17,058 research outputs found
Differential Emission Measure Determination of Collisionally Ionized Plasma: II. Application to Hot Stars
In a previous paper we have described a technique to derive constraints on
the differential emission measure (DEM) distribution, a measure of the
temperature distribution, of collisionally ionized hot plasmas from their X-ray
emission line spectra. We apply this technique to the Chandra/HETG spectra of
all of the nine hot stars available to us at the time this project was
initiated. We find that DEM distributions of six of the seven O stars in our
sample are very similar but that theta Ori has an X-ray spectrum characterized
by higher temperatures. The DEM distributions of both of B stars in our sample
have lower magnitudes than those of the O stars and one, tau Sco, is
characterized by higher temperatures than the other, beta Cru. These results
confirm previous work in which high temperatures have been found for theta Ori
and tau Sco and taken as evidence for channeling of the wind in magnetic
fields, the existence of which are related to the stars' youth. Our results
demonstrate the utility of our method for deriving temperature information for
large samples of X-ray emission line spectra.Comment: The contents of this paper were formerly part of astro-ph/0403603
which was split into two paper
New spectral classification technique for X-ray sources: quantile analysis
We present a new technique called "quantile analysis" to classify spectral
properties of X-ray sources with limited statistics. The quantile analysis is
superior to the conventional approaches such as X-ray hardness ratio or X-ray
color analysis to study relatively faint sources or to investigate a certain
phase or state of a source in detail, where poor statistics does not allow
spectral fitting using a model. Instead of working with predetermined energy
bands, we determine the energy values that divide the detected photons into
predetermined fractions of the total counts such as median (50%), tercile (33%
& 67%), and quartile (25% & 75%). We use these quantiles as an indicator of the
X-ray hardness or color of the source. We show that the median is an improved
substitute for the conventional X-ray hardness ratio. The median and other
quantiles form a phase space, similar to the conventional X-ray color-color
diagrams. The quantile-based phase space is more evenly sensitive over various
spectral shapes than the conventional color-color diagrams, and it is naturally
arranged to properly represent the statistical similarity of various spectral
shapes. We demonstrate the new technique in the 0.3-8 keV energy range using
Chandra ACIS-S detector response function and a typical aperture photometry
involving background subtraction. The technique can be applied in any energy
band, provided the energy distribution of photons can be obtained.Comment: 11 pages, 9 figures, accepted for publication in Ap
Evidence for a parsec scale X-ray jet from the accreting neutron star Circinus X-1
We analyzed the zero-order image of a 50 ks Chandra gratings observation of
Circinus X-1, taken in 2005 during the source's low-flux state. Circinus X-1 is
an accreting neutron star that exhibits ultra-relativistic arcsecond-scale
radio jets and diffuse arcminute-scale radio jets and lobes. The image shows a
clear excess along the general direction of the north-western counter-jet,
coincident with the radio emission, suggesting that it originates either in the
jet itself or in the shock the jet is driving into its environment. This makes
Circinus X-1 the first neutron star for which an extended X-ray jet has been
detected. The kinetic jet power we infer is significantly larger than the
minimum power required for the jet to inflate the large scale radio nebula.Comment: Added journal reference, corrected on reference and typo in labels
for Fig. 1; 5 pages, 3 figures, ApJ Letter, in pres
Dynamical Ne K Edge and Line Variations in the X-Ray Spectrum of the Ultra-compact Binary 4U 0614+091
We observed the ultra-compact binary candidate 4U 0614+091 for a total of 200
ksec with the high-energy transmission gratings onboard the \chandra X-ray
Observatory. The source is found at various intensity levels with spectral
variations present. X-ray luminosities vary between 2.0 \ergsec
and 3.5 \ergsec. Continuum variations are present at all times
and spectra can be well fit with a powerlaw component, a high kT blackbody
component, and a broad line component near oxygen. The spectra require
adjustments to the Ne K edge and in some occasions also to the Mg K edge. The
Ne K edge appears variable in terms of optical depths and morphology. The edge
reveals average blue- and red-shifted values implying Doppler velocities of the
order of 3500 \kms. The data show that Ne K exhibits excess column densities of
up to several 10 cm. The variability proves that the excess is
intrinsic to the source. The correponding disk velocities also imply an outer
disk radius of the order of cm consistent with an ultra-compact binary
nature. We also detect a prominent soft emission line complex near the \oviii
L position which appears extremely broad and relativistic effects from
near the innermost disk have to be included. Gravitationally broadened line
fits also provide nearly edge-on angles of inclination between 86 and
89. The emissions appear consistent with an ionized disk with
ionization parameters of the order of 10 at radii of a few 10 cm. The
line wavelengths with respect to \oviiia\ are found variably blue-shifted
indicating more complex inner disk dynamics.Comment: 24 pages, 8 figures, submitted to the Astrophyscial Main Journa
Optical properties of self-organized wurtzite InN/GaN quantum dots: A combined atomistic tight-binding and full configuration interaction calculation
In this work we investigate the electronic and optical properties of
self-assembled InN/GaN quantum dots. The one-particle states of the
low-dimensional heterostructures are provided by a tight-binding model that
fully includes the wurtzite crystal structure on an atomistic level. Optical
dipole and Coulomb matrix elements are calculated from these one-particle wave
functions and serve as an input for full configuration interaction
calculations. We present multi-exciton emission spectra and discuss in detail
how Coulomb correlations and oscillator strengths are changed by the
piezoelectric fields present in the structure. Vanishing exciton and biexciton
ground state emission for small lens-shaped dots is predicted.Comment: 3 pages, 2 figure
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