Fe lines in the 7.7--8.6 keV range are
ideal for the measurement of flare temperature and for detecting the presence
of low-energy nonthermal electrons present at flare impulsive stages. An
indication of what type of instruments to observe this region is given.Comment: 6 pages, 7 figures. Accepted for publication in Astronomy and
Astrophysic
The Solar Flare Chlorine Abundance from RESIK X-ray Spectra
The abundance of chlorine is determined from X-ray spectra obtained with the
RESIK instrument on {\em CORONAS-F} during solar flares between 2002 and 2003.
Using weak lines of He-like Cl, \ion{Cl}{16}, between 4.44 and 4.50 \AA, and
with temperatures and emission measures from {\em GOES} on an isothermal
assumption, we obtained on a scale . The uncertainty reflects an approximately factor 2 scatter in measured
line fluxes. Nevertheless our value represents what is probably the best solar
determination yet obtained. It is higher by factors of 1.8 and 2.7 than Cl
abundance estimates from an infrared sunspot spectrum and nearby \ion{H}{2}
regions. The constancy of the RESIK abundance values over a large range of
flares ({\em GOES} class from below C1 to X1) argues for any fractionation that
may be present in the low solar atmosphere to be independent of the degree of
solar activity.Comment: 13 pages, 3 colour figures. To be published, Astrophysical Journa
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
The Solar Photospheric-to-Coronal Fe abundance from X-ray Fluorescence Lines
The ratio of the Fe abundance in the photosphere to that in coronal flare
plasmas is determined by X-ray lines within the complex at 6.7~keV (1.9~\AA)
emitted during flares. The line complex includes the He-like Fe (\fexxv)
resonance line (6.70~keV) and Fe K lines (6.39, 6.40~keV), the
latter being primarily formed by the fluorescence of photospheric material by
X-rays from the hot flare plasma. The ratio of the Fe K lines to the
\fexxv\ depends on the ratio of the photospheric-to-flare Fe abundance,
heliocentric angle of the flare, and the temperature of the
flaring plasma. Using high-resolution spectra from X-ray spectrometers on the
{\em P78-1} and {\em Solar Maximum Mission} spacecraft, the Fe abundance in
flares is estimated to be and times the photospheric
Fe abundance, the {\em P78-1} value being preferred as it is more directly
determined. This enhancement is consistent with results from X-ray spectra from
the {\em RHESSI} spacecraft, but is significantly less than a factor 4 as in
previous work.Comment: Accepted for publication by MNRA
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
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