arXiv subjects

M. Malheiro

Publications and source records attributed to M. Malheiro.

At least 37 records · Page 2Linked to original sources

arXiv2017

Ultra-high energy cosmic rays from white dwarf pulsars and the Hillas criterion

The sources of ultra-high-energy cosmic rays ($E\gtrsim10^{19}$ eV) is still an open question in astroparticle physics and in the last years some efforts were made to understand its origin. In this work we consider white dwarf pulsars (WDPs) as possible sources of these ultra energetic particles. If some Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are considered as white dwarf pulsars, these sources can achieve large electromagnetic potentials on its surface and possibly accelerate particle almost of speed of light and with energies $E \sim 10^{20-21}$ eV. We show that the new scale of star surface magnetic field and size of SGRs/AXPs as very magnetized white dwarfs are well described in the Hillas diagram together with the AR Sorpii and AE Aquarii white dwarf pulsars recently found. SGRs/AXPs described as magnetars are much above the Hillas line and out of it is expected for normal neutron star pulsars.

astro-ph.HE↗

arXiv2016

On the nature of some SGRs and AXPs as rotation-powered neutron stars

We investigate the possibility that some SGRs/AXPs could be canonical rotation-powered pulsars using realistic NS structure parameters instead of fiducial values. We show that realistic NS parameters lowers the estimated value of the magnetic field and radiation efficiency, $L_X/\dot{E}_{\rm rot}$, with respect to estimates based on fiducial NS parameters. We show that nine SGRs/AXPs can be described as canonical pulsars driven by the NS rotational energy, for $L_X$ computed in the soft (2--10~keV) X-ray band. We compute the range of NS masses for which $L_X/\dot{E}_{\rm rot}<1$. We discuss the observed hard X-ray emission in three sources of the group of nine potentially rotation-powered NSs. This additional hard X-ray component dominates over the soft one leading to $L_X/\dot{E}_{\rm rot}>1$ in two of them. We show that 9 SGRs/AXPs can be rotation-powered NSs if we analyze their X-ray luminosity in the soft 2--10~keV band. Interestingly, four of them show radio emission and six have been associated with supernova remnants (including Swift J1834.9-0846 the first SGR observed with a surrounding wind nebula). These observations give additional support to our results of a natural explanation of these sources in terms of ordinary pulsars. Including the hard X-ray emission observed in three sources of the group of potential rotation-powered NSs, this number of sources with $L_X/\dot{E}_{\rm rot}<1$ becomes seven. It remains open to verification 1) the accuracy of the estimated distances and 2) the possible contribution of the associated supernova remnants to the hard X-ray emission.

astro-ph.HE↗

arXiv2016

Radial stability of anisotropic strange quark stars

The influence of the anisotropy in the equilibrium and stability of strange stars is investigated through the numerical solution of the hydrostatic equilibrium equation and the radial oscillation equation, both modified from their original version to include this effect. The strange matter inside the quark stars is described by the MIT bag model equation of state. For the anisotropy two different kinds of local anisotropic $σ=p_t-p_r$ are considered, where $p_t$ and $p_r$ are respectively the tangential and the radial pressure: one that is null at the star's surface defined by $p_r(R)=0$, and one that is nonnull on at the surface, namely, $σ_s=0$ and $σ_s\neq0$. In the case $σ_s=0$, the maximum mass value and the zero frequency of oscillation are found at the same central energy density, indicating that the maximum mass marks the onset of the instability. For the case $σ_s\neq0$, we show that the maximum mass point and the zero frequency of oscillation coincide in the same central energy density value only in a sequence of equilibrium configurations with the same value of $σ_s$. Thus, the stability star regions are determined always by the condition $dM/dρ_c>0$ only when the tangential pressure is maintained fixed at the star surface's $p_t(R)$. These results are also quite important to analyze the stability of other anisotropic compact objects such as neutron stars, boson stars and gravastars.

astro-ph.HE↗

arXiv2016

Gravitational waves in the $f(R,T)$ theory of gravity

There is a host of alternative theories of gravitation in the literature, among them the $f(R,T)$ recently elaborated by Harko and collaborators. In these theories the $R$ and $T$ are respectively the Ricci scalar and the trace of the energy momentum tensor. There is already in literature a series of studies of different forms of the $f(R,T)$ functions as well as their cosmological consequences. However, there is not so far in the literature studies related to the gravitational waves in $f(R,T)$ gravity. Here we consider such an issue, in particular studying the putative extra polarization models that can well appear in such theories. To do that, we consider different functional forms for $f(R,T)$.

gr-qc↗

arXiv2016

Stellar equilibrium configurations of compact stars in $f(R,T)$ gravity

In this article we study the hydrostatic equilibrium configuration of neutron stars and strange stars, whose fluid pressure is computed from the equations of state $p=ωρ^{5/3}$ and $p=0.28(ρ-4{\cal B})$, respectively, with $ω$ and ${\cal B}$ being constants and $ρ$ the energy density of the fluid. We start by deriving the hydrostatic equilibrium equation for the $f(R,T)$ theory of gravity, with $R$ and $T$ standing for the Ricci scalar and trace of the energy-momentum tensor, respectively. Such an equation is a generalization of the one obtained from general relativity, and the latter can be retrieved for a certain limit of the theory. For the $f(R,T)=R+2λT$ functional form, with $λ$ being a constant, we find that some physical properties of the stars, such as pressure, energy density, mass and radius, are affected when $λ$ is changed. We show that for a fixed central star energy density, the mass of neutron and strange stars can increase with $λ$. Concerning the star radius, it increases for neutron stars and it decreases for strange stars with the increment of $λ$. Thus, in $f(R,T)$ theory of gravity we can push the maximum mass above the observational limits. This implies that the equation of state cannot be eliminated if the maximum mass within General Relativity lies below the limit given by observed pulsars.

gr-qc↗

arXiv2015

General spin and pseudospin symmetries of the Dirac equation

In the 70's Smith and Tassie, and Bell and Ruegg independently found SU(2) symmetries of the Dirac equation with scalar and vector potentials. These symmetries, known as pseudospin and spin symmetries, have been extensively researched and applied to several physical systems. Twenty years after, in 1997, the pseudospin symmetry has been revealed by Ginocchio as a relativistic symmetry of the atomic nuclei when it is described by relativistic mean field hadronic models. The main feature of these symmetries is the suppression of the spin-orbit coupling either in the upper or lower components of the Dirac spinor, thereby turning the respective second-order equations into Schrödinger-like equations, i.e, without a matrix structure. In this paper we propose a generalization of these SU(2) symmetries for potentials in the Dirac equation with several Lorentz structures, which also allow for the suppression of the matrix structure of second-order equation equation of either the upper or lower components of the Dirac spinor. We derive the general properties of those potentials and list some possible candidates, which include the usual spin-pseudospin potentials, and also 2- and 1-dimensional potentials. An application for a particular physical system in two dimensions, electrons in graphene, is suggested.

quant-ph↗

arXiv2015

Equilibrium and stability of charged strange quark stars

The hydrostatic equilibrium and the stability against radial perturbation of charged strange quark stars composed of a charged perfect fluid are studied. For this purpose, it is considered that the perfect fluid follows the MIT bag model equation of state and the radial charge distribution follows a power-law. The hydrostatic equilibrium and the stability of charged strange stars are investigated through the numerical solutions of the Tolman-Oppenheimer-Volkoff equation and the Chandrasekhar's pulsation equation, being these equations modified from their original form to include the electrical charge. In order to appreciably affect the stellar structure, it is found that the total charge should be of order $10^{20}[\rm C]$, implying an electric field of around $10^{22}[\rm V/m]$. We found the electric charge that produces considerable effect on the structure and stability of the object is close to the star's surface. We obtain that for a range of central energy density the stability of the star decreases with the increment of the total charge and for a range of total mass the electric charge helps to grow the stability of the stars under study. We show that the central energy density used to reach the maximum mass value is the same used to determine the zero eigenfrequency of the fundamental mode when the total charge is fixed, thus indicating that the maximum mass point marks the onset of instability. In other words, when fixing the total charge, the conditions $\frac{dM}{dρ_c}>0$ and $\frac{dM}{dρ_c}<0$ are necessary and sufficient to determine the stable and unstable equilibrium configurations regions against radial oscillations.

astro-ph.SR↗

arXiv2014

Dynamical instability of white dwarfs and breaking of spherical symmetry under the presence of extreme magnetic fields

Massive, highly magnetized white dwarfs with fields up to $10^9$ G have been observed and theoretically used for the description of a variety of astrophysical phenomena. Ultramagnetized white dwarfs with uniform interior fields up to $10^{18}$ G, have been recently purported to obey a new maximum mass limit, $M_{\rm max}\approx 2.58~M_\odot$, which largely overcomes the traditional Chandrasekhar value, $M_{\rm Ch}\approx 1.44~M_\odot$. Such a much larger limit would make these astrophysical objects viable candidates for the explanation of the superluminous population of type Ia supernovae. We show that several macro and micro physical aspects such as gravitational, dynamical stability, breaking of spherical symmetry, general relativity, inverse $β$-decay, and pycnonuclear fusion reactions are of most relevance for the self-consistent description of the structure and assessment of stability of these objects. It is shown in this work that the first family of magnetized white dwarfs indeed satisfy all the criteria of stability, while the ultramagnetized white dwarfs are very unlikely to exist in nature since they violate minimal requests of stability. Therefore, the canonical Chandrasekhar mass limit of white dwarfs has to be still applied.

astro-ph.SR↗

arXiv2013

Polyakov-Nambu-Jona-Lasinio phase diagrams and quarkyonic phase from order parameters

We show that the magnitude of the order parameters in Polyakov-Nambu-Jona-Lasinio (PNJL) model, given by the quark condensate and the Polyakov loop, can be used as a criterium to clearly identify, without ambiguities, phases and boundaries of the strongly interacting matter, namely, the broken/restored chiral symmetry, and confinement/deconfinement regions. This structure is represented by the projection of the order parameters in the temperature-chemical potential plane, which allows a clear identification of pattern changes in the phase diagram. Such a criterium also enables the emergence of a quarkyonic phase even in the two-flavor system. We still show that this new phase diminishes due to the influence of an additional vector-type interaction in the PNJL phase diagrams, and is quite sensitive to the effect of the change of the $T_0$ parameter in the Polyakov potential. Finally, we show that the phases and boundaries constructed by our method indicate that the order parameters should be more strongly correlated, as in the case of entanglement PNJL (EPNJL) model. This result suggests a novel way to pursue further investigation of new interactions between the order parameters in order to improve the PNJL model.

hep-ph↗

arXiv2013

Magnetic Dipole Moment of SGRs and AXPs Described as Massive and Magnetic White Dwarfs

The Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are some of the most interesting groups of pulsars that have been intensively studied in the recent years. They are understood as neutron stars (NSs) with super strong magnetic fields, namely $B\gtrsim10^{14}$ G. However, in the last two years two SGRs with low magnetic fields $B\sim(10^{12}-10^{13})$ G have been detected. Moreover, three fast and very {\it magnetic} white dwarfs (WDs) have also been observed in the last years. Based on these new pulsar discoveries, we compare and contrast the magnetic fields, magnetic dipole moment, characteristic ages, and X-ray steady luminosities of these two SGRs (in the WD model) with three fast white dwarfs, to conclude that they show strong similarities corroborating an alternative description of several SGRs/AXPs as very massive and magnetic white dwarfs. The pulsar magnetic dipole moment $m$ depending only on the momentum of inertia $I$, and observational properties, such as the period $P$ and its first time derivative $\dot{P}$, can help to identify the scale of $I$ for SGRs/AXPs. We analyze the pulsar magnetic dipole moment $m$ of SGRs and AXPs when a model based on a massive fast rotating highly magnetized white dwarf is considered. We show that the values for $m$ obtained for several SGRs and AXPs are in agreement with the observed range $10^{34}{\rm emu}\leq m \leq10^{36}{\rm emu}$ of isolated and polar magnetic white dwarfs. This result together with the fact that for {\it magnetic} white dwarfs $B\sim(10^6-10^8)$ G their magnetic dipole moments are almost independent of the star rotation period ($10^{4}\lesssim P \lesssim10^{6} {\rm s}$) - a phenomenology not shared by neutron stars pulsars - suggests a possible {\it magnetic} white dwarf nature for some of SGRs/AXPs that have much smaller periods ($P\sim 10$ s).

astro-ph.SR↗

arXiv2013

Luminosity function of GRBs

The luminosity function (LF) statistics applied to the BATSE GRBs (sources of GUSBAD catalog) is the theme approached in this work. The LF is a strong statistical tool to extract useful information from astrophysical samples, where the key point of this statistical analysis is in the detector sensitivity, where we have performed careful analysis. We applied the tool of the LF statistics to three GRB classes predicted by the Fireshell model. One of the main differences between the Fireshell and Fireball models is the explanation of the GRB prompt emission, where the first claims that it is divided in two physical processes: 1) transparency of a optically thick Freshell producing the P-GRB emission and 2) interaction of a relativistic shell (composed by baryons electron-positron-photon) against the CBM (circumburst medium), producing the emission so-called extended afterglow peak (EAP). However, the Fireball model, the most quoted one, claims that the prompt emission is caused by interactions among several relativistic shells (composed by electron-positron-photon) with different Lorentz factors, a process called internal shocks. The transparency produces a strong short emission in the beginning, called P-GRB. Thus, to the Fireshell model, the prompt emission is P-GRB plus EAP. We produced, by LF statistics, predicted distributions of: peak flux, redshift and peak luminosity for the three GRB classes predicted by Fireshell model, we also used three GRB rates. We looked for differences among the distributions, and in fact we found. We performed a comparison between the distributions predicted and observed (with and without redshifts), where we had to build a list with 220 GRBs with known redshifts. We also estimated the effects of the Malmquist bias in all samples, and we looked for a correlation between the isotropic luminosity and the Band peak spectral energy.

astro-ph.HE↗

arXiv2013

A different nature between the radio AXPs in comparison to the others SGRs/AXPs

SGRs/AXPs are considered a subclass of pulsars powered by magnetic energy and not by rotation, as normal radio pulsars. They are understood as strongly magnetized neutron star, with large periods of rotation $P\sim(2-12)$ s, and large spin-down, with typical $\dot{P}\sim(10^{-13}-10^{-10})$ s/s in contrast to $\dot{P}\sim10^{-15}$ s for ordinary pulsars. Their persistent X-ray luminosity, as well as the bursts and flares typical of these sources, are instead believed to be powered by the decay of their ultrastrong magnetic field. SGRs/AXPs typically have a larger X-ray luminosity that can not be explained by their spin-down luminosity ($L_X>\dot{E}_{\rm rot}$), unlike rotation-powered pulsars. However, the recent discovery of radio-pulsed emission in four of this class of sources, where the spin-down rotational energy lost $\dot{E}_{\rm rot}$ is larger than the X-ray luminosity $L_X$ during the quiescent state - as in normal pulsars - opens the question of the nature of these radio sources in comparison to the others of this class. In this contribution, we show that the radio SGRs/AXPs obey a linear log-log relation between $L_X$ and $\dot{E}_{\rm rot}$, very similar to the one satisfied by X-ray and gamma-ray neutron star pulsars, suggesting their neutron star nature. Furthermore, we show that almost all the high-B pulsars are also near the line found for the radio AXPs. In contrast, for almost all the others SGRs/AXPs, $\log L_X$ does not vary too much as function of $\log \dot{E}_{\rm rot}$, a phenomenology not shared by X-ray neutron star pulsars, suggesting a different nature for these sources.

astro-ph.SR↗

arXiv2013

Influence of pions on the hadron-quark phase transition

In this work we present the features of the hadron-quark phase transition diagrams in which the pions are included in the system. To construct such diagrams we use two different models in the description of the hadronic and quark sectors. At the quark level, we consider two distinct parametrizations of the Polyakov-Nambu-Jona-Lasinio (PNJL) models. In the hadronic side, we use a well known relativistic mean-field (RMF) nonlinear Walecka model. We show that the effect of the pions on the hadron-quark phase diagrams is to move the critical end point (CEP) of the transitions lines. Such an effect also depends on the value of the critical temperature (T_0) in the pure gauge sector used to parametrize the PNJL models. Here we treat the phase transitions using two values for T_0, namely, T_0 = 270 MeV and T_0 = 190 MeV. The last value is used to reproduce lattice QCD data for the transition temperature at zero chemical potential.

hep-ph↗

arXiv2013

Describing SGRs/AXPs as fast and magnetized white dwarfs

Over the last decade, observational evidence has mounted that SGRs/AXPs belong to a particular class of pulsars. Furthermore, fast and very magnetic white dwarfs have been observed, and recently two SGRs with low magnetic fields $B\sim(10^{12}-10^{13})$ G, namely SGR 0418+5729 and Swift J1822.3-1606 were discovered with a period of $P\sim9.08$ s and $P\sim8.44$ s, respectively. These new discoveries opens the question of the nature of SGRs/AXPs, emerging alternative scenarios, in particular the white dwarf (WD) pulsar model. These astronomical observations have based an alternative description of the SGRs/AXPs expressed on rotating highly magnetized and very massive WDs. In this new description, several observational properties are easy understood and well explained as a consequence of the large radius of a massive white dwarf that manifests a new scale of mass density, moment of inertia, rotational energy, and magnetic dipole moment in comparison with the case of neutron stars. In this contribution, we will show that these recent discoveries of SGRs with low magnetic field share some properties with the recent detected fast WD pulsar AE Aquarii, and also with RXJ 0648.0-4418, and EUVE J0317-855, supporting the understanding of at least these SGRs with low-B as belonging to a class of very fast and magnetic massive WDs. Furthermore, these recent astronomical observations suggest that we should revisit the real nature of AXP/SGRs: are they really magnetars or very fast and massive white dwarfs?

astro-ph.SR↗

arXiv2013

Do SGRs/AXPs and radio AXPs have the same nature?

SGRs/AXPs are assumed to be a class of neutron stars (NS) powered by magnetic energy and not by rotation, as normal radio pulsars. However, the recent discovery of radio-pulsed emission in four of this class of sources, where the spin-down rotational energy lost $\dot{E}_{\rm rot}$ is larger than the X-ray luminosity $L_X$ during the quiescent state - as in normal pulsars - opens the question of the nature of these radio AXPs in comparison to the others of this class. In this work, we show that the radio AXPs obey a linear log-log relation between $L_X$ and $\dot{E}_{\rm rot}$, very similar to the one of normal X-ray pulsars, a correlation not seen for the others SGRs/AXPs. This result suggests a different nature between the radio AXPs comparing to the others SGRs/AXPs.

astro-ph.SR↗

arXiv2013

Spin and pseudospin symmetries of the Dirac equation with confining central potentials

We derive the node structure of the radial functions which are solutions of the Dirac equation with scalar $S$ and vector $V$ confining central potentials, in the conditions of exact spin or pseudospin symmetry, i.e., when one has $V=\pm S+C$, where $C$ is a constant. We show that the node structure for exact spin symmetry is the same as the one for central potentials which go to zero at infinity but for exact pseudospin symmetry the structure is reversed. We obtain the important result that it is possible to have positive energy bound solutions in exact pseudospin symmetry conditions for confining potentials of any shape, including naturally those used in hadron physics, from nuclear to quark models. Since this does not happen for potentials going to zero at large distances, used in nuclear relativistic mean-field potentials or in the atomic nucleus, this shows the decisive importance of the asymptotic behavior of the scalar and vector central potentials on the onset of pseudospin symmetry and on the node structure of the radial functions. Finally, we show that these results are still valid for negative energy bound solutions for anti-fermions.

nucl-th↗

arXiv2012

Similarities of SGRs with low magnetic field and white dwarf pulsars

Some of the most interesting types of astrophysical objects that have been intensively studied in the recent years are the Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) seen usually as neutron stars pulsars with super strong magnetic fields. However, in the last two years two SGRs with low magnetic fields have been detected. Moreover, fast and very magnetic white dwarf pulsars have also been observed in the last years. Based on these new pulsar discoveries, white dwarf pulsars have been proposed as an alternative explanation to the observational features of SGRs and AXPs. Here we present several properties of these SGRs/AXPs as WD pulsar, in particular the surface magnetic field and the magnetic dipole momentum.

astro-ph.SR↗

arXiv2012

Vector interaction strength in Polyakov-Nambu-Jona-Lasinio models from hadron-quark phase diagrams

We estimate the vector interaction strength of the Polyakov-Nambu-Jona-Lasinio (PNJL) parametrizations, assuming that its transition curves should be as close as possible of the recently studied RMF-PNJL hadron-quark phase diagrams. Such diagrams are obtained matching relativistic mean-field hadronic models, and the PNJL quark ones. By using this method we found for the magnitude of the vector interaction, often treated as a free parameter, a range of 7.66 GeV$^{-2}\lesssim G_V \lesssim 16.13$ GeV$^{-2}$, or equivalently, $1.52 \lesssim G_V/G_s \lesssim 3.2$, with $G_s$ being the scalar coupling constant of the model. These values are compatible but restricts the range of 4 GeV$^{-2}\lesssim G_V \lesssim 19$ GeV$^{-2}$, recently obtained from lattice QCD data through a different mean-field model approach.

nucl-th↗