arXiv subjects

Marcio G B de Avellar

Publications and source records attributed to Marcio G B de Avellar.

5 recordsLinked to original sources

arXiv2026

The mass of the neutron star in 4U 1820-30 revisited

We revisit the mass of the neutron star in the ultracompact binary 4U 1820--30 in light of a recently reported transient absorption feature at about 3.8 keV, interpreted as a gravitationally redshifted, highly ionized iron line and implying $1+z\simeq1.72$, a very high stellar compactness. We examine whether the mass-radius locus implied by this interpretation can be made compatible with external EoS-informed benchmarks and timing-based estimates. We map the compactness implied by the redshift onto the mass-radius plane, including rotational effects, and compare the resulting region with an EoS-informed 95\% reference contour derived from NICER data of several neutron-star systems. While we do not perform a statistically self-consistent joint mass-radius inference for 4U 1820--30, we present a quantitative conditional consistency test in a common $M-R$ framework, comparing published inputs under explicitly stated assumptions. We find that, if the maximum neutron-star mass is restricted to low values $(\leq 2.3\,M_{\odot})$, the redshift-implied locus shows at most marginal overlap with the EoS-informed contour, indicating substantial tension. Allowing a higher maximum mass enlarges the parameter space and can restore compatibility with that benchmark. However, such high masses remain in tension with previous touchdown-flux estimates, although they are not necessarily excluded by interpretations based on the highest detected quasi-periodic oscillation frequency. We conclude that the redshift interpretation of the 3.8 keV feature, the touchdown-flux estimates, and the QPO/ISCO interpretation do not naturally select the same mass-radius sector for this source under these assumptions. Reconciling them requires auxiliary assumptions with high leverage on the inferred compactness.

astro-ph.HE↗

arXiv2023

A visão da BBChain sobre o contexto tecnológico subjacente à adoção 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. -- Exploramos o uso da computação confidencial no contexto das CBDCs utilizando o framework CCF da Microsoft como exemplo. Via desenvolvimento de experimentos e comparação de diferentes abordagens e métricas de desempenho e segurança, buscamos avaliar a eficácia da computação confidencial para melhorar a privacidade, segurança e desempenho das CBDCs. Resultados preliminares sugerem que a computação confidencial pode ser uma solução promissora para os desafios tecnológicos enfrentados pelas CBDCs. Ao implementar a computação confidencial em DLTs, como o Hyperledger Besu, e utilizar frameworks como o CCF, aumentamos a confidencialidade e a privacidade das transações, mantendo a escalabilidade e a interoperabilidade necessárias para um sistema financeiro global e digital. Em conclusão, a computação confidencial pode reforçar significativamente o desenvolvimento do CBDC, promovendo um futuro financeiro seguro, privado e eficiente.

cs.DC↗

arXiv2017

The kilo Hertz quasi-periodic oscillations in neutron star low-mass X-ray binaries as tori oscillation modes. I

There have been many efforts to explain the dynamical mechanisms behind the phenomenology of quasi-periodic oscillations (QPOs) seen in the X-ray light curves of low-mass X-ray binaries. Up to now, none of the models can successfully explain all the frequencies observed in the power density spectrum of the light curve. Here we perform several hydrodynamic simulations of non-self-gravitating relativistic axisymmetric thick tori applied to the neutron star in the low-mass X-ray binary 4U 1636-53 and show how the observed oscillation modes triggered by different velocity perturbations give rise to a set of variability features similar to what we see in the observational X-ray data. When we match pairs of frequencies from the simulations of constant angular momentum ($l$) tori with the observed kilo Hertz QPOs, we find that certain combinations of frequencies lie on top of the observed relation, provided we assume a smaller mass for the neutron star than is generally assumed. However, constant-$l$ tori cannot match the entire range of frequencies observed for 4U 1636-53 due to physical constraints set by the torus size. We show that our model is consistent with the observed shift in QPO frequency that accompany state transitions of the accretion disk.

astro-ph.HE↗

arXiv2016

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.

astro-ph.HE↗

arXiv2013

Entropy, Disequilibrium and Complexity in Compact Stars: An information theory approach to understand their Composition

The composition of neutron stars is an issue that has been studied for decades. Yet we do not know exactly what these very compact objects are made of. At this stage of the technological development the best we can do is to constrain the range of equations of state via the mass-radius diagram. From the theoretical point of view the things are not easier. The theory of matter at high density and temperature is not well established. Keeping this in mind we applied the Information Theory as a novel way to restrict the range of possible equations of state for neutron stars. From our results, we conclude that if order costs energy, then nature should favour exotic strange quark stars over the hadronic neutron stars and that there is a trend for the types of compact stars of classified under the nomenclature "neutrons stars" to be at a state of minimum complexity.

astro-ph.SR↗