23 research outputs found
Importance of Compton scattering to radiation spectra of isolated neutron stars
Model atmospheres of isolated neutron stars with low magnetic field are
calculated with Compton scattering taking into account. Models with effective
temperatures 1, 3 and 5 MK, with two values of surface gravity log(g)g = 13.9
and 14.3), and different chemical compositions are calculated. Radiation
spectra computed with Compton scattering are softer than the computed with
Thomson scattering at high energies (E > 5 keV) for hot (T_eff > 1 MK)
atmospheres with hydrogen-helium composition. Compton scattering is more
significant to hydrogen models with low surface gravity. The emergent spectra
of the hottest (T_eff > 3 MK) model atmospheres can be described by diluted
blackbody spectra with hardness factors ~ 1.6 - 1.9. Compton scattering is less
important for models with solar abundance of heavy elements.Comment: Proceedings of the 363. WE-Heraeus Seminar on: Neutron Stars and
Pulsars (Posters and contributed talks) Physikzentrum Bad Honnef, Germany,
May.14-19, 2006, eds. W.Becker, H.H.Huang, MPE Report 291, pp.173-17
The thermal and kinematic Sunyaev-Zel'dovich effects revisited
This paper shows that a simple convolution integral expression based on the
mean value of the isotropic frequency distribution corresponding to photon
scattering off electrons leads to useful analytical expressions describing the
thermal Sunyaev-Zel'dovich effect. The approach, to first order in the Compton
parameter is able to reproduce the Kompaneets equation describing the effect.
Second order effects in the parameter induce a slight
increase in the crossover frequency.Comment: 7 pages, 2 figure
A Hydrodynamical Approach to CMB mu-distortions
Spectral distortion of the cosmic microwave background provides a unique
opportunity to probe primordial perturbations on very small scales by
performing large-scale measurements. We discuss in a systematic and pedagogic
way all the relevant physical phenomena involved in the production and
evolution of the mu-type spectral distortion. Our main results agree with
previous estimates (in particular we show that a recently found factor of 3/4
arises from relativistic corrections to the wave energy). We also discuss
several subleading corrections such as adiabatic cooling and the effects of
bulk viscosity, baryon loading and photon heat conduction. Finally we calculate
the transfer function for mu-distortions between the end of the mu-era and now.Comment: 45 page
Creation of the CMB spectrum: precise analytic solutions for the blackbody photosphere
The blackbody spectrum of CMB was created in the blackbody photosphere at
redshifts z>2x10^6. At these early times, the Universe was dense and hot enough
that complete thermal equilibrium between baryonic matter (electrons and ions)
and photons could be established. Any perturbation away from the blackbody
spectrum was suppressed exponentially. New physics, for example annihilation
and decay of dark matter, can add energy and photons to CMB at redshifts z>10^5
and result in a Bose-Einstein spectrum with a non-zero chemical potential
(). Precise evolution of the CMB spectrum around the critical redshift of
z~2x10^6 is required in order to calculate the -type spectral distortion
and constrain the underlying new physics. Although numerical calculation of
important processes involved (double Compton process, comptonization and
bremsstrahlung) is not difficult, analytic solutions are much faster and easier
to calculate and provide valuable physical insights. We provide precise (better
than 1%) analytic solutions for the decay of , created at an earlier
epoch, including all three processes, double Compton, Compton scattering on
thermal electrons and bremsstrahlung in the limit of small distortions. This is
a significant improvement over the existing solutions with accuracy ~10% or
worse. We also give a census of important sources of energy injection into CMB
in standard cosmology. In particular, calculations of distortions from
electron-positron annihilation and primordial nucleosynthesis illustrate in a
dramatic way the strength of the equilibrium restoring processes in the early
Universe. Finally, we point out the triple degeneracy in standard cosmology,
i.e., the and distortions from adiabatic cooling of baryons and
electrons, Silk damping and annihilation of thermally produced WIMP dark matter
are of similar order of magnitude (~ 10^{-8}-10^{-10})
Gauge fixing and the Hamiltonian for cylindrical spacetimes
We introduce a complete gauge fixing for cylindrical spacetimes in vacuo
that, in principle, do not contain the axis of symmetry. By cylindrically
symmetric we understand spacetimes that possess two commuting spacelike Killing
vectors, one of them rotational and the other one translational. The result of
our gauge fixing is a constraint-free model whose phase space has four
field-like degrees of freedom and that depends on three constant parameters.
Two of these constants determine the global angular momentum and the linear
momentum in the axis direction, while the third parameter is related with the
behavior of the metric around the axis. We derive the explicit expression of
the metric in terms of the physical degrees of freedom, calculate the reduced
equations of motion and obtain the Hamiltonian that generates the reduced
dynamics. We also find upper and lower bounds for this reduced Hamiltonian that
provides the energy per unit length contained in the system. In addition, we
show that the reduced formalism constructed is well defined and consistent at
least when the linear momentum in the axis direction vanishes. Furthermore, in
that case we prove that there exists an infinite number of solutions in which
all physical fields are constant both in the surroundings of the axis and at
sufficiently large distances from it. If the global angular momentum is
different from zero, the isometry group of these solutions is generally not
orthogonally transitive. Such solutions generalize the metric of a spinning
cosmic string in the region where no closed timelike curves are present.Comment: 12 pages, accepted for publication in Physical Review
Energy and Momentum Distributions of Kantowski and Sachs Space-time
We use the Einstein, Bergmann-Thomson, Landau-Lifshitz and Papapetrou
energy-momentum complexes to calculate the energy and momentum distributions of
Kantowski and Sachs space-time. We show that the Einstein and Bergmann-Thomson
definitions furnish a consistent result for the energy distribution, but the
definition of Landau-Lifshitz do not agree with them. We show that a signature
switch should affect about everything including energy distribution in the case
of Einstein and Papapetrou prescriptions but not in Bergmann-Thomson and
Landau-Lifshitz prescriptions.Comment: 12 page
DT/T beyond linear theory
The major contribution to the anisotropy of the temperature of the Cosmic
Microwave Background (CMB) radiation is believed to come from the interaction
of linear density perturbations with the radiation previous to the decoupling
time. Assuming a standard thermal history for the gas after recombination, only
the gravitational field produced by the linear density perturbations present on
a universe can generate anisotropies at low z (these
anisotropies would manifest on large angular scales). However, secondary
anisotropies are inevitably produced during the nonlinear evolution of matter
at late times even in a universe with a standard thermal history. Two effects
associated to this nonlinear phase can give rise to new anisotropies: the
time-varying gravitational potential of nonlinear structures (Rees-Sciama RS
effect) and the inverse Compton scattering of the microwave photons with hot
electrons in clusters of galaxies (Sunyaev-Zeldovich SZ effect). These two
effects can produce distinct imprints on the CMB temperature anisotropy. We
discuss the amplitude of the anisotropies expected and the relevant angular
scales in different cosmological scenarios. Future sensitive experiments will
be able to probe the CMB anisotropies beyong the first order primary
contribution.Comment: plain tex, 16 pages, 3 figures. Proceedings of the Laredo Advance
School on Astrophysics "The universe at high-z, large-scale structure and the
cosmic microwave background". To be publised by Springer-Verla
Reionization by active sources and its effects on the cosmic microwave background
We investigate the possible effects of reionization by active sources on the
cosmic microwave background. We concentrate on the sources themselves as the
origin of reionization, rather than early object formation, introducing an
extra period of heating motivated by the active character of the perturbations.
Using reasonable parameters, this leads to four possibilities depending on the
time and duration of the energy input: delayed last scattering, double last
scattering, shifted last scattering and total reionization. We show that these
possibilities are only very weakly constrained by the limits on spectral
distortions from the COBE FIRAS measurements. We illustrate the effects of
these reionization possibilities on the angular power spectrum of temperature
anisotropies and polarization for simple passive isocurvature models and simple
coherent sources, observing the difference between passive and active models.
Finally, we comment on the implications of this work for more realistic active
sources, such as causal white noise and topological defect models. We show for
these models that non-standard ionization histories can shift the peak in the
CMB power to larger angular scales.Comment: 21 pages LaTeX with 11 eps figures; replaced with final version
accepted for publication in Phys. Rev.
Exact Hypersurface-Homogeneous Solutions in Cosmology and Astrophysics
A framework is introduced which explains the existence and similarities of
most exact solutions of the Einstein equations with a wide range of sources for
the class of hypersurface-homogeneous spacetimes which admit a Hamiltonian
formulation. This class includes the spatially homogeneous cosmological models
and the astrophysically interesting static spherically symmetric models as well
as the stationary cylindrically symmetric models. The framework involves
methods for finding and exploiting hidden symmetries and invariant submanifolds
of the Hamiltonian formulation of the field equations. It unifies, simplifies
and extends most known work on hypersurface-homogeneous exact solutions. It is
shown that the same framework is also relevant to gravitational theories with a
similar structure, like Brans-Dicke or higher-dimensional theories.Comment: 41 pages, REVTEX/LaTeX 2.09 file (don't use LaTeX2e !!!) Accepted for
publication in Phys. Rev.