2,629 research outputs found
Magnetospheric particle acceleration and X-ray emission of pulsars
The available data on isolated X-ray pulsars, their wind nebulae, and the
supernova remnants which are connected to some of these sources are analyzed.
It is shown that electric fields of neutron stars tear off charged particles
from the surface of neutron star and trigger the acceleration of particles. The
charged particles are accelerated mainly in the field of magneto-dipole
radiation wave. Power and energy spectra of the charged particles depend on the
strength of the magneto-dipole radiation. Therefore, the X-ray radiation is
strongly dependent on the rate of rotational energy loss and weakly dependent
on the electric field intensity. Coulomb interaction between the charged
particles is the main factor for the energy loss and the X-ray spectra of the
charged particles.Comment: minor correction on table format, 20 pages (4 figures, 1 table),
submitted to International Journal of Modern Physics
Mutation, selection, and ancestry in branching models: a variational approach
We consider the evolution of populations under the joint action of mutation
and differential reproduction, or selection. The population is modelled as a
finite-type Markov branching process in continuous time, and the associated
genealogical tree is viewed both in the forward and the backward direction of
time. The stationary type distribution of the reversed process, the so-called
ancestral distribution, turns out as a key for the study of mutation-selection
balance. This balance can be expressed in the form of a variational principle
that quantifies the respective roles of reproduction and mutation for any
possible type distribution. It shows that the mean growth rate of the
population results from a competition for a maximal long-term growth rate, as
given by the difference between the current mean reproduction rate, and an
asymptotic decay rate related to the mutation process; this tradeoff is won by
the ancestral distribution.
Our main application is the quasispecies model of sequence evolution with
mutation coupled to reproduction but independent across sites, and a fitness
function that is invariant under permutation of sites. Here, the variational
principle is worked out in detail and yields a simple, explicit result.Comment: 45 pages,8 figure
THE WAIT-AND-SEE OPTION IN ASCENDING PRICE AUCTIONS
Cake-cutting protocols aim at dividing a ``cake'' (i.e., a divisible
resource) and assigning the resulting portions to several players in a way that
each of the players feels to have received a ``fair'' amount of the cake. An
important notion of fairness is envy-freeness: No player wishes to switch the
portion of the cake received with another player's portion. Despite intense
efforts in the past, it is still an open question whether there is a
\emph{finite bounded} envy-free cake-cutting protocol for an arbitrary number
of players, and even for four players. We introduce the notion of degree of
guaranteed envy-freeness (DGEF) as a measure of how good a cake-cutting
protocol can approximate the ideal of envy-freeness while keeping the protocol
finite bounded (trading being disregarded). We propose a new finite bounded
proportional protocol for any number n \geq 3 of players, and show that this
protocol has a DGEF of 1 + \lceil (n^2)/2 \rceil. This is the currently best
DGEF among known finite bounded cake-cutting protocols for an arbitrary number
of players. We will make the case that improving the DGEF even further is a
tough challenge, and determine, for comparison, the DGEF of selected known
finite bounded cake-cutting protocols.Comment: 37 pages, 4 figure
Molybdenum sound velocity and shear modulus softening under shock compression
We measured the longitudinal sound velocity in Mo shock compressed up to 4.4 Mbars on the Hugoniot. Its sound speed increases linearly with pressure up to 2.6 Mbars; the slope then decreases up to the melting pressure of âŒ3.8 Mbars. This suggests a decrease of shear modulus before the melt. A linear extrapolation of our data to 1 bar agrees with the ambient sound speed. The results suggest that Mo remains in the bcc phase on the Hugoniot up to the melting pressure. There is no statistically significant evidence for a previously reported bccâhcp phase transition on the Hugoniot
Topological chaos: what may this mean ?
We confront existing definitions of chaos with the state of the art in
topological dynamics. The article does not propose any new definition of chaos
but, starting from several topological properties that can be reasonably called
chaotic, tries to sketch a theoretical view of chaos. Among the main ideas in
this article are the distinction between overall chaos and partial chaos, and
the fact that some dynamical properties may be considered more chaotic than
others
Reply to âComment on âMolybdenum sound velocity and shear modulus softening under shock compressionâ â
We respond to the Comment by Errandonea et al. [Phys. Rev. B 92, 026101 (2015)] on their reinterpretation of our published data [Nguyen et al., Phys. Rev. B 89, 174109 (2014)]. In the original paper, we argued that there is no solid-solid phase transition along the Hugoniot at 2.1 Mbars. There is, however, a softening of the shear modulus starting at 2.6 Mbars. Errandonea et al. [Phys. Rev. B 92, 026101 (2015)] reinterpreted our data and concluded that there is a structural change near 2.3 Mbars on the Hugoniot. We will explore the differences and agreements in the two interpretations of our data
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