648 research outputs found
Future prospects for high resolution X-ray spectrometers
Capabilities of the X-ray spectroscopy payloads were compared. Comparison of capabilities of AXAF in the context of the science to be achieved is reported. The Einstein demonstrated the tremendous scientific power of spectroscopy to probe deeply the astrophysics of all types of celestial X-ray source. However, it has limitations in sensitivity and resolution. Each of the straw man instruments has a sensitivity that is at least an order of magnitude better than that of the Einstein FPSC. The AXAF promises powerful spectral capability
Implications of X-Ray Line Variations for 4U1822-371
4U 1822-371 is one of the proto-type accretion disk coronal sources with an
orbital period of about 5.6 hours. The binary is viewed almost edge-on at a
high inclination angle of 83 degrees, which makes it a unique candidate to
study binary orbital and accretion disk dynamics in high powered X-ray sources.
We observed the X-ray source in 4U 1822-371 with the Chandra High Energy
Transmission Grating Spectrometer (HETGS) for almost nine binary orbits. X-ray
eclipse times provide an update of the orbital ephemeris. We find that our
result follows the quadratic function implied by previous observations;
however, it suggests a flatter trend. Detailed line dynamics also confirm a
previous suggestion that the observed photo-ionized line emission originates
from a confined region in the outer edge of the accretion disk near the hot
spot. Line properties allow us to impose limits on the size of accretion disk,
the central corona, and the emission region. The photo-ionized plasma is
consistent with ionization parameters of log(xi) > 2, and when combined with
disk size and reasonable assumptions for the plasma density, this suggests
illuminating disk luminosities which are over an order of magnitude higher than
what is actually observed. That is, we do not directly observe the central
emitting X-ray source. The spectral continua are best fit by a flat power law
with a high energy cut-off and partial covering absorption (N_H ranging from
5.4-6.3x10^{22} cm^{-2}) with a covering fraction of about 50%. We discuss some
implications of our findings with respect to the photo-ionized line emission
for the basic properties of the X-ray source.Comment: Submitted to the Astrophysical Journa
Propuesta de una metodología estándar para la caracterización de la organogelificación de aceites comestibles con ceras
Saturated and trans fatty acids play a significant role in the plastic properties of food. However, health recommendations suggest limiting their intake. One approach which got the attention of researchers was to decrease the amount of saturated and trans fatty acids in food by the structuring of edible oils through the crystallization of waxes. The underlying mechanisms that lead to organogelation and the properties that characterize well-structured edible oil have been slow to fully understand due in part to a lack of standardization in their analysis which often makes the comparison between research results from different laboratories difficult. The aim of this work was to review previously reported methods for the characterization of organogelation using vegetable and animal waxes, and to propose a minimal standardization for an organogelation analysis.Los ácidos grasos saturados y trans juegan un rol significativo en las propiedades plásticas de los alimentos. Sin embargo, las recomendaciones de salud sugieren limitar su consumo. Un enfoque que han propuesto investigadores científicos es la disminución en la cantidad de grasas trans y saturadas en la alimentación por medio de la estructuración de aceites comestibles a partir de la cristalización de ceras. Los mecanismos por los cuales la organogelificación ocurre, así como las propiedades que caracterizan una buena estructuración del aceite, han sido descubiertos lentamente debido en parte a la falta de estandarización de los análisis implicados, lo que frecuentemente vuelve difícil la comparación entre resultados de investigación de distintos laboratorios. El objetivo de este trabajo es realizar una revisión de los principales métodos para la caracterización de la organogelificación y de los organogeles formados usando ceras vegetales y animales, proponiendo una estandarización mínima del análisis de organogelificación
Geometrical Evidence for Dark Matter: X-ray Constraints on the Mass of the Elliptical Galaxy NGC 720
(shortened for babbage) We describe (1) a new test for dark matter and
alternate theories of gravitation based on the relative geometries of the X-ray
and optical surface brightness distributions and an assumed form for the
potential of the optical light, (2) a technique to measure the shapes of the
total gravitating matter and dark matter of an ellipsoidal system which is
insensitive to the precise value of the temperature of the gas and to modest
temperature gradients, and (3) a method to determine the ratio of dark mass to
stellar mass that is dependent on the functional forms for the visible star,
gas and dark matter distributions, but independent of the distance to the
galaxy or the gas temperature. We apply these techniques to X-ray data from the
ROSAT Position Sensitive Proportional Counter (PSPC) of the optically-flattened
elliptical galaxy NGC 720. The X-ray isophotes are significantly elongated,
(90% confidence) for semi-major axis a\sim
100\arcsec. The major axes of the optical and X-ray isophotes are misaligned
by (90% confidence). We conclude that matter
distributed like the optical light cannot produce the observed ellipticities of
the X-ray isophotes independent of the gas pressure, the gas temperature, and
the value of the stellar mass.Comment: 63 pages, PostScrip
A Census of X-ray gas in NGC 1068: Results from 450ks of Chandra HETG Observation
We present models for the X-ray spectrum of the Seyfert 2 galaxy NGC 1068.
These are fitted to data obtained using the High Energy Transmission Grating
(HETG) on the Chandra X-ray observatory. The data show line and radiative
recombination continuum (RRC) emission from a broad range of ions and elements.
The models explore the importance of excitation processes for these lines
including photoionization followed by recombination, radiative excitation by
absorption of continuum radiation and inner shell fluorescence. The models show
that the relative importance of these processes depends on the conditions in
the emitting gas, and that no single emitting component can fit the entire
spectrum. In particular, the relative importance of radiative excitation and
photoionization/recombination differs according to the element and ion stage
emitting the line. This in turn implies a diversity of values for the
ionization parameter of the various components of gas responsible for the
emission, ranging from log(xi)=1 -- 3. Using this, we obtain an estimate for
the total amount of gas responsible for the observed emission. The mass flux
through the region included in the HETG extraction region is approximately 0.3
Msun/yr assuming ordered flow at the speed characterizing the line widths. This
can be compared with what is known about this object from other techniques.Comment: 39 pages, 12 figures, Ap. J. in pres
Ionization Structure and the Reverse Shock in E0102-72
The young oxygen-rich supernova remnant E0102-72 in the Small Magellanic
Cloud has been observed with the High Energy Transmission Grating Spectrometer
of Chandra. The high resolution X-ray spectrum reveals images of the remnant in
the light of individual emission lines of oxygen, neon, magnesium and silicon.
The peak emission region for hydrogen-like ions lies at larger radial distance
from the SNR center than the corresponding helium-like ions, suggesting passage
of the ejecta through the "reverse shock". We examine models which test this
interpretation, and we discuss the implications.Comment: 4 pages, 6 figures; To appear in "Young Supernova Remnants" (11th
Annual Astrophysics Conference in Maryland), S. S. Holt & U. Hwang (eds),
AIP, New York (2001
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