28 research outputs found
An XMM-Newton Observation of the Local Bubble Using a Shadowing Filament in the Southern Galactic Hemisphere
We present an analysis of the X-ray spectrum of the Local Bubble, obtained by
simultaneously analyzing spectra from two XMM-Newton pointings on and off an
absorbing filament in the Southern galactic hemisphere (b ~ -45 deg). We use
the difference in the Galactic column density in these two directions to deduce
the contributions of the unabsorbed foreground emission due to the Local
Bubble, and the absorbed emission from the Galactic halo and the extragalactic
background. We find the Local Bubble emission is consistent with emission from
a plasma in collisional ionization equilibrium with a temperature and an emission measure of 0.018 cm^{-6} pc. Our
measured temperature is in good agreement with values obtained from ROSAT
All-Sky Survey data, but is lower than that measured by other recent XMM-Newton
observations of the Local Bubble, which find
(although for some of these observations it is possible that the foreground
emission is contaminated by non-Local Bubble emission from Loop I). The higher
temperature observed towards other directions is inconsistent with our data,
when combined with a FUSE measurement of the Galactic halo O VI intensity. This
therefore suggests that the Local Bubble is thermally anisotropic.
Our data are unable to rule out a non-equilibrium model in which the plasma
is underionized. However, an overionized recombining plasma model, while
observationally acceptable for certain densities and temperatures, generally
gives an implausibly young age for the Local Bubble (\la 6 \times 10^5 yr).Comment: Accepted for publication in the Astrophysical Journal. 16 pages, 9
figure
An XMM-Newton Survey of the Soft X-ray Background. II. An All-Sky Catalog of Diffuse O VII and O VIII Emission Intensities
We present an all-sky catalog of diffuse O VII and O VIII line intensities,
extracted from archival XMM observations. The O VII and O VIII intensities are
typically ~2-11 and <~3 ph/cm^2/s/sr (LU), respectively, although much brighter
intensities were also recorded. Our data set includes 217 directions observed
multiple times by XMM. The time variation of the intensities from such
directions may be used to constrain SWCX models. The O VII and O VIII
intensities typically vary by <~5 and <~2 LU between repeat observations,
although several intensity enhancements of >10 LU were observed. We compared
our measurements with SWCX models. The heliospheric SWCX intensity is expected
to vary with ecliptic latitude and solar cycle. We found that the observed
oxygen intensities generally decrease from solar maximum to solar minimum, both
at high ecliptic latitudes (as expected) and at low ecliptic latitudes (not as
expected). The geocoronal SWCX intensity is expected to depend on the solar
wind proton flux and on the sightline's path through the magnetosheath. The
intensity variations seen in directions that have been observed multiple times
are in poor agreement with the predictions of a geocoronal SWCX model. The
oxygen lines account for ~40-50% of the 3/4 keV X-ray background that is not
due to unresolved AGN, in good agreement with a previous measurement. However,
this fraction is not easily explained by a combination of SWCX emission and
emission from hot plasma in the halo. The line intensities tend to increase
with longitude toward the inner Galaxy, possibly due to an increase in the
supernova rate in that direction or the presence of a halo of accreted material
centered on the Galactic Center. The variation of intensity with Galactic
latitude differs in different octants of the sky, and cannot be explained by a
single simple plane-parallel or constant-intensity halo model. (Abridged)Comment: Accepted for publication in the Astrophysical Journal Supplement
Series. 29 pages (main body of paper) plus 85 pages (full versions of Tables
1, 2, and 4 - these tables will be published as machine-readable tables in
the journal, and appear in abbreviated form in the main body of the paper).
12 figures. v2: Minor corrections, conclusions unaltere
The Origin of the Hot Gas in the Galactic Halo: Confronting Models with XMM-Newton Observations
We compare the predictions of three physical models for the origin of the hot
halo gas with the observed halo X-ray emission, derived from 26 high-latitude
XMM-Newton observations of the soft X-ray background between l=120\degr and
l=240\degr. These observations were chosen from a much larger set of
observations as they are expected to be the least contaminated by solar wind
charge exchange emission. We characterize the halo emission in the XMM-Newton
band with a single-temperature plasma model. We find that the observed halo
temperature is fairly constant across the sky (~1.8e6-2.3e6 K), whereas the
halo emission measure varies by an order of magnitude (~0.0005-0.006 cm^-6 pc).
When we compare our observations with the model predictions, we find that most
of the hot gas observed with XMM-Newton does not reside in isolated extraplanar
supernova remnants -- this model predicts emission an order of magnitude too
faint. A model of a supernova-driven interstellar medium, including the flow of
hot gas from the disk into the halo in a galactic fountain, gives good
agreement with the observed 0.4-2.0 keV surface brightness. This model
overpredicts the halo X-ray temperature by a factor of ~2, but there are a
several possible explanations for this discrepancy. We therefore conclude that
a major (possibly dominant) contributor to the halo X-ray emission observed
with XMM-Newton is a fountain of hot gas driven into the halo by disk
supernovae. However, we cannot rule out the possibility that the extended hot
halo of accreted material predicted by disk galaxy formation models also
contributes to the emission.Comment: 20 pages, 14 figures. New version accepted for publication in ApJ.
Changes include new section discussing systematic errors (Section 3.2),
improved method for characterizing our model spectra (4.2.2), changes to
discussion of other observations (5.1). Note that we can no longer rule out
possibility that extended hot halo of accreted material contributes to
observed halo emission (see 5.2.1
The origin of the hot gas in the galactic halo: Testing galactic fountain models' X-ray emission
We test the X-ray emission predictions of galactic fountain models against XMM-Newton measurements of the emission from the Milky Way's hot halo. These measurements are from 110 sight lines, spanning the full range of Galactic longitudes. We find that a magnetohydrodynamical simulation of a supernova-driven interstellar medium, which features a flow of hot gas from the disk to the halo, reproduces the temperature but significantly underpredicts the 0.5-2.0 keV surface brightness of the halo (by two orders of magnitude, if we compare the median predicted and observed values). This is true for versions of the model with and without an interstellar magnetic field. We consider different reasons for the discrepancy between the model predictions and the observations. We find that taking into account overionization in cooled halo plasma, which could in principle boost the predicted X-ray emission, is unlikely in practice to bring the predictions in line with the observations. We also find that including thermal conduction, which would tend to increase the surface brightnesses of interfaces between hot and cold gas, would not overcome the surface brightness shortfall. However, charge exchange emission from such interfaces, not included in the current model, may be significant. The faintness of the model may also be due to the lack of cosmic ray driving, meaning that the model may underestimate the amount of material transported from the disk to the halo. In addition, an extended hot halo of accreted material may be important, by supplying hot electrons that could boost the emission of the material driven out from the disk. Additional model predictions are needed to test the relative importance of these processes in explaining the observed halo emission.open2
The origin of the x-ray emission from the high-velocity cloud MS30.7-81.4-118
A soft X-ray enhancement has recently been reported toward the high-velocity cloud MS30.7-81.4-118 (MS30.7), a constituent of the Magellanic Stream. In order to investigate the origin of this enhancement, we have analyzed two overlapping XMM-Newton observations of this cloud. We find that the X-ray enhancement is 6??? or 100 pc across, and is concentrated to the north and west of the densest part of the cloud. We modeled the X-ray enhancement with a variety of spectral models. A single-temperature equilibrium plasma model yields a temperature of and a 0.4-2.0 keV luminosity of 7.9 ?? 1033 erg s-1. However, this model underpredicts the on-enhancement emission around 1 keV, which may indicate the additional presence of hotter plasma (T &#8819; 107 K), or that recombination emission is important. We examined several different physical models for the origin of the X-ray enhancement. We find that turbulent mixing of cold cloud material with hot ambient material, compression or shock heating of a hot ambient medium, and charge exchange reactions between cloud atoms and ions in a hot ambient medium all lead to emission that is too faint. In addition, shock heating in a cool or warm medium leads to emission that is too soft (for reasonable cloud speeds). We find that magnetic reconnection could plausibly power the observed X-ray emission, but resistive magnetohydrodynamical simulations are needed to test this hypothesis. If magnetic reconnection is responsible for the X-ray enhancement, the observed spectral properties could potentially constrain the magnetic field in the vicinity of the Magellanic Stream.open1
SIMULATIONS OF HIGH-VELOCITY CLOUDS. I. HYDRODYNAMICS AND HIGH-VELOCITY HIGH IONS
We present hydrodynamic simulations of high-velocity clouds (HVCs) traveling through the hot, tenuous medium in the Galactic halo. A suite of models was created using the FLASH hydrodynamics code, sampling various cloud sizes, densities, and velocities. In all cases, the cloud-halo interaction ablates material from the clouds. The ablated material falls behind the clouds where it mixes with the ambient medium to produce intermediate-temperature gas, some of which radiatively cools to less than 10,000K. Using a non-equilibrium ionization algorithm, we track the ionization levels of carbon, nitrogen, and oxygen in the gas throughout the simulation period. We present observation-related predictions, including the expected H I and high ion (C IV, N V, and O VI) column densities on sightlines through the clouds as functions of evolutionary time and off-center distance. The predicted column densities overlap those observed for Complex C. The observations are best matched by clouds that have interacted with the Galactic environment for tens to hundreds of megayears. Given the large distances across which the clouds would travel during such time, our results are consistent with Complex C having an extragalactic origin. The destruction of HVCs is also of interest; the smallest cloud (initial mass ≈ 120 M ???) lost most of its mass during the simulation period (60Myr), while the largest cloud (initial mass ≈ 4 ?? 105 M ???) remained largely intact, although deformed, during its simulation period (240Myr).open201
Revising the Local Bubble Model due to Solar Wind Charge Exchange X-ray Emission
The hot Local Bubble surrounding the solar neighborhood has been primarily
studied through observations of its soft X-ray emission. The measurements were
obtained by attributing all of the observed local soft X-rays to the bubble.
However, mounting evidence shows that the heliosphere also produces diffuse
X-rays. The source is solar wind ions that have received an electron from
another atom. The presence of this alternate explanation for locally produced
diffuse X-rays calls into question the existence and character of the Local
Bubble. This article addresses these questions. It reviews the literature on
solar wind charge exchange (SWCX) X-ray production, finding that SWCX accounts
for roughly half of the observed local 1/4 keV X-rays found at low latitudes.
This article also makes predictions for the heliospheric O VI column density
and intensity, finding them to be smaller than the observational error bars.
Evidence for the continued belief that the Local Bubble contains hot gas
includes the remaining local 1/4 keV intensity, the observed local O VI column
density, and the need to fill the local region with some sort of plasma. If the
true Local Bubble is half as bright as previously thought, then its electron
density and thermal pressure are 1/square-root(2) as great as previously
thought, and its energy requirements and emission measure are 1/2 as great as
previously thought. These adjustments can be accommodated easily, and, in fact,
bring the Local Bubble's pressure more in line with that of the adjacent
material. Suggestions for future work are made.Comment: 9 pages, refereed, accepted for publication in the proceedings of the
"From the Outer Heliosphere to the Local Bubble: Comparisons of New
Observations with Theory" conference and in Space Science Review
Patient-centered interventions to improve medication management and adherence: A qualitative review of research findings
Patient-centered approaches to improving medication adherence hold promise, but evidence of their effectiveness is unclear. This review reports the current state of scientific research around interventions to improve medication management through four patient-centered domains: shared decision-making, methods to enhance effective prescribing, systems for eliciting and acting on patient feedback about medication use and treatment goals, and medication-taking behavior
A Portrait of the Transcriptome of the Neglected Trematode, Fasciola gigantica—Biological and Biotechnological Implications
Fasciola gigantica (Digenea) is an important foodborne trematode that causes liver fluke disease (fascioliasis) in mammals, including ungulates and humans, mainly in tropical climatic zones of the world. Despite its socioeconomic impact, almost nothing is known about the molecular biology of this parasite, its interplay with its hosts, and the pathogenesis of fascioliasis. Modern genomic technologies now provide unique opportunities to rapidly tackle these exciting areas. The present study reports the first transcriptome representing the adult stage of F. gigantica (of bovid origin), defined using a massively parallel sequencing-coupled bioinformatic approach. From >20 million raw sequence reads, >30,000 contiguous sequences were assembled, of which most were novel. Relative levels of transcription were determined for individual molecules, which were also characterized (at the inferred amino acid level) based on homology, gene ontology, and/or pathway mapping. Comparisons of the transcriptome of F. gigantica with those of other trematodes, including F. hepatica, revealed similarities in transcription for molecules inferred to have key roles in parasite-host interactions. Overall, the present dataset should provide a solid foundation for future fundamental genomic, proteomic, and metabolomic explorations of F. gigantica, as well as a basis for applied outcomes such as the development of novel methods of intervention against this neglected parasite