3,430 research outputs found
Legendre expansion of the neutrino-antineutrino annihilation kernel: Influence of high order terms
We calculate the Legendre expansion of the rate of the process up to 3rd order extending previous results
of other authors which only consider the 0th and 1st order terms. Using
different closure relations for the moment equations of the radiative transfer
equation we discuss the physical implications of taking into account quadratic
and cubic terms on the energy deposition outside the neutrinosphere in a
simplified model. The main conclusion is that 2nd order is necessary in the
semi-transparent region and gives good results if an appropriate closure
relation is used.Comment: 14 pages, 4 figures. To be published in A&A Supplement Serie
Population Synthesis of Isolated Neutron Stars with magneto-rotational evolution II: from radio-pulsars to magnetars
Population synthesis studies constitute a powerful method to reconstruct the
birth distribution of periods and magnetic fields of the pulsar population.
When this method is applied to populations in different wavelengths, it can
break the degeneracy in the inferred properties of initial distributions that
arises from single-band studies. In this context, we extend previous works to
include -ray thermal emitting pulsars within the same evolutionary model as
radio-pulsars. We find that the cumulative distribution of the number of X-ray
pulsars can be well reproduced by several models that, simultaneously,
reproduce the characteristics of the radio-pulsar distribution. However, even
considering the most favourable magneto-thermal evolution models with fast
field decay, log-normal distributions of the initial magnetic field
over-predict the number of visible sources with periods longer than 12 s. We
then show that the problem can be solved with different distributions of
magnetic field, such as a truncated log-normal distribution, or a binormal
distribution with two distinct populations. We use the observational lack of
isolated NSs with spin periods P>12 s to establish an upper limit to the
fraction of magnetars born with B > 10^{15} G (less than 1\%). As future
detections keep increasing the magnetar and high-B pulsar statistics, our
approach can be used to establish a severe constraint on the maximum magnetic
field at birth of NSs.Comment: 12 pages, 11 figures, 5 table
Estimating causal networks in biosphere–atmosphere interaction with the PCMCI approach
Local meteorological conditions and biospheric activity are tightly coupled. Understanding these links is an essential prerequisite for predicting the Earth system under climate change conditions. However, many empirical studies on the interaction between the biosphere and the atmosphere are based on correlative approaches that are not able to deduce causal paths, and only very few studies apply causal discovery methods. Here, we use a recently proposed causal graph discovery algorithm, which aims to reconstruct the causal dependency structure underlying a set of time series. We explore the potential of this method to infer temporal dependencies in biosphere-atmosphere interactions. Specifically we address the following questions: How do periodicity and heteroscedasticity influence causal detection rates, i.e. the detection of existing and non-existing links? How consistent are results for noise-contaminated data? Do results exhibit an increased information content that justifies the use of this causal-inference method? We explore the first question using artificial time series with well known dependencies that mimic real-world biosphere-atmosphere interactions. The two remaining questions are addressed jointly in two case studies utilizing observational data. Firstly, we analyse three replicated eddy covariance datasets from a Mediterranean ecosystem at half hourly time resolution allowing us to understand the impact of measurement uncertainties. Secondly, we analyse global NDVI time series (GIMMS 3g) along with gridded climate data to study large-scale climatic drivers of vegetation greenness. Overall, the results confirm the capacity of the causal discovery method to extract time-lagged linear dependencies under realistic settings. The violation of the method's assumptions increases the likelihood to detect false links. Nevertheless, we consistently identify interaction patterns in observational data. Our findings suggest that estimating a directed biosphere-atmosphere network at the ecosystem level can offer novel possibilities to unravel complex multi-directional interactions. Other than classical correlative approaches, our findings are constrained to a few meaningful set of relations which can be powerful insights for the evaluation of terrestrial ecosystem models
Evolution of Proto-Neutron stars with kaon condensates
We present simulations of the evolution of a proto-neutron star in which
kaon-condensed matter might exist, including the effects of finite temperature
and trapped neutrinos. The phase transition from pure nucleonic matter to the
kaon condensate phase is described using Gibbs' rules for phase equilibrium,
which permit the existence of a mixed phase. A general property of neutron
stars containing kaon condensates, as well as other forms of strangeness, is
that the maximum mass for cold, neutrino-free matter can be less than the
maximum mass for matter containing trapped neutrinos or which has a finite
entropy. A proto-neutron star formed with a baryon mass exceeding that of the
maximum mass of cold, neutrino-free matter is therefore metastable, that is, it
will collapse to a black hole at some time during the Kelvin-Helmholtz cooling
stage.
The effects of kaon condensation on metastable stars are dramatic. In these
cases, the neutrino signal from a hypothetical galactic supernova (distance
kpc) will stop suddenly, generally at a level above the background in
the SuperK and SNO detectors, which have low energy thresholds and backgrounds.
This is in contrast to the case of a stable star, for which the signal
exponentially decays, eventually disappearing into the background. We find the
lifetimes of kaon-condensed metastable stars to be restricted to the range
40--70 s and weakly dependent on the proto-neutron star mass, in sharp contrast
to the significantly larger mass dependence and range (1--100 s) of
hyperon-rich metastable stars.Comment: 25 pages, 14 figures. Submitted to Astrophysical Journa
The influence of magnetic field geometry on magnetars X-ray spectra
Nowadays, the analysis of the X-ray spectra of magnetically powered neutron
stars or magnetars is one of the most valuable tools to gain insight into the
physical processes occurring in their interiors and magnetospheres. In
particular, the magnetospheric plasma leaves a strong imprint on the observed
X-ray spectrum by means of Compton up-scattering of the thermal radiation
coming from the star surface. Motivated by the increased quality of the
observational data, much theoretical work has been devoted to develop Monte
Carlo (MC) codes that incorporate the effects of resonant Compton scattering in
the modeling of radiative transfer of photons through the magnetosphere. The
two key ingredients in this simulations are the kinetic plasma properties and
the magnetic field (MF) configuration. The MF geometry is expected to be
complex, but up to now only mathematically simple solutions (self-similar
solutions) have been employed. In this work, we discuss the effects of new,
more realistic, MF geometries on synthetic spectra. We use new force-free
solutions in a previously developed MC code to assess the influence of MF
geometry on the emerging spectra. Our main result is that the shape of the
final spectrum is mostly sensitive to uncertain parameters of the
magnetospheric plasma, but the MF geometry plays an important role on the
angle-dependence of the spectra.Comment: 6 pages, 4 figures To appear in Proceedings of II Iberian Nuclear
Astrophysics Meeting held in Salamanca, September 22-23, 201
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