41,189 research outputs found
Klein-Gordon oscillator in a topologically nontrivial space-time
In this study, we analyze solutions of the wave equation for scalar particles
in a space-time with nontrivial topology. Solutions for the Klein--Gordon
oscillator are found considering two configurations of this space-time. In the
first one, it is assumed the space where the metric is
written in the usual inertial frame of reference. In the second case, we
consider a rotating reference frame adapted to the circle S1. We obtained
compact expressions for the energy spectrum and for the particles wave
functions in both configurations. Additionally, we show that the energy
spectrum of the solution associated to the rotating system has an additional
term that breaks the symmetry around
A Conserved Bach Current
The Bach tensor and a vector which generates conformal symmetries allow a
conserved four-current to be defined. The Bach four-current gives rise to a
quasilocal two-surface expression for power per luminosity distance in the
Vaidya exterior of collapsing fluid interiors. This is interpreted in terms of
entropy generation.Comment: to appear in Class. Quantum Gra
Tailored particle current in an optical lattice by a weak time-symmetric harmonic potential
Quantum ratchets exhibit asymptotic currents when driven by a time-periodic
potential of zero mean if the proper spatio-temporal symmetries are broken.
There has been recent debate on whether directed currents may arise for
potentials which do not break these symmetries. We show here that, in the
presence of degeneracies in the quasienergy spectrum, long-lasting directed
currents can be induced, even if the time reversal symmetry is not broken. Our
model can be realized with ultracold atoms in optical lattices in the
tight-binding regime, and we show that the time scale of the average current
can be controlled by extremely weak fields.Comment: 4 pages, 4 figure
Third and fourth degree collisional moments for inelastic Maxwell models
The third and fourth degree collisional moments for -dimensional inelastic
Maxwell models are exactly evaluated in terms of the velocity moments, with
explicit expressions for the associated eigenvalues and cross coefficients as
functions of the coefficient of normal restitution. The results are applied to
the analysis of the time evolution of the moments (scaled with the thermal
speed) in the free cooling problem. It is observed that the characteristic
relaxation time toward the homogeneous cooling state decreases as the
anisotropy of the corresponding moment increases. In particular, in contrast to
what happens in the one-dimensional case, all the anisotropic moments of degree
equal to or less than four vanish in the homogeneous cooling state for .Comment: 15 pages, 3 figures; v2: addition of two new reference
The XMM-Newton view of PG quasars: II. Properties of the Fe K-alpha line
The properties of the fluorescence Fe K-alpha emission lines of a sample of
38 quasars (QSOs) observed with XMM-Newton are studied. These objects are
included in the optically selected sample from the Palomar-Green (PG) Bright
Quasar Survey with an X-ray luminosity 1.3E43<L(2-10 keV)<5.1E45 ergs/s and
z<1.72. For each object in the sample, we investigated the presence of both
narrow and broad iron lines in detail. A total of 20 out of the 38 QSOs show
evidence of an Fe K-alpha emission line with a narrow profile. The majority of
the lines are consistent with an origin in low ionization material, which is
likely to be located in the outer parts of the accretion disk, the molecular
torus, and/or the Broad Line Region. The average properties of the narrow Fe
K-alpha emission line observed in the sample are similar to those of Seyfert
type galaxies as inferred from recent XMM-Newton and Chandra studies. A broad
line has been significantly detected in only three objects. Furthermore, we
studied the relationship between the equivalent width (EW) of the iron line and
the hard band X-ray luminosity for radio quiet quasars. The analysis indicates
that no clear correlation between the strength of the line and the hard X-ray
luminosity is present, and our results do not show compelling evidence for an
anticorrelation between these two quantities, i.e. the so-called X-ray Baldwin
effect.Comment: 10 pages, 3 figures, accepted by A&
ARES v2 - new features and improved performance
Aims: We present a new upgraded version of ARES. The new version includes a
series of interesting new features such as automatic radial velocity
correction, a fully automatic continuum determination, and an estimation of the
errors for the equivalent widths. Methods: The automatic correction of the
radial velocity is achieved with a simple cross-correlation function, and the
automatic continuum determination, as well as the estimation of the errors,
relies on a new approach to evaluating the spectral noise at the continuum
level. Results: ARES v2 is totally compatible with its predecessor. We show
that the fully automatic continuum determination is consistent with the
previous methods applied for this task. It also presents a significant
improvement on its performance thanks to the implementation of a parallel
computation using the OpenMP library.Comment: 4 pages, 2 Figures; accepted in A&A; ARES Webpage:
www.astro.up.pt/~sousasag/are
A long hard look at the minimum state of PG 2112+059 with XMM-Newton
XMM-Newton successfully detected the minimum state of PG 2112+059 during a
short snapshot observation and performed a long follow-up observation. The high
signal-to-noise spectra are modelled assuming different emission scenarios and
compared with archival spectra taken by XMM-Newton and Chandra.
The PG 2112+059 X-ray spectra acquired in May 2007 allowed the detection of a
weak iron fluorescent line, which is interpreted as being caused by reflection
from neutral material at some distance from the primary X-ray emitting source.
The X-ray spectra of PG 2112+059 taken at five different epochs during
different flux states can be interpreted within two different scenarios. The
first consists of two layers of ionised material with column densities of N_H
~5 x 10^22 cm^-2 and N_H ~3.5 x 10^23 cm^-2, respectively. The first layer is
moderately ionised and its ionisation levels follow the flux changes, while the
other layer is highly ionised and does not show any correlation with the flux
of the source. The spectra can also be interpreted assuming reflection by an
ionised accretion disk seen behind a warm absorber. The warm absorber
ionisation is consistent with being correlated with the flux of the source,
which provides an additional degree of self-consistency with the overall
reflection-based model. We explain the spectral variability with light bending
according to the models of Miniutti and Fabian and constrain the black hole
spin to be a/M > 0.86. Both scenarios also assume that a distant cold reflector
is responsible for the Fe K \alpha emission line.
Light bending provides an attractive explanation of the different states of
PG 2112+059 and may also describe the physical cause of the observed properties
of other X-ray weak quasars.Comment: 15 pages, 12 figures, A&A latex, accepted for publication in
Astronomy & Astrophysic
Bulk-Edge correspondence of entanglement spectrum in 2D spin ground states
General local spin ground states, described by a Valence Bond Solid (VBS)
on a two dimensional lattice are studied. The norm of these ground states is
mapped to a classical O(3) model on the same lattice. Using this
quantum-to-classical mapping we obtain the partial density matrix
associated with a subsystem of the original ground state. We show that
the entanglement spectrum of in a translation invariant lattice
is given by the spectrum of a quantum spin chain at the boundary of region ,
with local Heisenberg type interactions between spin 1/2 particles.Comment: 8 pages, 4 figures, one section and references adde
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