2,848 research outputs found
The MHD Alfven wave oscillation model of kHz Quasi Periodic Oscillations of Accreting X-ray Binaries
We ascribe the interpretation of the twin kilohertz Quasi Periodic
Oscillations (kHz QPOs) of X-ray spectra of Low Mass X-Ray Binaries (LMXBs) to
MHD Alfven wave oscillations in the different mass density regions of the
accreted matter at the preferred radius, and the upper kHz QPO frequency
coincides with the Keplerian frequency. The proposed model concludes that the
kHz QPO frequencies depend inversely on the preferred radius, and that
theoretical relation between the upper frequency (\nt) and the lower
frequency (\no) is \no \sim \nt^{2}, which is similar to the measured
empirical relation. The separation between the twin frequencies decreases
(increases) with increasing kHz QPO frequency if the lower kHz QPO frequency is
more (less) than 400 Hz.Comment: Accepted by Astron. & Astrophys., 4 pages, 4 figure
A model for upper kHz QPO coherence of accreting neutron star
{We investigate the coherence of the twin kilohertz quasi-periodic
oscillations (kHz QPOs) in the low-mass X-ray binary (LMXB) theoretically. The
profile of upper kHz QPO, interpreted as Keplerian frequency, is ascribed to
the radial extent of the kHz QPO emission region, associated with the
transitional layer at the magnetosphere-disk boundary, which corresponds to the
coherence of upper kHz QPO. The theoretical model for Q-factor of upper kHz QPO
is applied to the observational data of five Atoll and five Z sources, and the
consistence is implied.Comment: accepted by A&
X-ray Timing beyond the Rossi X-ray Timing Explorer
With its ability to look at bright galactic X-ray sources with
sub-millisecond time resolution, the Rossi X-ray Timing Explorer (RXTE)
discovered that the X-ray emission from accreting compact stars shows
quasi-periodic oscillations on the dynamical timescales of the strong field
region. RXTE showed also that waveform fitting of the oscillations resulting
from hot spots at the surface of rapidly rotating neutron stars constrain their
masses and radii. These two breakthroughs suddenly opened up a new window on
fundamental physics, by providing new insights on strong gravity and dense
matter. Building upon the RXTE legacy, in the Cosmic Vision exercise, testing
General Relativity in the strong field limit and constraining the equation of
state of dense matter were recognized recently as key goals to be pursued in
the ESA science program for the years 2015-2025. This in turn identified the
need for a large (10 m2 class) aperture X-ray observatory. In recognition of
this need, the XEUS mission concept which has evolved into a single launch L2
formation flying mission will have a fast timing instrument in the focal plane.
In this paper I will outline the unique science that will be addressed with
fast X-ray timing on XEUS.Comment: 12 pages, 6 figures, COSPAR Colloquium "Spectra & Timing of Compact
X-ray Binaries", January 17-20, 2005, Mumbai, India. Advances in Space
Research, 2006, in pres
On mass-constraints implied by the relativistic precession model of twin-peak quasi-periodic oscillations in Circinus X-1
Boutloukos et al. (2006) discovered twin-peak quasi-periodic oscillations
(QPOs) in 11 observations of the peculiar Z-source Circinus X-1. Among several
other conjunctions the authors briefly discussed the related estimate of the
compact object mass following from the geodesic relativistic precession model
for kHz QPOs. Neglecting the neutron star rotation they reported the inferred
mass M_0 = 2.2 +/- 0.3 M_\sun. We present a more detailed analysis of the
estimate which involves the frame-dragging effects associated with rotating
spacetimes. For a free mass we find acceptable fits of the model to data for
(any) small dimensionless compact object angular momentum j=cJ/GM^2. Moreover,
quality of the fit tends to increase very gently with rising j. Good fits are
reached when M ~ M_0[1+0.55(j+j^2)]. It is therefore impossible to estimate the
mass without the independent knowledge of the angular momentum and vice versa.
Considering j up to 0.3 the range of the feasible values of mass extends up to
3M_\sun. We suggest that similar increase of estimated mass due to rotational
effects can be relevant for several other sources.Comment: 10 pages, 9 figures (in colour
A Multi-Agent Reinforcement Learning Approach to Promote Cooperation in Evolutionary Games on Networks with Environmental Feedback
A prominent feature of biological organization in many species of social animals is the ability to achieve cooperation. However, despite its predominance in natural evolution, cooperative behaviors come at a cost, typically in the form of do ut des mechanisms (e.g., reciprocal altruism in vampire bats) with given thresholds for sharing resources or communication efforts. In this paper, we investigate the conditions of cooperation through the evolutionary dynamics of the prisoner's dilemma (PD) game as well as the learning dynamics resulting from the corresponding multi-agent reinforcement learning (MARL) model. In both cases, the interactions in the population are captured by a regular network and the impact of the players' actions is reflected through the evolution of an environmental resource, which also acts as a feedback on the dynamics. The following is a list of contributions: i) we provide a full characterization of the stability properties of the networked feedback-evolving PD game; ii) we determine a set of threshold values below which cooperation is promoted; iii) we develop the corresponding cross-learning model, which is a stateless MARL model, and we show that this model is equivalent to the networked PD game with environmental feedback.</p
Probing the temporal variability of Cygnus X-1 into the soft state
Building on results from previous studies of Cygnus~X-1, we analyze Rossi
X-ray Timing Explorer (RXTE) data taken when the source was in the soft and
transitional spectral states. We look at the power spectrum in the 0.01 -- 50
Hz range, using a model consisting of a cut-off power-law and two Lorentzian
components. We are able to constrain the relation between the characteristic
frequencies of the Lorentzian components, and show that it is consistent with a
power-law relation having the same index (1.2) as previously reported for the
hard state, but shifted by a factor ~2. Furthermore, it is shown that the
change in the frequency relation seen during the transitions can be explained
by invoking a shift of one Lorentzian component to a higher harmonic, and we
explore the possible support for this interpretation in the other component
parameters. With the improved soft state results we study the evolution of the
fractional variance for each temporal component. This approach indicates that
the two Lorentzian components are connected to each other, and unrelated to the
power-law component in the power spectrum, pointing to at least two separate
emission components.Comment: 11 pages, 10 figures, 1 electronic table. Accepted for publication in
A&
Future X-ray timing missions
Thanks to the Rossi X-ray Timing Explorer (RXTE), it is now widely recognized
that fast X-ray timing can be used to probe strong gravity fields around
collapsed objects and constrain the equation of state of dense matter in
neutron stars. We first discuss some of the outstanding issues which could be
solved with an X-ray timing mission building on the great successes of RXTE and
providing an order of magnitude better sensitivity. Then we briefly describe
the 'Experiment for X-ray timing and Relativistic Astrophysics' (EXTRA)
recently proposed to the European Space Agency as a follow-up to RXTE and the
related US mission 'Relativistic Astrophysics Explorer' (RAE).Comment: To be published in `Proceedings of the Third Microquasar Workshop:
Granada Workshop on galactic relativistic jet sources', Eds A. J.
Castro-Tirado, J. Greiner and J. M. Paredes, Astrophysics and Space Science,
in press. More about EXTRA can be found at:
http://www.cesr.fr/~barret/extra.htm
Coordinate-based neural representations for computational adaptive optics in widefield microscopy
Widefield microscopy is widely used for non-invasive imaging of biological
structures at subcellular resolution. When applied to complex specimen, its
image quality is degraded by sample-induced optical aberration. Adaptive optics
can correct wavefront distortion and restore diffraction-limited resolution but
require wavefront sensing and corrective devices, increasing system complexity
and cost. Here, we describe a self-supervised machine learning algorithm,
CoCoA, that performs joint wavefront estimation and three-dimensional
structural information extraction from a single input 3D image stack without
the need for external training dataset. We implemented CoCoA for widefield
imaging of mouse brain tissues and validated its performance with
direct-wavefront-sensing-based adaptive optics. Importantly, we systematically
explored and quantitatively characterized the limiting factors of CoCoA's
performance. Using CoCoA, we demonstrated the first in vivo widefield mouse
brain imaging using machine-learning-based adaptive optics. Incorporating
coordinate-based neural representations and a forward physics model, the
self-supervised scheme of CoCoA should be applicable to microscopy modalities
in general.Comment: 33 pages, 5 figure
A Cholesky decomposition-based implementation of relativistic two-component coupled-cluster methods for medium-sized molecules
A Cholesky decomposition (CD)-based implementation of relativistic
two-component coupled-cluster (CC) and equation-of-motion CC (EOM-CC) methods
using an exact two-component Hamiltonian augmented with atomic-mean-field
spin-orbit integrals (the X2CAMF scheme) is reported. The present CD-based
implementation of X2CAMF-CC and EOM-CC methods employs atomic-orbital-based
algorithms to avoid the construction of two-electron integrals and
intermediates involving three and four virtual indices. Our CD-based
implementation extends the applicability of X2CAMF-CC and EOM-CC methods to
medium-sized molecules with the possibility to correlate around 1000 spinors.
Benchmark calculations for uranium-containing small molecules have been
performed to assess the dependence of the CC results on the Cholesky threshold.
A Cholesky threshold of is shown to be sufficient to maintain
chemical accuracy. Example calculations to illustrate the capability of the
CD-based relativistic CC methods are reported for the bond-dissociation energy
of the uranium hexafluoride molecule, UF, with up to quadruple-zeta basis
sets, and the lowest excitation energy in solvated uranyl ion
[UO(HO)]
kHz Quasi Periodic Oscillations in Low Mass X-ray Binaries as Probes of General Relativity in the Strong Field Regime
We consider the interpretation of a pair of kHz Quasi Periodic Oscillations
(QPOs) in the Fourier spectra of two Low Mass X-Ray Binaries, Sco X-1 and
4U1608-52, hosting an old accreting neutron star. The observed frequency
difference of these QPOs decreaseas as their frequency increases, contrary to
simple beat frequency models, which predict a constant frequency difference. We
show that the behaviour of these QPOs is instead well matched in terms of the
fundamental frequencies (in the radial and azimuthal directions) for test
particle motion in the gravitational field of the neutron star, for reasonable
star masses, and nearly independent of the star spin. The radial frequency must
be much smaller than the azimuthal one, testifying that kHz QPOs are produced
close to the innermost stable orbit. These results are not reproduced through
the post--Newtonian (PN) approximation of General Relativity (GR). kHz QPOs
from X-ray binaries likely provide an accurate laboratory for strong field GR.Comment: to appear in Physical Review Letters, PRL Latex plus 2 figures in
standard PostScript forma
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