9,518 research outputs found
Low energy magnetic excitation spectrum of the unconventional ferromagnet CeRhB
The magnetic excitation spectrum of the unconventional ferromagnet
CeRhB was measured by inelastic neutron scattering on single
crystal sample in the magnetically ordered and paramagnetic phases. The
spin-wave excitation spectrum evidences high exchange interaction along the
c-axis about two orders of magnitude higher than the ones in the basal plane of
the hexagonal structure. Both strong out of plane and small in plane
anisotropies are found. This latter point confirms that considering the =5/2
multiplet alone is not adequate for describing the ground state of
CeRhB. Quasielastic scattering measured above is also
strongly anisotropic between the basal plane and the c-axis and suggests
localized magnetism.Comment: 8 Figure
Astrophysical science metrics for next-generation gravitational-wave detectors
The second generation of gravitational-wave detectors are being built and
tuned all over the world. The detection of signals from binary black holes is
beginning to fulfill the promise of gravitational-wave astronomy. In this work,
we examine several possible configurations for third-generation laser
interferometers in existing km-scale facilities. We propose a set of
astrophysically motivated metrics to evaluate detector performance. We measure
the impact of detector design choices against these metrics, providing a
quantitative cost-benefit analyses of the resulting scientific payoffs
Characterizing Multi-planet Systems with Classical Secular Theory
Classical secular theory can be a powerful tool to describe the qualitative
character of multi-planet systems and offer insight into their histories. The
eigenmodes of the secular behavior, rather than current orbital elements, can
help identify tidal effects, early planet-planet scattering, and dynamical
coupling among the planets, for systems in which mean-motion resonances do not
play a role. Although tidal damping can result in aligned major axes after all
but one eigenmode have damped away, such alignment may simply be fortuitous. An
example of this is 55 Cancri (orbital solution of Fischer et al., 2008) where
multiple eigenmodes remain undamped. Various solutions for 55 Cancri are
compared, showing differing dynamical groupings, with implications for the
coupling of eccentricities and for the partitioning of damping among the
planets. Solutions for orbits that include expectations of past tidal evolution
with observational data, must take into account which eigenmodes should be
damped, rather than expecting particular eccentricities to be near zero.
Classical secular theory is only accurate for low eccentricity values, but
comparison with other results suggests that it can yield useful qualitative
descriptions of behavior even for moderately large eccentricity values, and may
have advantages for revealing underlying physical processes and, as large
numbers of new systems are discovered, for triage to identify where more
comprehensive dynamical studies should have priority.Comment: Published in Celestial Mechanics and Dynamical Astronomy, 25 pages,
10 figure
A two dimensional finite difference time domain analysis of the quiet zone fields of an anechoic chamber
Prediction of anechoic chamber performance is a difficult problem. Electromagnetic anechoic chambers exist for a wide range of frequencies but are typically very large when measured in wavelengths. Three dimensional finite difference time domain (FDTD) modeling of anechoic chambers is possible with current computers but at frequencies lower than most chamber design frequencies. However, two dimensional FDTD (2D-FTD) modeling enables much greater detail at higher frequencies and offers significant insight into compact anechoic chamber design and performance. A major subsystem of an anechoic chamber for which computational electromagnetic analyses exist is the reflector. First, an analysis of the quiet zone fields of a low frequency anechoic chamber produced by a uniform source and a reflector in two dimensions using the FDTD method is presented. The 2D-FDTD results are compared with results from a three dimensional corrected physical optics calculation and show good agreement. Next, a directional source is substituted for the uniform radiator. Finally, a two dimensional anechoic chamber geometry, including absorbing materials, is considered, and the 2D-FDTD results for these geometries appear reasonable
Effects of substituting rare-earth ion R by non-magnetic impurities in - theory and numerical DMRG results
In this paper we study the effect of substituting R (rare-earth ion) by
non-magnetic ions in the spin-1 chain material . Using a
strong-coupling expansion and numerical density matrix renormalization group
calculations, we show that spin-wave bound states are formed at the impurity
site. Experimental consequences of the bound states are pointed out.Comment: 5 pages, 4 postscript figure
Observability and diagnostics in the X-ray band of shock-cloud interactions in supernova remnants
X-ray emitting features originating from the interaction of supernova shock
waves with small interstellar gas clouds are revealed in many X-ray
observations of evolved supernova remnants (e.g. Cygnus Loop and Vela), but
their interpretation is not straightforward.
We develop a self-consistent method for the analysis and interpretation of
shock-cloud interactions in middle-aged supernova remnants, which can provide
the key parameters of the system and the role of relevant physical effects like
the thermal conduction, without the need to run ad-hoc numerical simulations
and to bother of morphology details.
We explore all the possible values of the shock speed and cloud density
contrast relevant to middle-aged SNRs with a set of hydrodynamic simulations of
shock-cloud interaction, including the effects of thermal conduction and
radiative cooling. From the simulations, we synthesize spatially and spectrally
resolved focal-plane data as they would be collected with XMM-Newton/EPIC, an
X-ray instrument commonly used in these studies.
We devise and tune up two diagnostic tools, the first based on the
mean-photon energy vs. count rate scatter plot and the second on the spectral
analysis of the interaction region, that can be used to highlight the effects
of thermal conduction and to derive the shock speed in case of efficient
conduction at work. These tools can be used to ascertain information from X-ray
observations, without the need to develop detailed and ad-hoc numerical models
for the interpretation of the data.Comment: 9 pages, 7 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_13801.pd
Live to cheat another day: bacterial dormancy facilitates the social exploitation of beta-lactamases
The breakdown of antibiotics by β-lactamases may be cooperative, since resistant cells can detoxify their environment and facilitate the growth of susceptible neighbours. However, previous studies of this phenomenon have used artificial bacterial vectors or engineered bacteria to increase the secretion of β-lactamases from cells. Here, we investigated whether a broad-spectrum β-lactamase gene carried by a naturally occurring plasmid (pCT) is cooperative under a range of conditions. In ordinary batch culture on solid media, there was little or no evidence that resistant bacteria could protect susceptible cells from ampicillin, although resistant colonies could locally detoxify this growth medium. However, when susceptible cells were inoculated at high densities, late-appearing phenotypically susceptible bacteria grew in the vicinity of resistant colonies. We infer that persisters, cells that have survived antibiotics by undergoing a period of dormancy, founded these satellite colonies. The number of persister colonies was positively correlated with the density of resistant colonies and increased as antibiotic concentrations decreased. We argue that detoxification can be cooperative under a limited range of conditions: if the toxins are bacteriostatic rather than bacteridical; or if susceptible cells invade communities after resistant bacteria; or if dormancy allows susceptible cells to avoid bactericides. Resistance and tolerance were previously thought to be independent solutions for surviving antibiotics. Here, we show that these are interacting strategies: the presence of bacteria adopting one solution can have substantial effects on the fitness of their neighbours
Photoionized Lines in the X-ray Spectra of SMC X-1
We present a detailed spectral analysis of Chandra/ACIS-S CC mode
observations of the massive X-ray binary system SMC X-1. The system was
observed during both the high and low X-ray states of the roughly 60-day
superorbital period. The continuum spectra during both states are well
represented by a power law with photon index =0.9 and a blackbody of kT
= 0.15keV. The high state spectra are dominated by the continuum and
independent of orbital phase whereas the low state spectra show a strong
orbital dependence as well as line emission from O, Ne, Mg, Fe, and Si. This is
consistent with the states attributed to disk precession: during the high state
X-ray emission is dominated by the compact source which is abrubtly eclipsed
and during the low state the compact object is hidden by the disk and a larger,
less luminous scattering region is responsible for the X-ray emission. A
prominent Ne IX feature places a stringent limit (Log = 2.0-2.5) on the
ionization parameter which constrains the wind dynamics of the system. The Fe
line fluxes are related linearly to the blackbody fluxes indicating that both
originate in the same region or are excited by the same mechanism. There is
evidence for structure in the Fe-line that cannot be fully resolved by the
current observations. The pulse period measured during our observations,
0.70571470.00000027s shows that the uninterrupted spin-up trend of SMC X-1
continues. We discuss the implications of our results for models of SMC X-1.Comment: 31 pages including 10 figures. Accepted for publication in Ap
Continuous, Semi-discrete, and Fully Discretized Navier-Stokes Equations
The Navier--Stokes equations are commonly used to model and to simulate flow
phenomena. We introduce the basic equations and discuss the standard methods
for the spatial and temporal discretization. We analyse the semi-discrete
equations -- a semi-explicit nonlinear DAE -- in terms of the strangeness index
and quantify the numerical difficulties in the fully discrete schemes, that are
induced by the strangeness of the system. By analyzing the Kronecker index of
the difference-algebraic equations, that represent commonly and successfully
used time stepping schemes for the Navier--Stokes equations, we show that those
time-integration schemes factually remove the strangeness. The theoretical
considerations are backed and illustrated by numerical examples.Comment: 28 pages, 2 figure, code available under DOI: 10.5281/zenodo.998909,
https://doi.org/10.5281/zenodo.99890
Astrophysical science metrics for next-generation gravitational-wave detectors
The second generation of gravitational-wave (GW) detectors are being built and tuned all over the world. The detection of signals from binary black holes is beginning to fulfil the promise of GW astronomy. In this work, we examine several possible configurations for third-generation laser interferometers in existing km-scale facilities. We propose a set of astrophysically motivated metrics to evaluate detector performance. We measure the impact of detector design choices against these metrics, providing a quantitative cost-benefit analyses of the resulting scientific payoffs
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