29,870 research outputs found
Computing spectral sequences
In this paper, a set of programs enhancing the Kenzo system is presented.
Kenzo is a Common Lisp program designed for computing in Algebraic Topology, in
particular it allows the user to calculate homology and homotopy groups of
complicated spaces. The new programs presented here entirely compute Serre and
Eilenberg-Moore spectral sequences, in particular the groups and differential
maps for arbitrary r. They also determine when the spectral sequence has
converged and describe the filtration of the target homology groups induced by
the spectral sequence
On hadronic beam models for quasars and microquasars
Most of the hadronic jet models for quasars (QSOs) and microquasars (MQs)
found in literature represent beams of particles (e.g. protons). These
particles interact with the matter in the stellar wind of the companion star in
the system or with crossing clouds, generating gamma-rays via proton-proton
processes. Our aim is to derive the particle distribution in the jet as seen by
the observer, so that proper computation of the -ray and neutrino
yields can be done. We use relativistic invariants to obtain the transformed
expressions in the case of a power-law and power-law with a cutoff particle
distribution in the beam. We compare with previous expressions used earlier in
the literature. We show that formerly used expressions for the particle
distributions in the beam as seen by the observer are in error, differences
being strongly dependent on the viewing angle. For example, for
( is the Lorentz factor of the blob) and angles larger than , the earlier-used calculation entails an over-prediction (order of
magnitude or more) of the proton spectra for , whereas it always
over-predicts (two orders of magnitude) the proton spectrum at lower energies,
disregarding the viewing angle. All the results for photon and neutrino fluxes
in hadronic models in beams that have made use of the earlier calculation are
affected. Given that correct gamma-ray fluxes will be in almost any case
significantly diminished in comparison with published results, and that the
time of observations in Cherenkov facilities grows with the square of the
flux-reduction factor in a statistically limited result, the possibility of
observing hadronic beams is undermined.Comment: Accepted for publication in A&A Letter
The age-metallicity dependence for white dwarfs
We present a theoretical study on the metallicity dependence of the
initialtofinal mass relation and its influence on white dwarf age
determinations. We compute a grid of evolutionary sequences from the main
sequence to K on the white dwarf cooling curve, passing through
all intermediate stages. During the thermally-pulsing asymptotic giant branch
no third dredge-up episodes are considered and thus the photospheric C/O ratio
is below unity for sequences with metallicities larger than . We
consider initial metallicities from to , accounting for
stellar populations in the galactic disk and halo, with initial masses below
. We found a clear dependence of the shape of the
initialtofinal mass relation with the progenitor metallicity, where metal
rich progenitors result in less massive white dwarf remnants, due to an
enhancement of the mass loss rates associated to high metallicity values. By
comparing our theoretical computations with semi empirical data from globular
and old open clusters, we found that the observed intrinsic mass spread can be
accounted for by a set of initialtofinal mass relations characterized by
different metallicity values. Also, we confirm that the lifetime spent before
the white dwarf stage increases with metallicity. Finally, we estimate the mean
mass at the top of the white dwarf cooling curve for three globular clusters
NGC 6397, M4 and 47 Tuc, around , characteristic of old stellar
populations. However, we found different values for the progenitor mass, lower
for the metal poor cluster, NGC 6397, and larger for the younger and metal rich
cluster 47 Tuc, as expected from the metallicity dependence of the
initialtofinal mass relation.Comment: Accepted for publication in MNRA
Parametric instability of linear oscillators with colored time-dependent noise
The goal of this paper is to discuss the link between the quantum phenomenon
of Anderson localization on the one hand, and the parametric instability of
classical linear oscillators with stochastic frequency on the other. We show
that these two problems are closely related to each other. On the base of
analytical and numerical results we predict under which conditions colored
parametric noise suppresses the instability of linear oscillators.Comment: RevTex, 9 pages, no figure
Observation of a tricritical wedge filling transition in the 3D Ising model
In this Letter we present evidences of the occurrence of a tricritical
filling transition for an Ising model in a linear wedge. We perform Monte Carlo
simulations in a double wedge where antisymmetric fields act at the top and
bottom wedges, decorated with specific field acting only along the wegde axes.
A finite-size scaling analysis of these simulations shows a novel critical
phenomenon, which is distinct from the critical filling. We adapt to
tricritical filling the phenomenological theory which successfully was applied
to the finite-size analysis of the critical filling in this geometry, observing
good agreement between the simulations and the theoretical predictions for
tricritical filling.Comment: 5 pages, 3 figure
An inquiry into the nature of the gamma-ray source 3EG J1828+0142
The unidentified, low-latitude, -ray source 3EG J1828+0142 presents
high levels of variability and a steep spectral index . Here we
propose a model for this source where the high-energy emission is produced by a
galactic Kerr-Newman black hole. The model takes into account electron-positron
annihilation losses in the calculation of the expected spectral energy
distribution and can reproduce the observational features, including the
absence of a strong radio counterpart. We also report the discovery of a nearby
supernova remnant that could be associated with the original supernova
explosion that created the black hole. Several faint radio sources were also
detected in the radio field within the inner -ray confidence contour
and their spectral index estimated. Some of these sources could be the expected
weak radio counterpart.Comment: 11 pages, 2 figure
- …