12,118 research outputs found
Different kinds of long-term variability from Cygnus X-1
We present a study of the long-term variability of Cyg X-1 using data from
the RXTE/ASM and the RXTE/PCA during the time between the two soft states of
1996 and 2001/2002. This period has been characterized by many short ASM
flaring episodes which we have identified as "failed state transitions". The
150 d period which has been seen before and shortly after the 1996 soft state
is not obviously present in the ASM rate during most of this time. Applying
selection criteria from our pointed RXTE/PCA observations to exclude the
flaring episodes we show that the 150 d period can indeed still be
significantly detected in the hard state. Furthermore, while the ~420 d
timescale associated with the flaring is reduced in the selected hard state
count rate, it is still pronounced in the temporal evolution of the
corresponding hardness ratios. The Ryle radio flux is also consistent with the
150 d period being present but distorted during this time.Comment: 4 pages, 6 figures, to appear in Proceedings of "X-ray Timing 2003:
Rossi and Beyond", ed. P. Kaaret, F.K. Lamb, & J.H. Swan
Universality class of the depinning transition in the two-dimensional Ising model with quenched disorder
With Monte Carlo methods, we investigate the universality class of the
depinning transition in the two-dimensional Ising model with quenched random
fields. Based on the short-time dynamic approach, we accurately determine the
depinning transition field and both static and dynamic critical exponents. The
critical exponents vary significantly with the form and strength of the random
fields, but exhibit independence on the updating schemes of the Monte Carlo
algorithm. From the roughness exponents and , one
may judge that the depinning transition of the random-field Ising model belongs
to the new dynamic universality class with
and . The crossover from the second-order phase transition
to the first-order one is observed for the uniform distribution of the random
fields, but it is not present for the Gaussian distribution.Comment: 16 pages, 16 figures, 3 table
Statistical mechanics of spatial evolutionary games
We discuss the long-run behavior of stochastic dynamics of many interacting
players in spatial evolutionary games. In particular, we investigate the effect
of the number of players and the noise level on the stochastic stability of
Nash equilibria. We discuss similarities and differences between systems of
interacting players maximizing their individual payoffs and particles
minimizing their interaction energy. We use concepts and techniques of
statistical mechanics to study game-theoretic models. In order to obtain
results in the case of the so-called potential games, we analyze the
thermodynamic limit of the appropriate models of interacting particles.Comment: 19 pages, to appear in J. Phys.
Disordered Environments in Spatial Games
The Prisoner's dilemma is the main game theoretical framework in which the
onset and maintainance of cooperation in biological populations is studied. In
the spatial version of the model, we study the robustness of cooperation in
heterogeneous ecosystems in spatial evolutionary games by considering site
diluted lattices. The main result is that due to disorder, the fraction of
cooperators in the population is enhanced. Moreover, the system presents a
dynamical transition at , separating a region with spatial chaos from
one with localized, stable groups of cooperators.Comment: 6 pages, 5 figure
Field theory on --Minkowski space revisited: Noether charges and breaking of Lorentz symmetry
This paper is devoted to detailed investigations of free scalar field theory
on -Minkowski space. After reviewing necessary mathematical tools we
discuss in depth the Lagrangian and solutions of field equations. We analyze
the spacetime symmetries of the model and construct the conserved charges
associated with translational and Lorentz symmetry. We show that the version of
the theory usually studied breaks Lorentz invariance in a subtle way: There is
an additional trans-Planckian mode present, and an associated conserved charge
(the number of such modes) is not a Lorentz scalar.Comment: 22 pages, 1 figure, formulas in sect. III correcte
Beyond XSPEC: Towards Highly Configurable Analysis
We present a quantitative comparison between software features of the defacto
standard X-ray spectral analysis tool, XSPEC, and ISIS, the Interactive
Spectral Interpretation System. Our emphasis is on customized analysis, with
ISIS offered as a strong example of configurable software. While noting that
XSPEC has been of immense value to astronomers, and that its scientific core is
moderately extensible--most commonly via the inclusion of user contributed
"local models"--we identify a series of limitations with its use beyond
conventional spectral modeling. We argue that from the viewpoint of the
astronomical user, the XSPEC internal structure presents a Black Box Problem,
with many of its important features hidden from the top-level interface, thus
discouraging user customization. Drawing from examples in custom modeling,
numerical analysis, parallel computation, visualization, data management, and
automated code generation, we show how a numerically scriptable, modular, and
extensible analysis platform such as ISIS facilitates many forms of advanced
astrophysical inquiry.Comment: Accepted by PASP, for July 2008 (15 pages
Spectro-timing analysis of Cygnus X-1 during a fast state transition
We present the analysis of two long, quasi-uninterrupted RXTE observations of
Cygnus X-1 that span several days within a 10 d interval. The spectral
characteristics during this observation cover the region where previous
observations have shown the source to be most dynamic. Despite that the source
behavior on time scales of hours and days is remarkably similar to that on year
time scales. This includes a variety of spectral/temporal correlations that
previously had only been observed over Cyg X-1's long-term evolution.
Furthermore, we observe a full transition from a hard to a soft spectral state
that occurs within less than 2.5 hours - shorter than previously reported for
any other similar Cyg X-1 transition. We describe the spectra with a
phenomenological model dominated by a broken power law, and we fit the X-ray
variability power spectra with a combination of a cutoff power law and
Lorentzian components. The spectral and timing properties are correlated: the
power spectrum Lorentzian components have an energy-dependent amplitude, and
their peak frequencies increase with photon spectral index. Averaged over
3.2-10 Hz, the time lag between the variability in the 4.5-5.7 keV and 9.5-15
keV bands increases with decreasing hardness when the variability is dominated
by the Lorentzian components during the hard state. The lag is small when there
is a large power law noise contribution, shortly after the transition to the
soft state. Interestingly, the soft state not only shows the shortest lags, but
also the longest lags when the spectrum is at its softest and faintest. We
discuss our results in terms of emission models for black hole binaries.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
The Rest-Frame Instant Form of Relativistic Perfect Fluids and of Non-Dissipative Elastic Materials
For perfect fluids with equation of state , Brown gave an
action principle depending only on their Lagrange coordinates
without Clebsch potentials. After a reformulation on arbitrary spacelike
hypersurfaces in Minkowski spacetime, the Wigner-covariant rest-frame instant
form of these perfect fluids is given. Their Hamiltonian invariant mass can be
given in closed form for the dust and the photon gas. The action for the
coupling to tetrad gravity is given. Dixon's multipoles for the perfect fluids
are studied on the rest-frame Wigner hyperplane. It is also shown that the same
formalism can be applied to non-dissipative relativistic elastic materials
described in terms of Lagrangian coordinates.Comment: revtex file, 70 page
On the determination of the spin of the black hole in Cyg X-1 from X-ray reflection spectra
The spin of Cygnus X-1 is measured by fitting reflection models to Suzaku
data covering the energy band 0.9-400 keV. The inner radius of the accretion
disc is found to lie within 2 gravitational radii (r_g=GM/c^2) and a value for
the dimensionless black hole spin is obtained of 0.97^{+0.014}_{-0.02}. This
agrees with recent measurements using the continuum fitting method by Gou et
al. and of the broad iron line by Duro et al. The disc inclination is measured
at 23.7^{+6.7}_{-5.4} deg, which is consistent with the recent optical
measurement of the binary system inclination by Orosz et al of 27+/-0.8 deg. We
pay special attention to the emissivity profile caused by irradiation of the
inner disc by the hard power-law source. The X-ray observations and simulations
show that the index q of that profile deviates from the commonly used,
Newtonian, value of 3 within 3r_g, steepening considerably within 2r_g, as
expected in the strong gravity regime.Comment: 7 pages, 10 figures, MNRAS in pres
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