728 research outputs found
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
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
Chandra Observations of the X-ray Narrow-Line Region in NGC 4151
We present the first high-resolution X-ray spectrum of the Seyfert 1.5 galaxy
NGC 4151. Observations with the Chandra High Energy Transmission Grating
Spectrometer reveal a spectrum dominated by narrow emission lines from a
spatially resolved (1.6 kpc), highly ionized nebula. The X-ray narrow-line
region is composite, consisting of both photoionized and collisionally ionized
components. The X-ray emission lines have similar velocities, widths, and
spatial extent to the optical emission lines, showing that they arise in the
same region. The clouds in the narrow-line region must contain a large range of
ionization states in order to explain both the optical and X-ray photoionized
emission. Chandra data give the first direct evidence of X-ray line emission
from a hot plasma (T~1e7 K) which may provide pressure confinement for the
cooler (T=3e4 K) photoionized clouds.Comment: 13 pages, 3 figures, to be published in Astrophysical Journal Letter
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
Visits to rheumatologists for arthritis: The role of access to primary care physicians, geographic availability of rheumatologists, and socioeconomic status
Objective. This multilevel study examines access to rheumatologists for all arthritis and inflammatory arthritis, taking into account geographic availability of rheumatologists, access to primary care physicians (PCPs), and population characteristics (e.g., socioeconomic status [SES]). Methods. We analyzed data from the population (age ≤18 years) living in the 105 health planning areas in Ontario, Canada on visits to physicians for arthritis and musculoskeletal disorders. Using data from a survey of rheumatologists and Geographic Information System analysis, an index of geographic availability for rheumatologists was calculated, incorporating distance between the population and rheumatologist locations and the number of hours per week of rheumatologist care. Multilevel Poisson regression was used to examine factors associated with the rates of rheumatology visits for inflammatory arthritis and all arthritis. Results. Controlling for age and sex, the rheumatologist availability index was associated with visit rates for all arthritis, but not inflammatory arthritis. Patients living in areas with low access to PCPs or low SES were less likely to have office visits to rheumatologists for all arthritis and inflammatory arthritis. Conclusion. Besides potential deficiencies in rheumatology provision, there may be access barriers to rheumatology services, particularly for populations with low access to PCPs or low SES. This is of special concern for patients with inflammatory arthritis for whom rheumatologist care is necessary. In developing models of care for arthritis, this study points to the need to pay attention to areas with low PCP resources and areas of low SES, as well as the location and amount of rheumatology services available
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