41,424 research outputs found
Pulsar Polar Cap Heating and Surface Thermal X-Ray Emission I. Curvature Radiation Pair Fronts
We investigate the effect of pulsar polar cap (PC) heating produced by
positrons returning from the upper pair formation front. Our calculations are
based on a self-consistent treatment of the pair dynamics and the effect of
electric field screening by the returning positrons. We calculate the resultant
X-ray luminosities, and discuss the dependence of the PC heating efficiencies
on pulsar parameters, such as characteristic spin-down age, spin period, and
surface magnetic field strength. In this study we concentrate on the regime
where the pairs are produced in a magnetic field by curvature photons emitted
by accelerating electrons. Our theoretical results are not in conflict with the
available observational X-ray data and suggest that the effect of PC heating
should significantly contribute to the thermal X-ray fluxes from middle-aged
and old pulsars. The implications for current and future X-ray observations of
pulsars are briefly outlined.Comment: 28 pages, 7 figures, accepted for publication in Ap
A Note on Flux Induced Superpotentials in String Theory
Non-vanishing fluxes in M-theory and string theory compactifications induce a
superpotential in the lower dimensional theory. Gukov has conjectured the
explicit form of this superpotential. We check this conjecture for the
heterotic string compactified on a Calabi-Yau three-fold as well as for warped
M-theory compactifications on Spin(7) holonomy manifolds, by performing a
Kaluza-Klein reduction.Comment: 19 pages, no figure
Quantum Gravity Corrections for Schwarzschild Black Holes
We consider the Matrix theory proposal describing eleven-dimensional
Schwarzschild black holes. We argue that the Newtonian potential between two
black holes receives a genuine long range quantum gravity correction, which is
finite and can be computed from the supergravity point of view. The result
agrees with Matrix theory up to a numerical factor which we have not computed.Comment: 14 pages, Tex, no figure
Moduli Stabilisation in Heterotic Models with Standard Embedding
In this note we analyse the issue of moduli stabilisation in 4d models
obtained from heterotic string compactifications on manifolds with SU(3)
structure with standard embedding. In order to deal with tractable models we
first integrate out the massive fields. We argue that one can not only
integrate out the moduli fields, but along the way one has to truncate also the
corresponding matter fields. We show that the effective models obtained in this
way do not have satisfactory solutions. We also look for stabilised vacua which
take into account the presence of the matter fields. We argue that this also
fails due to a no-go theorem for Minkowski vacua in the moduli sector which we
prove in the end. The main ingredient for this no-go theorem is the constraint
on the fluxes which comes from the Bianchi identity.Comment: 20 pages, LaTeX; references adde
Compactifications of Heterotic Theory on Non-Kahler Complex Manifolds: I
We study new compactifications of the SO(32) heterotic string theory on
compact complex non-Kahler manifolds. These manifolds have many interesting
features like fewer moduli, torsional constraints, vanishing Euler character
and vanishing first Chern class, which make the four-dimensional theory
phenomenologically attractive. We take a particular compact example studied
earlier and determine various geometrical properties of it. In particular we
calculate the warp factor and study the sigma model description of strings
propagating on these backgrounds. The anomaly cancellation condition and
enhanced gauge symmetry are shown to arise naturally in this framework, if one
considers the effect of singularities carefully.
We then give a detailed mathematical analysis of these manifolds and
construct a large class of them. The existence of a holomorphic (3,0) form is
important for the construction. We clarify some of the topological properties
of these manifolds and evaluate the Betti numbers. We also determine the
superpotential and argue that the radial modulus of these manifolds can
actually be stabilized.Comment: 75 pages, Harvmac, no figures; v2: Some new results added, typos
corrected and references updated. Final version to appear in JHE
A Simplest Swimmer at Low Reynolds Number: Three Linked Spheres
We propose a very simple one-dimensional swimmer consisting of three spheres
that are linked by rigid rods whose lengths can change between two values. With
a periodic motion in a non-reciprocal fashion, which breaks the time-reversal
symmetry as well as the translational symmetry, we show that the model device
can swim at low Reynolds number. This model system could be used in
constructing molecular-size machines
Linear Sigma Models with Torsion
Gauged linear sigma models with (0,2) supersymmetry allow a larger choice of
couplings than models with (2,2) supersymmetry. We use this freedom to find a
fully linear construction of torsional heterotic compactifications, including
models with branes. As a non-compact example, we describe a family of metrics
which correspond to deformations of the heterotic conifold by turning on
H-flux. We then describe compact models which are gauge-invariant only at the
quantum level. Our construction gives a generalization of symplectic reduction.
The resulting spaces are non-Kahler analogues of familiar toric spaces like
complex projective space. Perturbatively conformal models can be constructed by
considering intersections.Comment: 40 pages, LaTeX, 1 figure; references added; a new section on
supersymmetry added; quantization condition revisite
Chiral Zeromodes on Vortex-type Intersecting Heterotic Five-branes
We solve the gaugino Dirac equation on a smeared intersecting five-brane
solution in E_8\times E_8 heterotic string theory to search for localized
chiral zeromodes on the intersection. The background is chosen to depend on the
full two-dimensional overall transverse coordinates to the branes. Under some
appropriate boundary conditions, we compute the complete spectrum of zeromodes
to find that, among infinite towers of Fourier modes, there exist only three
localized normalizable zeromodes, one of which has opposite chirality to the
other two. This agrees with the result previously obtained in the domain-wall
type solution, supporting the claim that there exists one net chiral zeromode
localized on the heterotic five-brane system.Comment: 10 pages, 2 figure
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