217 research outputs found
Collisional Plasma Models with APEC/APED: Emission Line Diagnostics of Hydrogen-like and Helium-like Ions
New X-ray observatories (Chandra and XMM-Newton) are providing a wealth of
high-resolution X-ray spectra in which hydrogen- and helium-like ions are
usually strong features. We present results from a new collisional-radiative
plasma code, the Astrophysical Plasma Emission Code (APEC), which uses atomic
data in the companion Astrophysical Plasma Emission Database (APED) to
calculate spectral models for hot plasmas. APED contains the requisite atomic
data such as collisional and radiative rates, recombination cross sections,
dielectronic recombination rates, and satellite line wavelengths. We compare
the APEC results to other plasma codes for hydrogen- and helium-like
diagnostics, and test the sensitivity of our results to the number of levels
included in the models. We find that dielectronic recombination with
hydrogen-like ions into high (n=6-10) principal quantum numbers affects some
helium-like line ratios from low-lying (n=2) transitions.Comment: 5 pages, 6 figures, accepted by ApJ Letter
The X-Ray Photoionized Wind in Cen X-3/V779 Cen
We analyze the ASCA spectrum of the Cen X-3 X-ray binary system in eclipse
using atomic models appropriate to recombination-dominated level population
kinetics in an overionized plasma. In order to estimate the wind
characteristics, we first fit the eclipse spectrum to a single-zone
photoionized plasma model. We then fit spectra from a range of orbital phases
using global models of photoionized winds from the companion star and the
accretion disk that account for the continuous distribution of density and
ionization state. We find that the spectrum can be reproduced by a density
distribution of the form derived by Castor, Abbot, & Klein (1975) for
radiation-driven winds with with the value of the mass-loss rate divided by the
terminal velocity consistent with values for isolated stars of the same stellar
type. This is surprising because the neutron star is very luminous (~10^38
erg/s) and the X-rays from the neutron star should ionize the wind and destroy
the ions that provide the opacity for the radiation-driven wind. Using the same
functional form for the density profile, we also fit the spectrum to a
spherically symmetric wind centered on the neutron star, a configuration chosen
to represent a disk wind. We argue that the relatively modest orbital variation
of the discrete spectrum rules out a disk wind hypothesis.Comment: ApJ accepte
Identification of an Extended Accretion Disk Corona in the Hercules X-1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances
We identify an accretion disk atmosphere and corona from the high resolution
X-ray spectrum of Hercules X-1, and we determine its detailed physical
properties. More than two dozen recombination emission lines (from Fe XXVI at
1.78 A to N VI at 29.08 A) and Fe K-alpha, K-beta fluorescence lines were
detected in a 50 ks observation with the Chandra High-Energy Transmission
Grating Spectrometer (HETGS). They allow us to measure the density,
temperature, spatial distribution, elemental composition, and kinematics of the
plasma. We exclude HZ Her as the source of the recombination emission. We
compare accretion disk model atmospheres with the observed spectrum in order to
constrain the stratification of density and ionization, disk atmosphere area,
elemental composition, and energetics. The atmospheric spectrum observed during
the low state is photoionized by the main-on X-ray continuum, indicating that
the disk is observed edge-on during the low state. We infer the mean number of
scatterings N of Ly-alpha and Ly-beta line photons from H-like ions. We derive
N < 69 for O VIII Ly_alpha_1, which rules out the presence of a mechanism
modeled by Sako (2003) to enhance N VII emission via a line overlap with O
VIII. The line optical depth diagnostics are consistent with a flattened
atmosphere. Our spectral analysis, the disk atmosphere model, and the presence
of intense N VII and N VI lines (plus N V in the UV), confirm the
over-abundance of nitrogen relative to other metals, which was shown to be
indicative of CNO cycle processing in a massive progenitor.Comment: 38 pages, 14 figures, accepted for publication in Ap
First Application of the Fe XVII I(17.10 A)/I(17.05 A) Line Ratio to Constrain the Plasma Density of a Cosmic X-ray Source
We show that the Fe XVII I(17.10 A)/I(17.05 A) line ratio observed in the
Chandra HETG spectrum of the intermediate polar EX Hydrae is significantly
smaller than that observed in the Sun or other late-type stars. Using the
Livermore X-ray Spectral Synthesizer, which calculates spectral models of
highly charged ions based on HULLAC atomic data, we find that the observed
I(17.10 A)/I(17.05 A) line ratio can be explained if the plasma density n_e >
3x10^{14} cm^{-3}. However, if photoexcitation is included in the level
population kinetics, the line ratio can be explained for any density if the
photoexcitation temperature T_bb > 55 kK. For photoexcitation to dominate the
Fe XVII level population kinetics, the relative size of the hotspot on the
white dwarf surface must be f < 2%. This constraint and the observed X-ray flux
requires a density n > 2x10^{14} cm^{-3} for the post-shock flow. Either way,
then, the Chandra HETG spectrum of EX Hya requires a plasma density which is
orders of magnitude greater than that observed in the Sun or other late-type
stars.Comment: 13 pages including 1 table and 4 encapsulated postscript figures;
LaTeX format, uses aastex.cls; accepted on 2001 June 27 for publication in
The Astrophysical Journa
The y activity from 11Li beta decay
The energies and absolute intensities of the Îł-rays from the ÎČ-decay of 11Li are measured. There is no sizable ÎČ branch to the 11Be ground state. Only (5.2 ± 1.4) % of the ÎČ-decay strength does not lead to ÎČ-delayed particle emission. New ÎČ-delayed neutron branches to excited states of 10Be are observed and the total delayed neutron emission probability is deduced
A new radiative cooling curve based on an up to date plasma emission code
This work presents a new plasma cooling curve that is calculated using the
SPEX package. We compare our cooling rates to those in previous works, and
implement the new cooling function in the grid-adaptive framework `AMRVAC'.
Contributions to the cooling rate by the individual elements are given, to
allow for the creation of cooling curves tailored to specific abundance
requirements. In some situations, it is important to be able to include
radiative losses in the hydrodynamics. The enhanced compression ratio can
trigger instabilities (such as the Vishniac thin-shell instability) that would
otherwise be absent. For gas with temperatures below 10,000 K, the cooling time
becomes very long and does not affect the gas on the timescales that are
generally of interest for hydrodynamical simulations of circumstellar plasmas.
However, above this temperature, a significant fraction of the elements is
ionised, and the cooling rate increases by a factor 1000 relative to lower
temperature plasmas.Comment: 11 pages, 6 figures. Typos fixed to match version on A&A
'forthcoming' website. Tables in text format online available at
http://www.phys.uu.nl/~schure/coolin
Dielectronic Recombination (via N=2 --> N'=2 Core Excitations) and Radiative Recombination of Fe XX: Laboratory Measurements and Theoretical Calculations
We have measured the resonance strengths and energies for dielectronic
recombination (DR) of Fe XX forming Fe XIX via N=2 --> N'=2 (Delta_N=0) core
excitations. We have also calculated the DR resonance strengths and energies
using AUTOSTRUCTURE, HULLAC, MCDF, and R-matrix methods, four different
state-of-the-art theoretical techniques. On average the theoretical resonance
strengths agree to within <~10% with experiment. However, the 1 sigma standard
deviation for the ratios of the theoretical-to-experimental resonance strengths
is >~30% which is significantly larger than the estimated relative experimental
uncertainty of <~10%. This suggests that similar errors exist in the calculated
level populations and line emission spectrum of the recombined ion. We confirm
that theoretical methods based on inverse-photoionization calculations (e.g.,
undamped R-matrix methods) will severely overestimate the strength of the DR
process unless they include the effects of radiation damping. We also find that
the coupling between the DR and radiative recombination (RR) channels is small.
We have used our experimental and theoretical results to produce
Maxwellian-averaged rate coefficients for Delta_N=0 DR of Fe XX. For kT>~1 eV,
which includes the predicted formation temperatures for Fe XX in an optically
thin, low-density photoionized plasma with cosmic abundances, our experimental
and theoretical results are in good agreement. We have also used our R-matrix
results, topped off using AUTOSTRUCTURE for RR into J>=25 levels, to calculate
the rate coefficient for RR of Fe XX. Our RR results are in good agreement with
previously published calculations.Comment: To be published in ApJS. 65 pages with 4 tables and lots of figure
Fission-Residues Produced in the Spallation Reaction 238U+p at 1 A GeV
Fission fragments from 1 A GeV 238U projectiles irradiating a hydrogen target
were investigated by using the fragment separator FRS for magnetic selection of
reaction products including ray-tracing and DE-ToF techniques. The momentum
spectra of 733 identified fragments were analysed to provide isotopic
production cross sections, fission-fragment velocities and recoil momenta of
the fissioning parent nuclei. Besides their general relevance, these quantities
are also demanded for applications. Calculations and simulations with codes
commonly used and recently developed or improved are compared to the data.Comment: 60 pages, 21 figures, 4 tables, 2 appendices (15 pages
The Structure and X-ray Recombination Emission of a Centrally Illuminated Accretion Disk Atmosphere and Corona
We model an accretion disk atmosphere and corona photoionized by a central
X-ray continuum source. We calculate the opacity and radiation transfer for an
array of disk radii, to obtain the two-dimensional structure of the disk and
its X-ray recombination emission. The atmospheric structure is insensitive to
the viscosity alpha. We find a feedback mechanism between the disk structure
and the central illumination, which expands the disk and increases the solid
angle subtended by the atmosphere. We model the disk of a neutron star X-ray
binary. We map the temperature, density, and ionization structure of the disk,
and we simulate the high resolution spectra observable with the Chandra and
XMM-Newton grating spectrometers. The X-ray emission lines from the disk
atmosphere are detectable, especially for high-inclination binary systems. The
grating observations of two classes of X-ray binaries already reveal important
spectral similarities with our models. The line spectrum is very sensitive to
the structure of each atmospheric layer, and it probes the heating mechanisms
in the disk. The model spectrum is dominated by double-peaked lines of H-like
and He-like ions, plus weak Fe L. Species with a broad range of ionization
levels coexist at each radius: from Fe XXVI in the hot corona, to C VI at the
base of the atmosphere. The choice of stable solutions affects the spectrum,
since a thermal instability is present in the regime where the X-ray
recombination emission is most intense.Comment: 32 pages, incl. 26 figures, accepted for publication in Ap
- âŠ