3,819 research outputs found
Promotion of Cooperation by Selective Group Extinction
Multilevel selection is an important organizing principle that crucially
underlies evolutionary processes from the emergence of cells to eusociality and
the economics of nations. Previous studies on multilevel selection assumed that
the effective higher-level selection emerges from lower-level reproduction.
This leads to selection among groups, although only individuals reproduce. We
introduce selective group extinction, where groups die with a probability
inversely proportional to their group fitness. When accounting for this the
critical benefit-to-cost ratio is substantially lowered. Because in game theory
and evolutionary dynamics the degree of cooperation crucially depends on this
ratio above which cooperation emerges previous studies may have substantially
underestimated the establishment and maintenance of cooperation.Comment: Accepted for publication in New Journal of Physic
Stationary point approach to the phase transition of the classical XY chain with power-law interactions
The stationary points of the Hamiltonian H of the classical XY chain with
power-law pair interactions (i.e., decaying like r^{-{\alpha}} with the
distance) are analyzed. For a class of "spinwave-type" stationary points, the
asymptotic behavior of the Hessian determinant of H is computed analytically in
the limit of large system size. The computation is based on the Toeplitz
property of the Hessian and makes use of a Szeg\"o-type theorem. The results
serve to illustrate a recently discovered relation between phase transitions
and the properties of stationary points of classical many-body Hamiltonian
functions. In agreement with this relation, the exact phase transition energy
of the model can be read off from the behavior of the Hessian determinant for
exponents {\alpha} between zero and one. For {\alpha} between one and two, the
phase transition is not manifest in the behavior of the determinant, and it
might be necessary to consider larger classes of stationary points.Comment: 9 pages, 6 figure
Implications of automatic photon quenching on compact gamma-ray sources
Aims: We investigate photon quenching in compact non-thermal sources. This
involves photon-photon annihilation and lepton synchrotron radiation in a
network that can become non-linear. As a result the gamma-ray luminosity of a
source cannot exceed a critical limit that depends only on the radius of the
source and on the magnetic field. Methods: We perform analytic and numerical
calculations that verify previous results and extend them so that the basic
properties of photon quenching are investigated. Results: We apply the above to
the 2006 TeV observations of quasar 3C279 and obtain the parameter space of
allowed values for the radius of the emitting source, its magnetic field
strength and the Doppler factor of the flow. We argue that the TeV observations
favour either a modest Doppler factor and a low magnetic field or a high
Doppler factor and a high magnetic field.Comment: 10 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Temporal signatures of leptohadronic feedback mechanisms in compact sources
The hadronic model of Active Galactic Nuclei and other compact high energy
astrophysical sources assumes that ultra-relativistic protons,
electron-positron pairs and photons interact via various hadronic and
electromagnetic processes inside a magnetized volume, producing the
multiwavelength spectra observed from these sources. A less studied property of
such systems is that they can exhibit a variety of temporal behaviours due to
the operation of different feedback mechanisms. We investigate the effects of
one possible feedback loop, where \gamma-rays produced by photopion processes
are being quenched whenever their compactness increases above a critical level.
This causes a spontaneous creation of soft photons in the system that result in
further proton cooling and more production of \gamma-rays, thus making the loop
operate. We perform an analytical study of a simplified set of equations
describing the system, in order to investigate the connection of its temporal
behaviour with key physical parameters. We also perform numerical integration
of the full set of kinetic equations verifying not only our analytical results
but also those of previous numerical studies. We find that once the system
becomes `supercritical', it can exhibit either a periodic behaviour or a damped
oscillatory one leading to a steady state. We briefly point out possible
implications of such a supercriticality on the parameter values used in Active
Galactic Nuclei spectral modelling, through an indicative fitting of the VHE
emission of blazar 3C 279.Comment: 19 pages, 20 figures, accepted for publication in MNRA
Entanglement entropy in quantum spin chains with broken reflection symmetry
We investigate the entanglement entropy of a block of L sites in quasifree
translation-invariant spin chains concentrating on the effect of reflection
symmetry breaking. The majorana two-point functions corresponding to the
Jordan-Wigner transformed fermionic modes are determined in the most general
case; from these it follows that reflection symmetry in the ground state can
only be broken if the model is quantum critical. The large L asymptotics of the
entropy is calculated analytically for general gauge-invariant models, which
has, until now, been done only for the reflection symmetric sector. Analytical
results are also derived for certain non-gauge-invariant models, e.g., for the
Ising model with Dzyaloshinskii-Moriya interaction. We also study numerically
finite chains of length N with a non-reflection-symmetric Hamiltonian and
report that the reflection symmetry of the entropy of the first L spins is
violated but the reflection-symmetric Calabrese-Cardy formula is recovered
asymptotically. Furthermore, for non-critical reflection-symmetry-breaking
Hamiltonians, we find an anomaly in the behavior of the "saturation entropy" as
we approach the critical line. The paper also provides a concise but extensive
review of the block entropy asymptotics in translation invariant quasifree spin
chains with an analysis of the nearest neighbor case and the enumeration of the
yet unsolved parts of the quasifree landscape.Comment: 12 pages and 4 figure
Boundary correlation function of fixed-to-free bcc operators in square-lattice Ising model
We calculate the boundary correlation function of fixed-to-free boundary
condition changing operators in the square-lattice Ising model. The correlation
function is expressed in four different ways using block Toeplitz
determinants. We show that these can be transformed into a scalar Toeplitz
determinant when the size of the matrix is even. To know the asymptotic
behavior of the correlation function at large distance we calculate the
asymptotic behavior of this scalar Toeplitz determinant using the Szeg\"o's
theorem and the Fisher-Hartwig theorem. At the critical temperature we confirm
the power-law behavior of the correlation function predicted by conformal field
theory
Metastable precursors during the oxidation of the Ru(0001) surface
Using density-functional theory, we predict that the oxidation of the
Ru(0001) surface proceeds via the accumulation of sub-surface oxygen in
two-dimensional islands between the first and second substrate layer. This
leads locally to a decoupling of an O-Ru-O trilayer from the underlying metal.
Continued oxidation results in the formation and stacking of more of these
trilayers, which unfold into the RuO_2(110) rutile structure once a critical
film thickness is exceeded. Along this oxidation pathway, we identify various
metastable configurations. These are found to be rather close in energy,
indicating a likely lively dynamics between them at elevated temperatures,
which will affect the surface chemical and mechanical properties of the
material.Comment: 11 pages including 9 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Opacity in compact extragalactic radio sources and its effect on radio-optical reference frame alignment
Accurate alignment of the radio and optical celestial reference frames
requires detailed understanding of physical factors that may cause offsets
between the positions of the same object measured in different spectral bands.
Opacity in compact extragalactic jets (due to synchrotron self-absorption and
external free-free absorption) is one of the key physical phenomena producing
such an offset, and this effect is well-known in radio astronomy ("core
shift"). We have measured the core shifts in a sample of 29 bright compact
extragalactic radio sources observed using very long baseline interferometry
(VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and
estimate that the average shift between radio and optical positions of distant
quasars would be of the order of 0.1-0.2 mas. This shift exceeds positional
accuracy of GAIA and SIM. We suggest two possible approaches to carefully
investigate and correct for this effect in order to align accurately the radio
and optical positions. Both approaches involve determining a Primary Reference
Sample of objects to be used for tying the radio and optical reference frames
together.Comment: 4 pages, 1 figure; to appear in IAU Symposium 248 Proceedings, "A
Giant Step: from Milli- to Micro-arcsecond Astrometry", eds. W.-J. Jin, I.
Platais, M. Perryma
Modeling the Emission Processes in Blazars
Blazars are the most violent steady/recurrent sources of high-energy
gamma-ray emission in the known Universe. They are prominent emitters of
electromagnetic radiation throughout the entire electromagnetic spectrum. The
observable radiation most likely originates in a relativistic jet oriented at a
small angle with respect to the line of sight. This review starts out with a
general overview of the phenomenology of blazars, including results from a
recent multiwavelength observing campaign on 3C279. Subsequently, issues of
modeling broadband spectra will be discussed. Spectral information alone is not
sufficient to distinguish between competing models and to constrain essential
parameters, in particular related to the primary particle acceleration and
radiation mechanisms in the jet. Short-term spectral variability information
may help to break such model degeneracies, which will require snap-shot
spectral information on intraday time scales, which may soon be achievable for
many blazars even in the gamma-ray regime with the upcoming GLAST mission and
current advances in Atmospheric Cherenkov Telescope technology. In addition to
pure leptonic and hadronic models of gamma-ray emission from blazars,
leptonic/hadronic hybrid models are reviewed, and the recently developed
hadronic synchrotron mirror model for TeV gamma-ray flares which are not
accompanied by simultaneous X-ray flares (``orphan TeV flares'') is revisited.Comment: Invited Review at "The Multimessenger Approach to Gamma-Ray Sources",
Barcelona, Spain, July 2006; submitted to Astrophysics and Space Science. 10
pages, including 6 eps figures. Uses Springer's ApSS macro
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