383 research outputs found
Constraints on hot metals in the Vicinity of the Galaxy
We have searched for evidence of soft X-ray absorption by hot metals in the
vicinity of the Galaxy in the spectra of a small sample of fifteen Type I AGN
observed with the high resolution X-ray gratings on board Chandra. This is an
extension of our previous survey of hot OVII and OVIII absorbing gas in the
vicinity of the Galaxy. The strongest absorption signatures within a few
hundred km/s of their rest-frame energies are most likely due to warm absorbing
outflows from the nearest AGN, which are back-lighting the local hot gas. We
emphasize that absorption signatures in the spectra of some distant AGN that
are kinematically consistent with the recessional velocity of the AGN are most
likely to be due to hot local gas. Along the sightline towards PG 1211+143, PDS
456 and MCG-6-30-15 there is a very large absorbing Fe column density which is
kinematically consistent with absorption by hot, local Fe. The sightlines to
these three AGN pass through the limb of the Northern Polar Spur (NPS), a local
bubble formed from several supernovae which, if rich in Fe, may account for a
large local Fe column.
We obtain limits on the column density of local, highly ionized N, Ne, Mg, Si
along all of the sightlines in our sample. We correlate the column density
limits with those of highly ionized O along the same sightlines. Assuming the
hot local gas is in collisionally ionized equilibrium, we obtain limits on the
temperature and relative abundances of the metals in the hot local gas. Our
limits on the ionic column densities in the local hot gas seem to be consistent
with those observed in the hot halo gas of edge-on normal spiral galaxies.Comment: 9 pages,2 figures, MNRAS (accepted
On rapid migration and accretion within disks around supermassive black holes
Galactic nuclei should contain a cluster of stars and compact objects in the
vicinity of the central supermassive black hole due to stellar evolution, minor
mergers and gravitational dynamical friction. By analogy with protoplanetary
migration, nuclear cluster objects (NCOs) can migrate in the accretion disks
that power active galactic nuclei by exchanging angular momentum with disk gas.
Here we show that an individual NCO undergoing runaway outward migration
comparable to Type III protoplanetary migration can generate an accretion rate
corresponding to Seyfert AGN or quasar luminosities. Multiple migrating NCOs in
an AGN disk can dominate traditional viscous disk accretion and at large disk
radii, ensemble NCO migration and accretion could provide sufficient heating to
prevent the gravitational instability from consuming disk gas in star
formation. The magnitude and energy of the X-ray soft excess observed at
~0.1-1keV in Seyfert AGN could be explained by a small population of
~10^{2}-10^{3} accreting stellar mass black holes or a few ULXs. NCO migration
and accretion in AGN disks are therefore extremely important mechanisms to add
to realistic models of AGN disks.Comment: 6 pages, 2 figures, MNRAS Letters (accepted
The Kinematics and Physical Conditions pf the Ionized Gas in Markarian 509. II. STIS Echelle Observations
We present observations of the UV absorption lines in the luminous Seyfert 1
galaxy Mrk 509, obtained with the medium resolution (lambda/Delta-lambda ~
40,000) echelle gratings of the Space Telescope Imaging Spectrograph on the
Hubble Space Telescope. The spectra reveal the presence of eight kinematic
components of absorption in Ly-alpha, C IV, and N V, at radial velocities of
-422, -328, -259, -62, -22, +34, +124, and +210 km s^-1 with respect to an
emission-line redshift of z = 0.03440, seven of which were detected in an
earlier Far Ultraviolet Spectrographic Explorer (FUSE) spectrum. The component
at -22 km s^-1 also shows absorption by Si IV. The covering factor and velocity
width of the Si IV lines were lower than those of the higher ionization lines
for this component, which is evidence for two separate absorbers at this
velocity. We have calculated photoionization models to match the UV column
densities in each of these components. Using the predicted O VI column
densities, we were able to match the O VI profiles observed in the FUSE
spectrum. Based on our results, none of the UV absorbers can produce the X-ray
absorption seen in simultaneous Chandra observations; therefore, there must be
more highly ionized gas in the radial velocity ranges covered by the UV
absorbers.Comment: 30 pages, three figures (Figure 1 is in color). Accepted for
publication in the Astrophysical Journa
Predicting dentists decisions: a choice-based conjoint analysis of Medicaid participation
Objectives: Private practice dentists are the major source of care for the dental
safety net; however, the proportion of dentists who participate in state Medicaid
programs is low, often due to poor perceptions of the program’s administration
and patient population. Using a discrete choice experiment and a series of
hypothetical scenarios, this study evaluated trade-offs dentists make when deciding
to accept Medicaid patients.
Methods: An online choice-based conjoint survey was sent to 272 general dentists
in Iowa. Hypothetical scenarios presented factors at systematically varied levels.
The primary determination was whether dentists would accept a new Medicaid
patient in each scenario. Using an ecological model of behavior, determining
factors were selected from the categories of policy, administration, community, and
patient population to estimate dentists’ relative preferences.
Results: 62 percent of general dentists responded to the survey. The probability of
accepting a new Medicaid patient was highest (81 percent) when reimbursement
rates were 85 percent of the dentist’s fees, patients never missed appointments,
claims were approved on first submission, and no other practices in the area
accepted Medicaid. Although dentists preferred higher reimbursement rates, 56
percent would still accept a new Medicaid patient when reimbursement decreased
to 55 percent if they were told that the patient would never miss appointments and
claims would be approved on initial submission.
Conclusions: This study revealed trade-offs that dentists make when deciding to
participate in Medicaid. Findings indicate that states can potentially improve
Medicaid participation without changing reimbursement rates by making
improvements in claims processing and care coordination to reduce missed
appointments.Funding for this project came from an Innovation Fund for
Oral Health award from the DentaQuest Foundation (Boston,
MA)
Tracing a Disk Wind in NGC 3516
X-ray spectra of AGN often contain signatures indicative of absorption in
multiple layers of gas whose ionization-state and covering fraction may vary
with time. It has been unclear to date how much of the observed X-ray spectral
and timing behavior in AGN can be attributed to variations in absorption,
versus variations in the strengths of emission or reflection components.
Diagnostics of the inner regions of AGN cannot be reliably performed until the
origin of observed effects is understood. We investigate the role of the X-ray
absorbers in the Seyfert 1 galaxy NGC 3516. Time-averaged and flux-selected
spectroscopy is used to examine the behavior of NGC 3516 observed in Chandra
HETG and XMM data from Oct 2006. New H-like and He-like emission and absorption
features discovered in the Fe K regime reveal a previously unknown zone of
circumnuclear gas in NGC 3516 with log xi ~ 4.3 and column density 1E23 cm^-2.
A lower-ionization layer with log xi ~2 and of similar column density is
confirmed from previous observations, this layer has a covering fraction around
50%, and changes in covering provide a simple explanation of a deep dip in the
light curve that we interpret as an eclipse of the continuum due to passage of
a cloud across the sight line within half a day. These inner zones of absorbing
gas are detected to have outflow velocities in the range 1000-2000 km/s, this,
and constraints on radial location are consistent with an origin as part of a
disk wind in NGC 3516.Comment: 10 pages, 8 figures, accepted by A&A Feb 26 2008 Version 2: Figs 1 &
2 were corrected for a problem with ACIS section of the data. Little impact
on text of the pape
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