1,525 research outputs found
A new approach to the study of quasi-normal modes of rotating stars
We propose a new method to study the quasi-normal modes of rotating
relativistic stars. Oscillations are treated as perturbations in the frequency
domain of the stationary, axisymmetric background describing a rotating star.
The perturbed quantities are expanded in circular harmonics, and the resulting
2D-equations they satisfy are integrated using spectral methods in the
(r,theta)-plane. The asymptotic conditions at infinity, needed to find the mode
frequencies, are implemented by generalizing the standing wave boundary
condition commonly used in the non rotating case. As a test, the method is
applied to find the quasi-normal mode frequencies of a slowly rotating star.Comment: 24 pages, 7 figures, submitted to Phys. Rev.
String-Like Lagrangians from a Generalized Geometry
This note will use Hitchin's generalized geometry and a model of axionic
gravity developed by Warren Siegel in the mid-nineties to show that the
construction of Lagrangians based on the inner product arising from the pairing
of a vector and its dual can lead naturally to the low-energy Lagrangian of the
bosonic string.Comment: Conclusions basically unchanged, but presentation streamlined
significantly. Published versio
On the geometry of double field theory
Double field theory was developed by theoretical physicists as a way to
encompass -duality. In this paper, we express the basic notions of the
theory in differential-geometric invariant terms, in the framework of
para-Kaehler manifolds. We define metric algebroids, which are vector bundles
with a bracket of cross sections that has the same metric compatibility
property as a Courant bracket. We show that a double field gives rise to two
canonical connections, whose scalar curvatures can be integrated to obtain
actions. Finally, in analogy with Dirac structures, we define and study
para-Dirac structures on double manifolds.Comment: The paper will appear in J. Math. Phys., 201
Black hole particle emission in higher-dimensional spacetimes
In models with extra dimensions, a black hole evaporates both in the bulk and
on the visible brane, where standard model fields live. The exact emissivities
of each particle species are needed to determine how the black hole decay
proceeds. We compute and discuss the absorption cross-sections, the relative
emissivities and the total power output of all known fields in the evaporation
phase. Graviton emissivity is highly enhanced as the spacetime dimensionality
increases. Therefore, a black hole loses a significant fraction of its mass in
the bulk. This result has important consequences for the phenomenology of black
holes in models with extra dimensions and black hole detection in particle
colliders.Comment: 4 pages, RevTeX 4. v3: Misprints in Tables correcte
Multi-mode TES bolometer optimization for the LSPE-SWIPE instrument
In this paper we explore the possibility of using transition edge sensor
(TES) detectors in multi-mode configuration in the focal plane of the Short
Wavelength Instrument for the Polarization Explorer (SWIPE) of the
balloon-borne polarimeter Large Scale Polarization Explorer (LSPE) for the
Cosmic Microwave Background (CMB) polarization. This study is motivated by the
fact that maximizing the sensitivity of TES bolometers, under the augmented
background due to the multi-mode design, requires a non trivial choice of
detector parameters. We evaluate the best parameter combination taking into
account scanning strategy, noise constraints, saturation power and operating
temperature of the cryostat during the flight.Comment: in Journal of Low Temperature Physics, 05 January 201
Relativistic models of magnetars: structure and deformations
We find numerical solutions of the coupled system of Einstein-Maxwell's
equations with a linear approach, in which the magnetic field acts as a
perturbation of a spherical neutron star. In our study, magnetic fields having
both poloidal and toroidal components are considered, and higher order
multipoles are also included. We evaluate the deformations induced by different
field configurations, paying special attention to those for which the star has
a prolate shape. We also explore the dependence of the stellar deformation on
the particular choice of the equation of state and on the mass of the star. Our
results show that, for neutron stars with mass M = 1.4 Msun and surface
magnetic fields of the order of 10^15 G, a quadrupole ellipticity of the order
of 10^(-6) - 10^(-5) should be expected. Low mass neutron stars are in
principle subject to larger deformations (quadrupole ellipticities up to
10^(-3) in the most extreme case). The effect of quadrupolar magnetic fields is
comparable to that of dipolar components. A magnetic field permeating the whole
star is normally needed to obtain negative quadrupole ellipticities, while
fields confined to the crust typically produce positive quadrupole
ellipticities.Comment: 25 pages, 9 figures, submitted to MNRA
Gravitational signals emitted by a point mass orbiting a neutron star: effects of stellar structure
The effects that the structure of a neutron star would have on the
gravitational emission of a binary system are studied in a perturbative regime,
and in the frequency domain. Assuming that a neutron star is perturbed by a
point mass moving on a close, circular orbit, we solve the equations of stellar
perturbations in general relativity to evaluate the energy lost by the system
in gravitational waves. We compare the energy output obtained for different
stellar models with that found by assuming that the perturbed object is a black
hole with the same mass, and we discuss the role played by the excitation of
the stellar modes. Ouresults indicate that the stellar structure begins to
affect the emitted power when the orbital velocity is v >0.2c (about 185 Hz for
a binary system composed of two canonical neutron stars). We show that the
differences between different stellar models and a black hole are due mainly to
the excitation of the quasinormal modes of the star. Finally, we discuss to
what extent and up to which distance the perturbative approach can be used to
describe the interaction of a star and a pointlike massive body.Comment: 22 pages, 6 figures, to appear in Phys. Rev. D. Revised version,
added one table and extended discussio
Gauging the Wess-Zumino term of a sigma model with boundary
We investigate the gauging of the Wess-Zumino term of a sigma model with
boundary. We derive a set of obstructions to gauging and we interpret them as
the conditions for the Wess-Zumino term to extend to a closed form in a
suitable equivariant relative de Rham complex. We illustrate this with the
two-dimensional sigma model and we show that the new obstructions due to the
boundary can be interpreted in terms of Courant algebroids. We specialise to
the case of the Wess-Zumino-Witten model, where it is proved that there always
exist suitable boundary conditions which allow gauging any subgroup which can
be gauged in the absence of a boundary. We illustrate this with two natural
classes of gaugings: (twisted) diagonal subgroups with boundary conditions
given by (twisted) conjugacy classes, and chiral isotropic subgroups with
boundary conditions given by cosets.Comment: 18 pages (minor changes in response to referee report
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