14,598 research outputs found
The inner disk radius in the propeller phase and accretion-propeller transition of neutron stars
We have investigated the critical conditions required for a steady propeller
effect for magnetized neutron stars with optically thick, geometrically thin
accretion disks. We have shown through simple analytical calculations that a
steady-state propeller mechanism cannot be sustained at an inner disk radius
where the viscous and magnetic stresses are balanced. The radius calculated by
equating these stresses is usually found to be close to the conventional Alfven
radius for spherical accretion, r_A. Our results show that: (1) a steady
propeller phase can be established with a maximum inner disk radius that is at
least \sim 15 times smaller than r_A depending on the mass-flow rate of the
disk, rotational period and strength of the magnetic dipole field of the star,
(2) the critical accretion rate corresponding to the accretion-propeller
transition is orders of magnitude lower than the rate estimated by equating r_A
to the co-rotation radius. Our results are consistent with the properties of
the transitional millisecond pulsars which show transitions between the
accretion powered X-ray pulsar and the rotational powered radio pulsar states.Comment: 6 pages, accepted for publication in MNRA
Performances of the NA48 Liquid Krypton calorimeter
The NA48 experiments aims at a precise measurement of direct CP violation in
the neutral Kaon system. This puts stringent requirements on the
electromagnetic calorimeter used to detect photons of average energy 25 GeV.
The choice of NA48 is a quasi homogeneous Liquid Krypton calorimeter with fast
readout. The operation of this device and the performances achieved are
described.Comment: 15 page
Design optimization for cost and quality: The robust design approach
Designing reliable, low cost, and operable space systems has become the key to future space operations. Designing high quality space systems at low cost is an economic and technological challenge to the designer. A systematic and efficient way to meet this challenge is a new method of design optimization for performance, quality, and cost, called Robust Design. Robust Design is an approach for design optimization. It consists of: making system performance insensitive to material and subsystem variation, thus allowing the use of less costly materials and components; making designs less sensitive to the variations in the operating environment, thus improving reliability and reducing operating costs; and using a new structured development process so that engineering time is used most productively. The objective in Robust Design is to select the best combination of controllable design parameters so that the system is most robust to uncontrollable noise factors. The robust design methodology uses a mathematical tool called an orthogonal array, from design of experiments theory, to study a large number of decision variables with a significantly small number of experiments. Robust design also uses a statistical measure of performance, called a signal-to-noise ratio, from electrical control theory, to evaluate the level of performance and the effect of noise factors. The purpose is to investigate the Robust Design methodology for improving quality and cost, demonstrate its application by the use of an example, and suggest its use as an integral part of space system design process
Equivariant Fields in an Gauge Theory with new Spontaneously Generated Fuzzy Extra Dimensions
We find new spontaneously generated fuzzy extra dimensions emerging from a
certain deformation of supersymmetric Yang-Mills (SYM) theory with cubic
soft supersymmetry breaking and mass deformation terms. First, we determine a
particular four dimensional fuzzy vacuum that may be expressed in terms of a
direct sum of product of two fuzzy spheres, and denote it in short as . The direct sum structure of the vacuum is revealed
by a suitable splitting of the scalar fields in the model in a manner that
generalizes our approach in \cite{Seckinson}. Fluctuations around this vacuum
have the structure of gauge fields over ,
and this enables us to conjecture the spontaneous broken model as an effective
gauge theory on the product manifold . We support this interpretation by
examining the theory and determining all of the
equivariant fields in the model, characterizing its low energy degrees of
freedom. Monopole sectors with winding numbers are accessed from after suitable projections and subsequently equivariant fields in these
sectors are obtained. We indicate how Abelian Higgs type models with vortex
solutions emerge after dimensionally reducing over the fuzzy monopole sectors
as well. A family of fuzzy vacua is determined by giving a systematic treatment
for the splitting of the scalar fields and it is made manifest that suitable
projections of these vacuum solutions yield all higher winding number fuzzy
monopole sectors. We observe that the vacuum configuration identifies with the bosonic part of the product of two fuzzy
superspheres with supersymmetry and elaborate on this
feature.Comment: 38+1 pages, published versio
Trajectories with suppressed tensor-to-scalar ratio in Aligned Natural Inflation
In Aligned Natural Inflation, an alignment between different potential terms
produces an inflaton excursion greater than the axion scales in the potential.
We show that, starting from a general potential of two axions with two aligned
potential terms, the effective theory for the resulting light direction is
characterized by four parameters: an effective potential scale, an effective
axion constant, and two extra parameters (related to ratios of the axion scales
and the potential scales in the field theory). For all choices of these
extra parameters, the model can support inflation along valleys (in the
field space) that end in minima of the potential. This leads to a
phenomenology similar to that of single field Natural Inflation. For a
significant range of the extra two parameters, the model possesses also higher
altitude inflationary trajectories passing through saddle points of the
field potential, and disconnected from any minimum. These plateaus end when
the heavier direction becomes unstable, and therefore all of inflation takes
place close to the saddle point, where - due to the higher altitude - the
potential is flatter (smaller parameter). As a consequence, a
tensor-to-scalar ratio can be
easily achieved in the allowed region, well within the latest
CMB contours
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