4,170 research outputs found
A lattice spring model of heterogeneous materials with plasticity
A three-dimensional lattice spring model of a heterogeneous material is presented. For small deformations, the model is shown to recover the governing equations for an isotropic elastic medium. The model gives reasonable agreement with theoretical predictions for the elastic fields generated by a spherical inclusion, although for small particle sizes the discretization of the underlying lattice causes some departures from the predicted values. Plasticity is introduced by decreasing the elastic moduli locally whilst maintaining stress continuity. Results are presented for a spherical inclusion in a plastic matrix and are found to be in good agreement with the predictions of Wilner (1988 J. Mech. Phys. Solids 36 141-65).</p
Understanding the fidelity effect when evaluating games with children
There have been a number of studies that have compared evaluation results from prototypes of different fidelities but very few of these are with children. This paper reports a comparative study of three prototypes ranging from low fidelity to high fidelity within the context of mobile games, using a between subject design with 37 participants aged 7 to 9. The children played a matching game on either an iPad, a paper prototype using screen shots of the actual game or a sketched version. Observational data was captured to establish the usability problems, and two tools from the Fun Toolkit were used to measure user experience. The results showed that there was little difference for user experience between the three prototypes and very few usability problems were unique to a specific prototype. The contribution of this paper is that children using low-fidelity prototypes can effectively evaluate games of this genre and style
The "universal" radio/X-ray flux correlation : the case study of the black hole GX 339-4
The existing radio and X-ray flux correlation for Galactic black holes in the
hard and quiescent states relies on a sample which is mostly dominated by two
sources (GX 339-4 and V404 Cyg) observed in a single outburst. In this paper,
we report on a series of radio and X-ray observations of the recurrent black
hole GX 339-4 with the Australia Telescope Compact Array, the Rossi X-ray
Timing Explorer and the Swift satellites. With our new long term campaign, we
now have a total of 88 quasi-simultaneous radio and X-ray observations of GX
339-4 during its hard state, covering a total of seven outbursts over a
15--year period. Our new measurements represent the largest sample for a
stellar mass black hole, without any bias from distance uncertainties, over the
largest flux variations and down to a level that could be close to quiescence,
making GX 339-4 the reference source for comparison with other accreting
sources (black holes, neutrons stars, white dwarfs and active galactic nuclei).
Our results demonstrate a very strong and stable coupling between radio and
X-ray emission, despite several outbursts of different nature and separated by
a period of quiescence. The radio and X-ray luminosity correlation of the form
L_X ~L_Rad^0.62 +/-0.01 confirms the non-linear coupling between the jet and
the inner accretion flow powers and better defines the standard correlation
track in the radio-X-ray diagram for stellar mass black holes. We further note
epochs of deviations from the fit that significantly exceed the measurement
uncertainties, especially during the formation and destruction of the compact
jets ...[abridged]. We incorporated our new data in a more global study of
black hole candidates strongly supporting a scale invariance in the
jet-accretion coupling of accreting black holes, and confirms the existence of
two populations of sources in the radio/X-ray diagram.Comment: Paper accepted in MNRAS. 18 pages, 9 figure
Anderson impurity in pseudo-gap Fermi systems
We use the numerical renormalization group method to study an Anderson
impurity in a conduction band with the density of states varying as rho(omega)
\propto |omega|^r with r>0. We find two different fixed points: a local-moment
fixed point with the impurity effectively decoupled from the band and a
strong-coupling fixed point with a partially screened impurity spin. The
specific heat and the spin-susceptibility show powerlaw behaviour with
different exponents in strong-coupling and local-moment regime. We also
calculate the impurity spectral function which diverges (vanishes) with
|omega|^{-r} (|\omega|^r) in the strong-coupling (local moment) regime.Comment: 8 pages, LaTeX, 4 figures includes as eps-file
Large-N limit of a magnetic impurity in unconventional density waves
We investigate the effect of unconventional density wave (UDW) condensate on
an Anderson impurity using large-N technique at T=0. In accordance with
previous treatments of a Kondo impurity in pseudogap phases, we find that Kondo
effect occurs only in a certain range of parameters. The f-electron density of
states reflects the influence of UDW at low energies and around the maximum of
the density wave gap. The static spin susceptibility diverges at the critical
coupling, indicating the transition from strong to weak coupling. In the
dynamic spin susceptibility an additional peak appears showing the presence the
UDW gap. Predictions concerning non-linear density of states are made. Our
results apply to other unconventional condensates such as d-wave
superconductors and d-density waves as well.Comment: 9 pages, 7 figure
Ground State Properties of Anderson Impurity in a Gapless Host
Using the Bethe ansatz method, we study the ground state properties of a
Anderson impurity in a ``gapless'' host, where a density of band
states vanishes at the Fermi level as . As
in metals, the impurity spin is proven to be screened at arbitrary parameters
of the system. However, the impurity occupancy as a function of the bare
impurity energy is shown to acquire novel qualitative features which
demonstrate a nonuniversal behavior of the system. The latter explains why the
Kondo screening is absent (or exists only at quite a large electron-impurity
coupling) in earlier studies based on scaling arguments.Comment: 5 pages, no figure, RevTe
Imaging X-ray, Optical, and Infrared Observations of the Transient Anomalous X-ray Pulsar XTE J1810-197
We report X-ray imaging, timing, and spectral studies of XTE J1810-197, a
5.54s pulsar discovered by Ibrahim et al. (2003) in recent RXTE observations.
In a set of short exposures with the Chandra HRC camera we detect a strongly
modulated signal (55+/-4% pulsed fraction) with the expected period located at
(J2000) 18:09:51.08, -19:43:51.7, with a uncertainty radius of 0.6 arcsec (90%
C.L.). Spectra obtained with XMM-Newton are well fitted by a two-component
model that typically describes anomalous X-ray pulsars (AXPs), an absorbed
blackbody plus power law with parameters kT = 0.67+/-0.01 keV, Gamma=3.7+/-0.2,
N_H=(1.05+/-0.05)E22 cm^-2, and Fx(0.5-10 keV) = 3.98E-11 ergs/cm2/s.
Alternatively, a 2T blackbody fit is just as acceptable. The location of CXOU
J180951.1-194351 is consistent with a point source seen in archival Einstein,
Rosat, & ASCA images, when its flux was nearly two orders-of-magnitude fainter,
and from which no pulsations are found. The spectrum changed dramatically
between the "quiescent" and "active" states, the former can be modeled as a
softer blackbody. Using XMM timing data, we place an upper limit of 0.03 lt-s
on any orbital motion in the period range 10m-8hr. Optical and infrared images
obtained on the SMARTS 1.3m telescope at CTIO show no object in the Chandra
error circle to limits V=22.5, I=21.3, J=18.9, & K=17.5. Together, these
results argue that CXOU J180951.1-194351 is an isolated neutron star, one most
similar to the transient AXP AX J1844.8-0256. Continuing study of XTE J1810-197
in various states of luminosity is important for understanding and possibly
unifying a growing class of isolated, young neutron stars that are not powered
by rotation.Comment: 12 pages, 7 figures, AAS LaTex, uses emulateapj5.sty. Updated to
include additional archival data and a new HRC observation. To appear in The
Astrophysical Journa
Guia para utilização da CDU: um guia introdutório para o uso e aplicação da Classificação Decimal Universal
143 p.Apresenta a estrutura da CDU, com aplicações para seu uso. Com capítulo sobre aplicações online por A. Buxton
Multi-Wavelength Coverage of State Transitions in the New Black Hole X-Ray Binary Swift J1910.2-0546
Understanding how black holes accrete and supply feedback to their
environment is one of the outstanding challenges of modern astrophysics. Swift
J1910.2-0546 is a candidate black hole low-mass X-ray binary that was
discovered in 2012 when it entered an accretion outburst. To investigate the
binary configuration and the accretion morphology we monitored the evolution of
the outburst for ~3 months at X-ray, UV, optical (B,V,R,I), and near-infrared
(J,H,K) wavelengths using Swift and SMARTS. The source evolved from a hard to a
soft X-ray spectral state with a relatively cold accretion disk that peaked at
~0.5 keV. A Chandra/HETG spectrum obtained during this soft state did not
reveal signatures of an ionized disk wind. Both the low disk temperature and
the absence of a detectable wind could indicate that the system is viewed at
relatively low inclination. The multi-wavelength light curves revealed two
notable features that appear to be related to X-ray state changes. Firstly, a
prominent flux decrease was observed in all wavebands ~1-2 weeks before the
source entered the soft state. This dip occurred in (0.6-10 keV) X-rays ~6 days
later than at longer wavelengths, which could possibly reflect the viscous time
scale of the disk. Secondly, about two weeks after the source transitioned back
into the hard state, the UV emission significantly increased while the X-rays
steadily decayed. We discuss how these observations may reflect changes in the
accretion flow morphology, perhaps related to the quenching/launch of a jet or
the collapse/recovery of a hot flow.Comment: 8 pages, 5 figures, 1 table. To be published in Ap
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