524 research outputs found
Combinatorial 3-manifolds with transitive cyclic symmetry
In this article we give combinatorial criteria to decide whether a transitive
cyclic combinatorial d-manifold can be generalized to an infinite family of
such complexes, together with an explicit construction in the case that such a
family exists. In addition, we substantially extend the classification of
combinatorial 3-manifolds with transitive cyclic symmetry up to 22 vertices.
Finally, a combination of these results is used to describe new infinite
families of transitive cyclic combinatorial manifolds and in particular a
family of neighborly combinatorial lens spaces of infinitely many distinct
topological types.Comment: 24 pages, 5 figures. Journal-ref: Discrete and Computational
Geometry, 51(2):394-426, 201
The accretion environment in Vela X-1 during a flaring period using XMM-Newton
We present analysis of 100 ks contiguous XMM-Newton data of the prototypical
wind accretor Vela X-1. The observation covered eclipse egress between orbital
phases 0.134 and 0.265, during which a giant flare took place, enabling us to
study the spectral properties both outside and during the flare. This giant
flare with a peak luminosity of erg
s allows estimates of the physical parameters of the accreted structure
with a mass of g.
We have been able to model several contributions to the observed spectrum
with a phenomenological model formed by three absorbed power laws plus three
emission lines. After analysing the variations with orbital phase of the column
density of each component, as well as those in the Fe and Ni fluorescence
lines, we provide a physical interpretation for each spectral component.
Meanwhile, the first two components are two aspects of the principal accretion
component from the surface of the neutron star, and the third component seems
to be the \textit{X-ray light echo} formed in the stellar wind of the
companion.Comment: Accepted. Astronomy and Astrophysics, 201
Formation of phase lags at the cyclotron energies in the pulse profiles of magnetized, accreting neutron stars
Context: Accretion-powered X-ray pulsars show highly energy-dependent and
complex pulse-profile morphologies. Significant deviations from the average
pulse profile can appear, in particular close to the cyclotron line energies.
These deviations can be described as energy-dependent phase lags, that is, as
energy-dependent shifts of main features in the pulse profile. Aims: Using a
numerical study we explore the effect of cyclotron resonant scattering on
observable, energy-resolved pulse profiles. Methods: We generated the
observable emission as a function of spin phase, using Monte Carlo simulations
for cyclotron resonant scattering and a numerical ray-tracing routine
accounting for general relativistic light-bending effects on the intrinsic
emission from the accretion columns. Results: We find strong changes in the
pulse profile coincident with the cyclotron line energies. Features in the
pulse profile vary strongly with respect to the average pulse profile with the
observing geometry and shift and smear out in energy additionally when assuming
a non-static plasma. Conclusions: We demonstrate how phase lags at the
cyclotron energies arise as a consequence of the effects of angular
redistribution of X-rays by cyclotron resonance scattering in a strong magnetic
field combined with relativistic effects. We also show that phase lags are
strongly dependent on the accretion geometry. These intrinsic effects will in
principle allow us to constrain a system's accretion geometry.Comment: 4 pages, 4 figures; updated reference lis
A Closed Contour of Integration in Regge Calculus
The analytic structure of the Regge action on a cone in dimensions over a
boundary of arbitrary topology is determined in simplicial minisuperspace. The
minisuperspace is defined by the assignment of a single internal edge length to
all 1-simplices emanating from the cone vertex, and a single boundary edge
length to all 1-simplices lying on the boundary. The Regge action is analyzed
in the space of complex edge lengths, and it is shown that there are three
finite branch points in this complex plane. A closed contour of integration
encircling the branch points is shown to yield a convergent real wave function.
This closed contour can be deformed to a steepest descent contour for all sizes
of the bounding universe. In general, the contour yields an oscillating wave
function for universes of size greater than a critical value which depends on
the topology of the bounding universe. For values less than the critical value
the wave function exhibits exponential behaviour. It is shown that the critical
value is positive for spherical topology in arbitrary dimensions. In three
dimensions we compute the critical value for a boundary universe of arbitrary
genus, while in four and five dimensions we study examples of product manifolds
and connected sums.Comment: 16 pages, Latex, To appear in Gen. Rel. Gra
Addition of cetuximab to first-line chemotherapy in patients with advanced non-small-cell lung cancer: a cost-utility analysis
Background: Adding cetuximab to standard chemotherapy results in a moderate increase of overall survival in patients with advanced non-small-cell lung cancer (NSCLC), but the cost-effectiveness is unknown. Materials and methods: A Markov model was constructed based on the results of the First-Line ErbituX in lung cancer randomized trial, adding cetuximab to cisplatin-vinorelbine first-line chemotherapy in patients with advanced NSCLC. The primary outcome was the incremental cost-effectiveness ratio (ICER) of adding cetuximab, expressed as cost per quality-adjusted life year (QALY) gained, and relative to a willingness-to-pay threshold of €60 000/QALY. The impact of cetuximab intermittent dosing schedules on the ICER was also evaluated. Results: Adding cetuximab to standard chemotherapy leads to a gain of 0.07 QALYs per patient at an additional cost of €26 088. The ICER for adding cetuximab to chemotherapy was €376 205 per QALY gained. Intermittent cetuximab dosing schedules resulted in ICERs per QALY gained between €31 300 and €83 100, under the assumption of equal efficacy. Conclusions: From a health economic perspective, the addition of cetuximab to standard first-line chemotherapy in patients with epidermal growth factor receptor-expressing advanced NSCLC cannot be recommended to date, due to a high ICER compared with other health care interventions. Treatment schedules resulting in more favorable cost-utility ratios should be evaluate
Interatomic Coulombic Decay following Photoionization of the Helium Dimer: Observation of Vibrational Structure
Using synchrotron radiation we simultaneously ionize and excite one helium
atom of a helium dimer (He_2) in a shakeup process. The populated states of the
dimer ion (i.e. He^[*+](n = 2; 3)-He) are found to deexcite via interatomic
coulombic decay. This leads to the emission of a second electron from the
neutral site and a subsequent coulomb explosion. In this letter we present a
measurement of the momenta of fragments that are created during this reaction.
The electron energy distribution and the kinetic energy release of the two He^+
ions show pronounced oscillations which we attribute to the structure of the
vibrational wave function of the dimer ion.Comment: 8 pages, 5 figure
Single photon double ionization of the helium dimer
We show that a single photon can ionize the two helium atoms of the helium
dimer in a distance up to 10 {\deg}A. The energy sharing among the electrons,
the angular distributions of the ions and electrons as well as comparison with
electron impact data for helium atoms suggest a knock-off type double
ionization process. The Coulomb explosion imaging of He_2 provides a direct
view of the nuclear wave function of this by far most extended and most diffuse
of all naturally existing molecules.Comment: 10 pages, 5 figure
Triangulations and Severi varieties
We consider the problem of constructing triangulations of projective planes
over Hurwitz algebras with minimal numbers of vertices. We observe that the
numbers of faces of each dimension must be equal to the dimensions of certain
representations of the automorphism groups of the corresponding Severi
varieties. We construct a complex involving these representations, which should
be considered as a geometric version of the (putative) triangulations
Be-Phenomenon in Neutron Star X-ray Binaries
In this work we provide a brief insight into two aspects of Be/X-ray binaries, which are probably involved in production of X-ray outbursts: the evolution of the Be star disk, in particular of its size, and the binary geometry which drives gravitational interaction. Simultaneous X-ray and optical data will aid our investigation of the evolution of Be stars in binaries and the X-ray outburst mechanism
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