98 research outputs found
Stochastic Electron Acceleration During the NIR and X-ray Flares in Sagittarius A*
Recent near-IR (NIR) and X-ray observations of Sagittarius A*'s spectrum have
yielded several strong constraints on the transient energization mechanism,
justifying a re-examination of the stochastic acceleration model proposed
previously for these events. We here demonstrate that the new results are fully
consistent with the acceleration of electrons via the transit-time damping
process. But more importantly, these new NIR and X-ray flares now can constrain
the source size, the gas density, the magnetic field, and the wave energy
density in the turbulent plasma. Future simultaneous multi-wavelength
observations with good spectral information will, in addition, allow us to
study their temporal evolution, which will eventually lead to an accurate
determination of the behavior of the plasma just minutes prior to its
absorption by the black hole.Comment: 13 pages, 4 figures, Submitted to ApJ
Polarimetric Imaging of the Massive Black Hole at the Galactic Center
The radio source Sgr A* in the Galactic center emits a polarized spectrum at
millimeter and sub-millimeter wavelengths that is strongly suggestive of
relativistic disk accretion onto a massive black hole. We use the
well-constrained mass of Sgr A* and a magnetohydrodynamic model of the
accretion flow to match both the total flux and polarization from this object.
Our results demonstrate explicitly that the shift in the position angle of the
polarization vector, seen at wavelengths near the peak of the mm to sub-mm
emission from this source, is a signal of relativistic accretion flow in a
strong gravitational field. We provide maps of the polarized emission to
illustrate how the images of polarized intensity from the vicinity of the black
hole would appear in upcoming observations with very long baseline radio
interferometers (VLBI). Our results suggest that near-term VLBI observations
will be able to directly image the polarized Keplerian portion of the flow near
the horizon of the black hole.Comment: 12 pages, 2 figures, Accepted for publciation in ApJ Letter
Is Thermal Expansion Driving the Initial Gas Ejection in NGC 6251?
In this paper, we explore the possibility that the radiative properties of
the most compact region in NGC 6251* may be understood in the same sense as Sgr
A*, though with some telling differences that may hint at the nature of jet
formation. We show that observations of this object with ASCA, ROSAT, HST and
VLBI together may be hinting at a picture in which Bondi-Hoyle accretion from
an ambient ionized medium feeds a standard disk accreting at ~ 4.0*10^{22} g
s^{-1}. Somewhere near the event horizon, this plasma is heated to >10^{11} K,
where it radiates via thermal synchrotron (producing a radio component) and
self-Comptonization (accounting for a nonthermal X-ray flux). This temperature
is much greater than its virial value and the hot cloud expands at roughly the
sound speed (~0.1c), after which it begins to accelerate on a parsec scale to
relativistic velocities. In earlier work, the emission from the extended jet
has been modeled successfully using nonthermal synchrotron self-Compton
processes, with a self-absorbed inner core. In the picture we are developing
here, the initial ejection of matter is associated with a self-absorbed thermal
radio component that dominates the core emission on the smallest scales. The
nonthermal particle distributions responsible for the emission in the extended
jet are then presumably energized, e.g., via shock acceleration, within the
expanding, hot gas. The power associated with this plasma represents an
accretion efficiency of about 0.54, requiring dissipation in a prograde disk
around a rapidly spinning black hole (with spin parameter a~1).Comment: 17 pages, 1 figures, to appear in Ap
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Comparative Metabolomics of Early Development of the Parasitic Plants Phelipanche aegyptiaca and Triphysaria versicolor.
Parasitic weeds of the family Orobanchaceae attach to the roots of host plants via haustoria capable of drawing nutrients from host vascular tissue. The connection of the haustorium to the host marks a shift in parasite metabolism from autotrophy to at least partial heterotrophy, depending on the level of parasite dependence. Species within the family Orobanchaceae span the spectrum of host nutrient dependency, yet the diversity of parasitic plant metabolism remains poorly understood, particularly during the key metabolic shift surrounding haustorial attachment. Comparative profiling of major metabolites in the obligate holoparasite Phelipanche aegyptiaca and the facultative hemiparasite Triphysaria versicolor before and after attachment to the hosts revealed several metabolic shifts implicating remodeling of energy and amino acid metabolism. After attachment, both parasites showed metabolite profiles that were different from their respective hosts. In P. aegyptiaca, prominent changes in metabolite profiles were also associated with transitioning between different tissue types before and after attachment, with aspartate levels increasing significantly after the attachment. Based on the results from 15N labeling experiments, asparagine and/or aspartate-rich proteins were enriched in host-derived nitrogen in T. versicolor. These results point to the importance of aspartate and/or asparagine in the early stages of attachment in these plant parasites and provide a rationale for targeting aspartate-family amino acid biosynthesis for disrupting the growth of parasitic weeds
Polarized mm And sub-mm Emission From Sgr A* At The Galactic Center
The recent detection of significant linear polarization at mm and sub-mm
wavelengths in the spectrum of Sgr A* (if confirmed) will be a useful probe of
the conditions within several Schwarzschild radii () of the event horizon
at the Galactic Center. Hydrodynamic simulations of gas flowing in the vicinity
of this object suggest that the infalling gas circularizes when it approaches
within of the black hole. We suggest that the sub-mm ``excess'' of
emission seen in the spectrum of Sgr A* may be associated with radiation
produced within the inner Keplerian region and that the observed polarization
characteristics provide direct evidence for this phenomenon. The overall
spectrum from this region, including the high-energy component due to
bremsstrahlung and inverse Compton scattering processes, is at or below the
recent {\it Chandra} measurement, and may account for the X-ray source if it
turns out to be the actual counterpart to Sgr A*.Comment: 12 pages, 2 figures. published in APJ Letter
Probing the Density in the Galactic Center Region: Wind-Blown Bubbles and High-Energy Proton Constraints
Recent observations of the Galactic center in high-energy gamma-rays (above
0.1TeV) have opened up new ways to study this region, from understanding the
emission source of these high-energy photons to constraining the environment in
which they are formed. We present a revised theoretical density model of the
inner 5pc surrounding Sgr A* based on the fact that the underlying structure of
this region is dominated by the winds from the Wolf-Rayet stars orbiting Sgr
A*. An ideal probe and application of this density structure is this high
energy gamma-ray emission. We assume a proton-scattering model for the
production of these gamma-rays and then determine first whether such a model is
consistent with the observations and second whether we can use these
observations to further constrain the density distribution in the Galactic
center.Comment: 36 pages including 17 figures, submitted to ApJ, comments welcom
Role of dual breakwaters and trenches on efficiency of an oscillating water column
In this paper, the effects of double-submerged breakwaters and trenches on the hydrodynamic performance of an oscillating water column (OWC) are investigated. The multi-domain boundary element method is used to tackle the physical problem of wave scattering and radiation from the device. The role of the height of the breakwaters, depth of the trenches, width of the breakwaters and trenches, spacing between the structures, length of the OWC chamber, and other wave and structural parameters is investigated on the efficiency of OWC. The study reveals that there is an oscillating pattern of the efficiency curve in the presence of single or double breakwater/trenches; this pattern is absent when the bottom is flat. Moreover, compared to single or no breakwaters/trenches, the occurrence of full OWC efficiency is higher in the presence of double breakwaters/trenches. Furthermore, the amplitude of the oscillating pattern in the efficiency curve increases with an increase in the height and depth of the breakwaters and trenches, respectively. For some particular wave and structural parameters, zero OWC efficiency occurs nearly 0ℎ=3.4 within 0<0ℎ<5 (k0 wave number and h water depth). This zero efficiency moves toward small wave numbers as the spacing between OWC and rigid breakwater/trench increases. The radiation conductance of OWC decreases with an increase in the barrier height. The findings outline the structural criteria that can be employed to build and deploy an effective OWC device
Spin-Induced Disk Precession in the Supermassive Black Hole at the Galactic Center
Sgr A* is a compact radio source at the Galactic Center, thought to be the
radiative manifestation of a supermassive black hole.
At least a portion of its spectrum--notably the mm/sub-mm ``bump''--appears to
be produced within the inner portion () of a hot, magnetized
Keplerian flow, whose characteristics are also consistent with the
linear polarization detected from this source at mm wavelengths. (The
Schwarzschild radius, , for an object of this mass is cm, or roughly 1/20 A.U.) The recent detection of a 106-day
cycle in Sgr A*'s radio variability adds significant intrigue to this picture,
since it may signal a precession of the disk induced by the spin of the
black hole. The dynamical time scale near the marginally stable orbit around an
object with this mass is mins. Thus, since the physical conditions
associated with the disk around Sgr A* imply rigid-body rotation, a precession
period of 106 days may be indicative of a small black hole spin if the
circularized flow is confined to a region , for which . The precession of a larger structure would require
a bigger black hole spin. We note that a small value of () would
be favored if the non-thermal ( cm) portion of Sgr A*'s spectrum is
powered with energy extracted via a Blandford-Znajek type of process, for which
the observed luminosity would correspond to an outer disk radius . Such a small disk size is also suggested by earlier hydrodynamical
simulations, and is implied by Sgr A*'s spectral and polarimetric
characteristics.Comment: 12 pages, 1 figure. Accepted for publicaton in ApJ Letter
Stochastic Acceleration in the Galactic Center HESS Source
Stochastic acceleration of electrons interacting resonantly with a turbulent
magnetic field in a small accretion torus appears to be the likely mechanism
responsible for much of Sagittarius A*'s millimeter and shorter wavelength
spectrum. The longer wavelength radiation is produced at larger radii by
electrons either diffusing from smaller scales or accelerated in situ. An
important prediction of this model is the ejection of a significant flux of
relativistic protons from a magnetic-field-dominated acceleration site into the
wind-shocked medium surrounding the black hole. Recently, several air Cerenkov
telescopes, notably HESS, have detected TeV emission from the Galactic center,
with characteristics hinting at a p-p-induced pion decay process for the
\gamma-ray emission. Given (1) the size of this acceleration region measured in
the radio band and (2) the wind-injected ISM mapped with Chandra using the
diffuse X-rays, it is feasible to test the idea that protons accelerated within
\~20 Schwarzschild radii of the black hole produce the TeV emission farther
out. We show a fraction of TeV protons scattering about once within ~3 pc of
Sagittarius A* and the proton power (~10^37 erg s^-1) produced in concert with
the 7 mm radio emission matches the TeV luminosity well. This model explains
why the TeV source does not vary on a timescale of a year or less. The particle
cascade generated by the p-p scatterings also produces bremsstrahlung, inverse
Compton, and synchrotron emission at longer wavelengths from secondary
particles. We compare these with current measurements and demonstrate that
GLAST will detect this source during its one-year all-sky survey.Comment: 19 pages, 3 figure
Health-related quality of life as measured with EQ-5D among populations with and without specific chronic conditions: A population-based survey in Shaanxi province, China
© 2013 Tan et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Introduction: The aim of this study was to examine health-related quality of life (HRQoL) as measured by EQ-5D and to investigate the influence of chronic conditions and other risk factors on HRQoL based on a distributed sample located in Shaanxi Province, China. Methods: A multi-stage stratified cluster sampling method was performed to select subjects. EQ-5D was employed to measure the HRQoL. The likelihood that individuals with selected chronic diseases would report any problem in the EQ-5D dimensions was calculated and tested relative to that of each of the two reference groups. Multivariable linear regression models were used to investigate factors associated with EQ VAS. Results: The most frequently reported problems involved pain/discomfort (8.8%) and anxiety/depression (7.6%). Nearly half of the respondents who reported problems in any of the five dimensions were chronic patients. Higher EQ VAS scores were associated with the male gender, higher level of education, employment, younger age, an urban area of residence, access to free medical service and higher levels of physical activity. Except for anemia, all the selected chronic diseases were indicative of a negative EQ VAS score. The three leading risk factors were cerebrovascular disease, cancer and mental disease. Increases in age, number of chronic conditions and frequency of physical activity were found to have a gradient effect. Conclusion: The results of the present work add to the volume of knowledge regarding population health status in this area, apart from the known health status using mortality and morbidity data. Medical, policy, social and individual attention should be given to the management of chronic diseases and improvement of HRQoL. Longitudinal studies must be performed to monitor changes in HRQoL and to permit evaluation of the outcomes of chronic disease intervention programs. © 2013 Tan et al.National Nature Science Foundation (No. 8107239
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