695 research outputs found
Thermodynamics and short-range correlations of the XXZ chain close to its triple point
The XXZ quantum spin chain has a triple point in its ground state
- phase diagram. This first order critical point is located at the
joint end point of the two second order phase transition lines marking the
transition from the gapless phase to the fully polarized phase and to the
N\'eel ordered phase, respectively. We explore the magnetization and the
short-range correlation functions in its vicinity using the exact solution of
the model. In the critical regime above the triple point we observe a strong
variation of all physical quantities on a low energy scale of order
induced by the transversal quantum fluctuations. We interpret this phenomenon
starting from a strong-coupling perturbation theory about the highly degenerate
ground state of the Ising chain at the triple point. From the perturbation
theory we identify the relevant scaling of the magnetic field and of the
temperature. Applying the scaling to the exact solutions we obtain explicit
formulae for the magnetization and short-range correlation functions at low
temperatures.Comment: 18 pages, 7 figures, v2: figures rearranged, v3: a typo correcte
Can biochar link forest restoration with commercial agriculture?
For the potential benefits of biochar to be fully realized, it must be produced economically in high volumes and close to stable markets. The author is part of a research team conducting an examination of the economic feasibility of linking forest restoration with commercial agriculture in a specific region of Oregon, USA through large-scale production of biochar made from forest biomass at a dedicated plant site. The focus of this presentation is the biochar production economics and carbon impacts associated with two different biochar conversion technologies, four different biochar plant configuration scenarios, and 2 potential plant locations.
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Automated Inspection Device for Explosive Charge in Shells - AIDECS
Certain defects in the explosive charge of an artillery shell can cause the projectile to explode prematurely in the barrel of the launcher from which it is fired. The sensitivity of the radiographic technique presently used is limited by the large influence of the steel shell casing on the transmitted radiation. A filmless radiometric technique utilizing the basic radiation principle of Compton scattering, which will detect cavities in the explosive filler with minimal interference from the steel casing, has been identified and tested. By scanning the shell with a beam of radiation and observing the Compton scattering through a unique collimating system, it has been possible to detect voids as small as 1/16 inch in cross section. The hardware consists of the source, beam collimator, detector collimator, and a large plastic scintillator detector system. The projectile is inserted into the beam path and moved through a fixed scanning pattern by a mechanical handling system. The scanning sequence is computer contra ll ed and results in a threedimensional data matrix giving a direct representation of density within the projectile. Voids are identified and classified by computer analysis, and shell acceptability decisions are automatically generated. An engineering prototype system is currently being assembled and tested. (A production prototype conceptual design is concurrently under development.) This new technique will replace an existing film radiography inspection procedure and eliminate the need for human interpretation of the defects, while providing more consistent and reliable inspections at lower costs
XMM Follow-Up Observations of Three Swift BAT-Selected Active Galactic Nuclei
We present XMM-Newton observations of three AGN taken as part of a hunt to
find very heavily obscured Compton-thick AGN. For obscuring columns greater
than 10^25 cm^-2, AGN are only visible at energies below 10 keV via
reflected/scattered radiation, characterized by a flat power-law. We therefore
selected three objects (ESO 417-G006, IRAS 05218-1212, and MCG -01-05-047) from
the Swift BAT hard X-ray survey catalog with Swift X-ray Telescope XRT 0.5-10
keV spectra with flat power-law indices as candidate Compton-thick sources for
follow-up observations with the more sensitive instruments on XMM-Newton. The
XMM spectra, however, rule out reflection-dominated models based on the
weakness of the observed Fe K-alpha lines. Instead, the spectra are well-fit by
a model of a power-law continuum obscured by a Compton-thin absorber, plus a
soft excess. This result is consistent with previous follow-up observations of
two other flat-spectrum BAT-detected AGN. Thus, out of the six AGN in the
22-month BAT catalog with apparently flat Swift XRT spectra, all five that have
had follow-up observations are not likely Compton-thick. We also present new
optical spectra of two of these objects, IRAS 05218-1212 and MCG -01-05-047.
Interestingly, though both these AGN have similar X-ray spectra, their optical
spectra are completely different, adding evidence against the simplest form of
the geometric unified model of AGN. IRAS 05218-1212 appears in the optical as a
Seyfert 1, despite the ~8.5x10^22 cm^-2 line-of-sight absorbing column
indicated by its X-ray spectrum. MCG -01-05-047's optical spectrum shows no
sign of AGN activity; it appears as a normal galaxy.Comment: 18 pages including 4 figures, accepted by Ap
Dynamic properties of the spin-1/2 XY chain with three-site interactions
We consider a spin-1/2 XY chain in a transverse (z) field with multi-site
interactions. The additional terms introduced into the Hamiltonian involve
products of spin components related to three adjacent sites. A Jordan-Wigner
transformation leads to a simple bilinear Fermi form for the resulting
Hamiltonian and hence the spin model admits a rigorous analysis. We point out
the close relationships between several variants of the model which were
discussed separately in previous studies. The ground-state phases (ferromagnet
and two kinds of spin liquid) of the model are reflected in the dynamic
structure factors of the spin chains, which are the main focus in this study.
First we consider the zz dynamic structure factor reporting for this quantity a
closed-form expression and analyzing the properties of the two-fermion
(particle-hole) excitation continuum which governs the dynamics of transverse
spin component fluctuations and of some other local operator fluctuations. Then
we examine the xx dynamic structure factor which is governed by many-fermion
excitations, reporting both analytical and numerical results. We discuss some
easily recognized features of the dynamic structure factors which are
signatures for the presence of the three-site interactions.Comment: 28 pages, 10 fugure
Spatially resolved origin of mm-wave linear polarization in the nuclear region of 3C 84
We report results from a deep polarization imaging of the nearby radio galaxy 3C 84 (NGC 1275). The source was observed with the Global Millimeter VLBI Array (GMVA) at 86 GHz at an ultra-high angular resolution of 50μas (corresponding to 250R). We also add complementary multi-wavelength data from the Very Long Baseline Array (VLBA; 15 & 43 GHz) and from the Atacama Large Millimeter/submillimeter Array (ALMA; 97.5, 233.0, and 343.5 GHz). At 86 GHz, we measure a fractional linear polarization of ~ 2% in the VLBI core region. The polarization morphology suggests that the emission is associated with an underlying limb-brightened jet. The fractional linear polarization is lower at 43 and 15 GHz (~ 0.3-0.7% and < 0.1%, respectively). This suggests an increasing linear polarization degree towards shorter wavelengths on VLBI scales. We also obtain a large rotation measure (RM) of ~ 10⁵⁻⁶ rad/m² in the core at ≳43 GHz. Moreover, the VLBA 43 GHz observations show a variable RM in the VLBI core region during a small flare in 2015. Faraday depolarization and Faraday conversion in an inhomogeneous and mildly relativistic plasma could explain the observed linear polarization characteristics and the previously measured frequency dependence of the circular polarization. Our Faraday depolarization modeling suggests that the RM most likely originates from an external screen with a highly uniform RM distribution. To explain the large RM value, the uniform RM distribution, and the RM variability, we suggest that the Faraday rotation is caused by a boundary layer in a transversely stratified jet. Based on the RM and the synchrotron spectrum of the core, we provide an estimate for the magnetic field strength and the electron density of the jet plasma.Accepted manuscrip
On the possible generation of the young massive open clusters Stephenson2 and BDSB122 by Omega Centauri
A massive objects such as a globular cluster passing through the disk of a
galaxy can trigger star formation. We test the hypothesis that the most massive
globular cluster in the Galaxy, Centauri, which crossed the disk
approximately Myr ago, may have triggered the formation of the open
clusters Stephenson 2 and BDSB 122. The orbits of Centauri, Stephenson
2 and BDSB 122 are computed for the three-component model of Johnston,
Hernquist & Bolte, which considers the disk, spheroidal and halo gravitational
potentials. With the re-constructed orbit of Centauri, we show that
the latest impact site is consistent, within important uncertainties, with the
birth-site of the young massive open clusters BDSB 122 and Stephenson 2. Within
uncertainties, this scenario is consistent with the time-scale of their
backwards motion in the disk, shock wave propagation and delay for star
formation. Together with open cluster formation associated to density waves in
spiral arms, the present results are consistent with the idea that massive
globular clusters as additional progenitors of open clusters, the massive ones
in particular.Comment: 6 pages, 4 figures; accepted by A&
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