8,728 research outputs found
Collisional relaxation of electrons in a warm plasma and accelerated nonthermal electron spectra in solar flares
Extending previous studies of nonthermal electron transport in solar flares
which include the effects of collisional energy diffusion and thermalization of
fast electrons, we present an analytic method to infer more accurate estimates
of the accelerated electron spectrum in solar flares from observations of the
hard X-ray spectrum. Unlike for the standard cold-target model, the spatial
characteristics of the flaring region, especially the necessity to consider a
finite volume of hot plasma in the source, need to be taken into account in
order to correctly obtain the injected electron spectrum from the
source-integrated electron flux spectrum (a quantity straightforwardly obtained
from hard X-ray observations). We show that the effect of electron
thermalization can be significant enough to nullify the need to introduce an
{\it ad hoc} low-energy cutoff to the injected electron spectrum in order to
keep the injected power in non-thermal electrons at a reasonable value. Rather
the suppression of the inferred low-energy end of the injected spectrum
compared to that deduced from a cold-target analysis allows the inference from
hard X-ray observations of a more realistic energy in injected non-thermal
electrons in solar flares.Comment: accepted for publication in Ap
On the variation of solar flare coronal x-ray source sizes with energy
Observations with {\em RHESSI} have enabled the detailed study of the
structure of dense hard X-ray coronal sources in solar flares. The variation of
source extent with electron energy has been discussed in the context of
streaming of non-thermal particles in a one-dimensional cold-target model, and
the results used to constrain both the physical extent of, and density within,
the electron acceleration region. Here we extend this investigation to a more
physically realistic model of electron transport that takes into account the
finite temperature of the ambient plasma, the initial pitch-angle distribution
of the accelerated electrons, and the effects of collisional pitch-angle
scattering. The finite temperature results in the thermal diffusion of
electrons, that leads to the observationally-inferred value of the acceleration
region volume being an overestimate of its true value. The different directions
of the electron trajectories, a consequence of both the non-zero injection
pitch-angle and scattering within the target, cause the projected propagation
distance parallel to the guiding magnetic field to be reduced, so that a
one-dimensional interpretation can overestimate the actual density by a factor
of up to . The implications of these results for the determination of
acceleration region properties (specific acceleration rate, filling factor,
etc.) are discussed.Comment: 45 pages, 9 figures, accepted for publication in Ap
Constrained viscosity solution to the HJB equation arising in perpetual American employee stock options pricing
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Thermally stimulated relaxation and behaviors of oxygen vacancies in SrTiO<inf>3</inf> single crystals with (100), (110) and (111) orientations
Abstract
The strontium titanate (SrTiO3) single crystals with different orientations of (100), (110) and (111) were investigated using thermally stimulated depolarization current (TSDC) measurements, which has been proved to be an effective strategy to fundamentally study the relationship between relaxation phenomena and defect chemistry in dielectrics. The origins of different relaxations in SrTiO3 crystals were identified and the activation energy of oxygen vacancies was estimated from TSDC measurements. It was further found that oxygen-treated SrTiO3 crystals exhibit different relaxation behaviors. Noticeable changes of thermal relaxation associated with oxygen vacancies have taken place in relation to the crystalline anisotropy. The SrTiO3 (110) samples display higher concentration and activation energy of oxygen vacancies. First-principles calculations were carried out on SrTiO3 (110) crystals to study the effect of oxygen vacancy on different surface microstructure. From the resulting minimum formation energy of 0.63 eV, it demonstrates that the oxygen vacancies tend to form on the TiO-terminated surfaces. Considering the band structure, oxygen vacancies near the surface contribute to the transition of crystal from insulator to metallic characteristic.</jats:p
The -based dimensionless accretion rate and its connection with the corona for AGN
With respect to the full width at half-maximum (), the broad line dispersion ()
was preferred as a velocity tracer to calculate the single-epoch supermassive
black hole mass () suggested by \cite{Yu2020b}. For a compiled
sample of 311 broad-line active galactic nuclei (AGN) with measured hard X-ray
photon index (), and the optical Fe II relative
strength () are measured from their optical spectra, which are used
to calculate -based virial and dimensionless
accretion rate (). With respect to ,
it is found that the mean value of -based is
on average larger by 0.26 dex, and the mean value of -based is on average smaller by 0.51 dex. It is
found that there exists a non-linear relationship between the Eddington ratio
() and , i.e., . This non-linear relationship
comes from the accretion efficiency , which is smaller for AGN with
higher . We find a strong bivariate correlation of the
fraction of energy released in the corona with
and \mbh, . The flat slope of favours the shear
stress tensor of the accretion disk being proportional to the geometric mean of
gas pressure and total pressure. We find a strong bivariate relation of
with and , . The hard X-ray
spectrum becomes softer with increasing of , although the scatter is
large.Comment: 18 pages, 9 figures, ApJ, in pres
A comparison of the local spiral structure from Gaia DR2 and VLBI maser parallaxes
Context. The Gaia mission has released the second data set (Gaia DR2), which
contains parallaxes and proper motions for a large number of massive, young
stars. Aims. We investigate the spiral structure in the solar neighborhood
revealed by Gaia DR2 and compare it with that depicted by VLBI maser
parallaxes. Methods. We examined three samples with different constraints on
parallax uncertainty and distance errors and stellar spectral types: (1) all OB
stars with parallax errors of less than 10%; (2) only O-type stars with 0.1 mas
errors imposed and with parallax distance errors of less than 0.2 kpc; and (3)
only O-type stars with 0.05 mas errors imposed and with parallax distance
errors of less than 0.3 kpc. Results. In spite of the significant distance
uncertainties for stars in DR2 beyond 1.4 kpc, the spiral structure in the
solar neighborhood demonstrated by Gaia agrees well with that illustrated by
VLBI maser results. The O-type stars available from DR2 extend the spiral arm
models determined from VLBI maser parallaxes into the fourth Galactic quadrant,
and suggest the existence of a new spur between the Local and Sagittarius arms.Comment: 4 pages, 3 figures, 1 table, accepted for publication in A&
Performance Analysis and Optimization of Sizable 6-Axis Force Sensor Based on Stewart Platform
Appropriateness of antibiotic prescriptions in ambulatory care in China: a nationwide descriptive database study
BACKGROUND: Inappropriate antibiotic use greatly accelerates antimicrobial resistance. The appropriateness of antibiotic prescriptions is well evaluated, using big observational data, in some high-income countries, whereas the evidence of this appropriateness is scarce in China. We aimed to assess the appropriateness of antibiotic prescriptions in ambulatory care settings in China to inform future antimicrobial stewardship. METHODS: We used data from the Beijing Data Center for Rational Use of Drugs, which was a national database designed for monitoring rationality of drug use. 139 hospitals that uploaded diagnosis and prescription information were included from 28 provincial-level regions of mainland China. Outpatient prescriptions were classified as appropriate, potentially appropriate, inappropriate, or not linked to any diagnosis for antibiotic use by following a published classification scheme. Antibiotic prescription rates for various diagnosis categories and proportions of inappropriate antibiotic prescriptions for different subgroups were estimated. Antibiotic prescribing patterns and proportions of individual antibiotics prescribed for different diagnosis categories were analysed and reported. FINDINGS: Between Oct 1, 2014, and April 30, 2018, 18 848 864 (10·9%) of 172 704 117 outpatient visits ended with antibiotic prescriptions. For conditions for which antibiotic use was appropriate, potentially appropriate, and inappropriate, 42·2%, 30·6%, and 7·6% of visits were associated with antibiotic prescriptions, respectively. Of all 18 848 864 antibiotic prescriptions, 9 689 937 (51·4%) were inappropriate, 5 354 224 (28·4%) were potentially appropriate, 2 893 102 (15·3%) were appropriate, and 911 601 (4·8%) could not be linked to any diagnosis. A total of 23 266 494 individual antibiotics were prescribed, of which 18 620 086 (80·0%) were broad-spectrum and the top four most prescribed antibiotics were third-generation cephalosporins (5 056 058 [21·7%]), second-generation cephalosporins (3 823 410 [16·4%]), macrolides (3 554 348 [15·3%]), and fluoroquinolones (3 285 765 [14·1%]). INTERPRETATION: Inappropriate antibiotic prescribing was highly prevalent nationwide in China. Over half of the antibiotic prescriptions were inappropriate in secondary-level and tertiary-level hospitals, suggesting an urgent need for outpatient antibiotic stewardship aimed at optimising antibiotic prescribing to achieve the goals set in China's 2016 national action plan to contain antimicrobial resistance. FUNDING: The National Natural Science Foundation of China
Oscillatory surface dichroism of an insulating topological insulator Bi2Te2Se
Using circular dichroism-angle resolved photoemission spectroscopy
(CD-ARPES), we report a study of the effect of angular momentum transfer
between polarized photons and topological surface states on the surface of
highly bulk insulating topological insulator Bi2Te2Se. The photoelectron
dichroism is found to be strongly modulated by the frequency of the helical
photons including a dramatic sign-flip. Our results suggest that the observed
dichroism and its sign-flip are consequences of strong coupling between the
photon field and the spin-orbit nature of the Dirac modes on the surface. Our
studies reveal the intrinsic dichroic behavior of topological surface states
and point toward the potential utility of bulk insulating topological
insulators in device applications.Comment: 5 pages, 4 figure
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