Marcio G B de Avellar

Time lags of the kilohertz quasi-periodic oscillations in the low-mass X-ray binaries 4U 1608–52 and 4U 1636–53

We studied the time lags and the coherence of the X-ray light curves of the neutron star low mass X-ray binaries 4U 1608-52 and 4U 1636-53. These quantities are frequency-dependent measures of the time or phase delay and of the degree of linear correlation between two X-ray light curves in two different energies bands; they encode information about the size and geometry of the medium that produces them.

EXACT AND QUASI-EXACT MODELS OF STRANGE STARS

We construct and compare in this work 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 the differences among the models are explicitly shown and discussed. In particular, we present a model starting with a Gaussian ansatz for the density profile that provides a very accurate and almost complete analytical integration of the problem, modulo a small difference for one of the metric potentials.

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 1-1300 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 dependences upon frequency and energy can be used to extract size scales and physical conditions of the medium that produces the lags.

Trends of stellar entropy along stellar evolution

This paper is devoted to discuss the difference in the thermodynamic entropy budget {\it per baryon} in each type of stellar object found in Universe. We track and discuss the actual {\it decrease} of the stored baryonic thermodynamic entropy from the most primitive molecular cloud up to the final fate of matter in the black holes, passing through evolved states of matter as found in white dwarfs and neutron stars. We then discuss the case of actual stars of different masses throughout their {\it evolution}, clarifying the role of virial equilibrium condition for the decrease of the entropy and related issues. Finally, we discuss how gravity ultimately drives composition, hence structural changes along the stellar evolution all the way until the ultimate collapse to black holes, which may increase dramatically their entropy because of the gravitational contribution itself.

Microbial habitability of Europa sustained by radioactive sources

There is an increasing interest in the icy moons of the Solar System due to their potential habitability and as targets for future exploratory missions, which include astrobiological goals. Several studies have reported new results describing the details of these moons’ geological settings; however, there is still a lack of information regarding the deep subsurface environment of the moons. The purpose of this article is to evaluate the microbial habitability of Europa constrained by terrestrial analogue environments and sustained by radioactive energy provided by natural unstable isotopes. The geological scenarios are based on known deep environments on Earth, and the bacterial ecosystem is based on a sulfate-reducing bacterial ecosystem found 2.8 km below the surface in a basin in South Africa. The results show the possibility of maintaining the modeled ecosystem based on the proposed scenarios and provides directions for future models and exploration missions for a more complete evaluation of the habitability of Europa and of icy moons in general.

Information Theoretical Methods as Discerning Quantifiers of the Equations of State of Neutron Stars

In this work we use the statistical measures of information entropy, disequilibrium and complexity to discriminate different approaches and parametrizations for different equations of state for quark stars. We confirm the usefulness of such quantities to quantify the role of interactions in such stars. We find that within this approach, a quark matter equation of state such as SU(2) NJL with vectorial coupling and phase transition is slightly favoured and deserves deeper studies.