20 research outputs found
Exact and Quasi-exact Models of Strange Stars
We construct and compare a variety of simple models for strange stars,
namely, hypothetical self-bound objects made of a cold stable version of the
quark-gluon plasma. Exact, quasi-exact and numerical models are examined to
find the most economical description for these objects. A simple and successful
parametrization of them is given in terms of the central density, and many
differences among the models are explicitly shown and discussed.Comment: 20 pp. 15 figures, to appear in IJMP
Phase lags of quasi-periodic oscillations across source states in the low-mass X-ray binary 4U 1636-53
While there are many dynamical mechanisms and models that try to explain the
origin and phenomenology of the quasi-periodic oscillations (QPOs) seen in the
X-ray light curves of low-mass X-ray binaries, few of them address how the
radiative processes occurring in these extreme environments give rise to the
rich set of variability features actually observed in these light curves. A
step towards this end comes from the study of the energy and frequency
dependence of the phase lags of these QPOs. Here we used a methodology that
allowed us to study, for the first time, the dependence of the phase lags of
all QPOs in the range of 1 Hz to 1300 Hz detected in the low-mass X-ray binary
4U 1636-53 upon energy and frequency as the source changes its states as it
moves through the colour-colour diagram. Our results suggest that within the
context of models of up-scattering Comptonization, the phase lags dependencies
upon frequency and energy can be used to extract size scales and physical
conditions of the medium that produces the lags
The amplitude of the kilohertz quasi-periodic oscillations in 4U 1636-53 in the frequency-energy space
We present for the neutron-star low-mass X-ray binary 4U 163653, and for
the first time for any source of kilohertz quasi-periodic oscillations (kHz
QPOs), the two-dimensional behaviour of the fractional rms amplitude of the kHz
QPOs in the parameter space defined by QPO frequency and photon energy. We find
that the rms amplitude of the lower kHz QPO increases with energy up to
keV and then decreases at higher energies, while the rms amplitude of
the upper kHz QPO either continues increasing or levels off at high energies.
The rms amplitude of the lower kHz QPO increases and then decreases with
frequency, peaking at Hz, while the amplitude of the upper kHz QPO
decreases with frequency, with a local maximum at around Hz, and is
consistent with becoming zero at the same QPO frequency, Hz, in all
energy bands, thus constraining the neutron-star mass at , under the assumption that this QPO reflects the Keplerian frequency
at the inner edge of the accretion disc. We show that the slope of the rms
energy spectrum is connected to the changing properties of the kHz QPOs in
different energy bands as its frequencies change. Finally, we discuss a
possible mechanism responsible for the radiative properties of the kHz QPOs
and, based on a model in which the QPO arises from oscillations in a
Comptonising cloud of hot electrons, we show that the properties of the kHz
QPOs can constrain the thermodynamic properties of the inner accretion flow
Magnetic field decay in black widow pulsars
We study in this work the evolution of the magnetic field in 'redback-black widow' pulsars. Evolutionary calculations of these 'spider' systems suggest that first the accretion operates in the redback stage, and later the companion star ablates matter due to winds from the recycled pulsar. It is generally believed that mass accretion by the pulsar results in a rapid decay of the magnetic field when compared to the rate of an isolated neutron star. We study the evolution of the magnetic field in black widow pulsars by solving numerically the induction equation using the modified Crank-Nicolson method with intermittent episodes of mass accretion on to the neutron star. Our results show that the magnetic field does not fall below a minimum value ('bottom field') in spite of the long evolution time of the black widow systems, extending the previous conclusions for much younger low-mass X-ray binary systems. We find that in this scenario, the magnetic field decay is dominated by the accretion rate, and that the existence of a bottom field is likely related to the fact that the surface temperature of the pulsar does not decay as predicted by the current cooling models. We also observe that the impurity of the pulsar crust is not a dominant factor in the decay of magnetic field for the long evolution time of black widow systems.Instituto de AstrofÃsica de La Plat
A vis\~ao da BBChain sobre o contexto tecnol\'ogico subjacente \`a ado\c{c}\~ao do Real Digital
We explore confidential computing in the context of CBDCs using Microsoft's
CCF framework as an example. By developing an experiment and comparing
different approaches and performance and security metrics, we seek to evaluate
the effectiveness of confidential computing to improve the privacy, security,
and performance of CBDCs. Preliminary results suggest that confidential
computing could be a promising solution to the technological challenges faced
by CBDCs. Furthermore, by implementing confidential computing in DLTs such as
Hyperledger Besu and utilizing frameworks such as CCF, we increase transaction
confidentiality and privacy while maintaining the scalability and
interoperability required for a global digital financial system. In conclusion,
confidential computing can significantly bolster CBDC development, fostering a
secure, private, and efficient financial future.
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Exploramos o uso da computa\c{c}\~ao confidencial no contexto das CBDCs
utilizando o framework CCF da Microsoft como exemplo. Via desenvolvimento de
experimentos e compara\c{c}\~ao de diferentes abordagens e m\'etricas de
desempenho e seguran\c{c}a, buscamos avaliar a efic\'acia da computa\c{c}\~ao
confidencial para melhorar a privacidade, seguran\c{c}a e desempenho das CBDCs.
Resultados preliminares sugerem que a computa\c{c}\~ao confidencial pode ser
uma solu\c{c}\~ao promissora para os desafios tecnol\'ogicos enfrentados pelas
CBDCs. Ao implementar a computa\c{c}\~ao confidencial em DLTs, como o
Hyperledger Besu, e utilizar frameworks como o CCF, aumentamos a
confidencialidade e a privacidade das transa\c{c}\~oes, mantendo a
escalabilidade e a interoperabilidade necess\'arias para um sistema financeiro
global e digital. Em conclus\~ao, a computa\c{c}\~ao confidencial pode
refor\c{c}ar significativamente o desenvolvimento do CBDC, promovendo um futuro
financeiro seguro, privado e eficiente.Comment: Comments: 11 pages, 8 figures, in (Brazilian) Portugues