arXiv subjects

J. C. N. de Araujo

Publications and source records attributed to J. C. N. de Araujo.

At least 19 recordsLinked to original sources

arXiv2026

Neutron stars in $f(Q) = Q +ξQ^2$ gravity

Modified theories of gravity based on the symmetric teleparallel framework have recently attracted considerable attention as viable alternatives to General Relativity. In this context, f(Q) gravity, in which the gravitational interaction is encoded in the nonmetricity scalar Q, provides a consistent geometrical formulation that differs from the standard curvature-based description. In this work, we investigate the structure of neutron stars within a family of f(Q) gravity models by employing realistic equations of state, namely FPS, SLy, ENG and MPA1. Using the covariant formulation of f(Q) gravity, we derive the corresponding Tolman-Oppenheimer-Volkoff equations and apply them to model compact stellar configurations. Numerical integration of the field equations provides the mass-radius relations and the maximum masses supported by each equation of state, enabling a direct comparison with current observational constraints. Furthermore, we analyze the behavior of the nonmetricity scalar both inside and outside the stellar object, providing additional insight into the gravitational structure of compact stars in this framework.

gr-qc↗

arXiv2024

Prospects for the Observation of Continuous Gravitational Waves from Deformed Fast-spinning White Dwarfs

There has been a growing interest within the astrophysics community in highly magnetized and fast-spinning white dwarfs (WDs), commonly referred to as HMWDs. WDs with these characteristics are quite uncommon and possess magnetic fields $\geqslant 10^6$ G, along with short rotation periods ranging from seconds to just a few minutes. Based on our previous work, we analyze the emission of Gravitational Waves (GWs) in HMWDs through two mechanisms: matter accretion and magnetic deformation, which arise due to the asymmetry surrounding the star's rotational axis. Here, we perform a thorough self-consistent analysis, accounting for rotation and employing a realistic equation of state to investigate the stability of stars. Our investigation focuses on the emission of gravitational radiation from six rapidly spinning WDs: five of them are situated within binary systems, while one is an AXP, proposed as a magnetic accreting WD. Furthermore, we apply the matter accretion mechanism alongside the magnetic deformation mechanism to assess the influence of one process on the other. Our discoveries indicate that these WDs could potentially act as GW sources for BBO and DECIGO, depending on specific parameters, such as their mass, the angle ($α$) between the magnetic and rotational axes, and the accumulated mass ($δm$) at their magnetic poles, which is influenced by the effect of matter accretion. However, detecting this particular class of stars using the LISA and TianQin space detectors seems unlikely due to the challenging combination of parameters such as a large $δm$, a large $α$ angle and a small WD mass value.

astro-ph.SR↗

arXiv2023

On the optical transients from double white-dwarf mergers

Double white-dwarf (DWD) mergers are relevant astrophysical sources expected to produce massive, highly-magnetized WDs, supernovae (SNe) Ia, and neutron stars (NSs). Although they are expected to be numerous sources in the sky, their detection has evaded the most advanced transient surveys. This article characterizes the optical transient expected from DWD mergers in which the central remnant is a stable (sub-Chandrasekhar) WD. We show that the expansion and cooling of the merger's dynamical ejecta lead to an optical emission peaking at $1$-$10$ d post-merger, with luminosities of $10^{40}$-$10^{41}$ erg s$^{-1}$. We present simulations of the light-curves, spectra, and the color evolution of the transient. We show that these properties, together with the estimated rate of mergers, are consistent with the absence of detection, e.g., by The Zwicky Transient Facility (ZTF). More importantly, we show that the Legacy Survey of Space and Time (LSST) of the Vera C. Rubin Observatory will likely detect a few/several hundred per year, opening a new window to the physics of WDs, NSs, and SN Ia.

astro-ph.SR↗

arXiv2023

Solving Tolman-Oppenheimer-Volkoff equations in $f(T)$ gravity: a novel approach applied to polytropic equations of state

The Teleparallel Theory is an alternative theory of gravity equivalent to General Relativity (GR) and with non-vanishing torsion $T$. Some extensions of this theory, the so-called $f(T)$ models, have been subject of many recent works. The purpose of our work in the end is to consider recent results for a specific family of $f(T)$ models by using their corresponding Tolman-Oppenheimer-Volkof to describe compact objects such as neutron stars. By performing numerical calculations, it is possible to find, among other things, the maximum mass allowed by the model for a neutron star for a given equation of state (EOS), which would also allow us to evaluate which models are in accordance with observations. To begin with, the present work, the second in the series, considers polytropic EOSs since they can offer a simpler and satisfactory description for the compact objects. In addition, with these EOSs, we can already assess how different the $f(T)$ theories are in relation to GR with respect to the stellar structure. The results already known to GR must be reproduced to some extent and, eventually, we can find models that allow higher maximum masses than Relativity itself, which could explain, for example, the secondary component of the event GW190814. This particular issue will be subject of a forthcoming paper, the third in the series, where realistic EOSs are considered.

gr-qc↗

arXiv2022

Solving Tolman-Oppenheimer-Volkoff equations in $f(T)$ gravity: a novel approach

The torsion models have stood out among the proposals for an alternative description of gravity. The simplest of them, the Teleparallel theory, is equivalent to General Relativity and there are many studies that seek to study its extension to more general functions of the torsion $T$. The purpose of {our study } is to consider a family of $f(T)$ models and apply their corresponding Tolman-Oppenheimer-Volkof equations to compact objects such as neutron stars. Consequently, through a numerical analysis, calculate, among other things, the maximum mass allowed by the model for a neutron star, which would also allow us to evaluate which models are in accordance with observations. In the present paper, the first in the series, we show explicitly the set of equations that must be solved, and how to solve it, in order to model compact stars in $f(T)$ gravity without the need to adopt any particular form for the metric functions or consider any perturbative approach, as has been done in some works in the literature.

gr-qc↗

arXiv2022

The double white dwarf merger progenitors of SDSS J2211+1136 and ZTF J1901+1458

Double white dwarf (DWD) mergers are possibly the leading formation channel of massive, rapidly rotating, high-field magnetic white dwarfs (HFMWDs). However, the direct link connecting a DWD merger to any observed HFMWD is still missing. We here show that the HFMWDs SDSS J221141.80+113604.4 (hereafter J2211+1136) and ZTF J190132.9+145808.7 (hereafter J1901+1458), might be DWD merger products. J2211+1136 is a $1.27\, M_\odot$ WD with a rotation period of $70.32$ s and a surface magnetic field of $15$ MG. J1901+1458 is a $1.327$--$1.365\, M_\odot$ WD with a rotation period of $416.20$ s, and a surface magnetic field in the range $600$--$900$ MG. With the assumption of single-star evolution, the currently measured WD masses and surface temperatures, the cooling ages of J2211+1136 and J1901+1458 are, respectively, $2.61$--$2.85$ Gyr and $10$--$100$ Myr. We hypothesize that these WDs are DWD merger products and compute the evolution of the post-merged configuration formed by a central WD surrounded by a disk. We show that the post-merger system evolves through three phases depending on whether accretion, mass ejection (propeller), or magnetic braking dominates the torque onto the central WD. We calculate the time the WD spends in each of these phases and obtain the accretion rate and disk mass for which the WD rotational age, i.e., the total time elapsed since the merger to the instant where the WD central remnant reaches the current measured rotation period, agrees with the estimated WD cooling age. We infer the mass values of the primary and secondary WD components of the DWD merger that lead to a post-merger evolution consistent with the observations.

astro-ph.SR↗

arXiv2019

Reconstruction of energy conditions from observations and implications for extended theories of gravity

The attempt to describe the recent accelerated expansion of the Universe includes different propositions for dark energy models and modified gravity theories. Establish their features in order to discriminate and even rule out part of these models using observational data is a fundamental issue of cosmology. In the present work we consider a class of extended theories of gravity (ETG) that are minimally coupled to the ordinary matter fields. In this context, and assuming a homogeneous and isotropic spacetime, we derive the energy conditions for this ETG class. We then put constraints on these conditions using a model-independent approach to reconstruct the deceleration function along with the Joint Light-curve Analysis (JLA) supernova sample, 11 baryon acoustic oscillation and 22 cosmic-chronometer measurements. We also consider an additional condition imposing the strong energy condition only on the ordinary matter. This is to guarantee the presence of only attractive matter in the energy-momentum tensor, at least in the redshift range of the observations, i.e., the recent accelerated expansion of the Universe is due solely to the modifications in the gravity theory. The main result of this work is a general reconstruction of the energy conditions valid for every considered ETG.

astro-ph.CO↗

arXiv2017

Cosmological constant constraints from observation-derived energy condition bounds and their application to bimetric massive gravity

Among the various possibilities to probe the theory behind the recent accelerated expansion of the universe, the energy conditions (ECs) are of particular interest, since it is possible to confront and constrain the many models, including different theories of gravity, with observational data. In this context, we use the ECs to probe any alternative theory whose extra term acts as a cosmological constant. For this purpose, we apply a model-independent approach to reconstruct the recent expansion of the universe. Using Type Ia supernova, baryon acoustic oscillations and cosmic-chronometer data, we perform a Markov Chain Monte Carlo analysis to put constraints on the effective cosmological constant $Ω^0_{\rm eff}$. By imposing that the cosmological constant is the only component that possibly violates the ECs, we derive lower and upper bounds for its value. For instance, we obtain that $0.59 < Ω^0_{\rm eff} < 0.91$ and $0.40 < Ω^0_{\rm eff} < 0.93$ within, respectively, $1σ$ and $3σ$ confidence levels. In addition, about 30\% of the posterior distribution is incompatible with a cosmological constant, showing that this method can potentially rule it out as a mechanism for the accelerated expansion. We also study the consequence of these constraints for two particular formulations of the bimetric massive gravity. Namely, we consider the Visser's theory and the Hassan and Roses's massive gravity by choosing a background metric such that both theories mimic General Relativity with a cosmological constant. Using the $Ω^0_{\rm eff}$ observational bounds along with the upper bounds on the graviton mass we obtain constraints on the parameter spaces of both theories.

gr-qc↗

arXiv2016

Primordial Gravitational Waves in Running Vacuum Cosmologies

We investigate the cosmological production of gravitational waves in a nonsingular flat cosmology powered by a "running vacuum" energy density described by $ρ_Λ\equivρ_Λ(H)$, a phenomenological expression potentially linked with the renormalization group approach in quantum field theory in curved spacetimes. The model can be interpreted as a particular case of the class recently discussed by Perico et al. (Phys. Rev. D {\bf 88}, 063531, 2013) which is termed complete in the sense that the cosmic evolution occurs between two extreme de Sitter stages (early and late time de Sitter phases). {The gravitational wave equation is derived and its time-dependent part numerically integrated since the primordial de Sitter stage. The generated spectrum of gravitons is also compared with the standard calculations where an abrupt transition, from the early de Sitter to the radiation phase, is usually assumed.} It is found that the stochastic background of gravitons is very similar to the one predicted by the cosmic concordance model plus inflation except at higher frequencies ($ν\gtrsim 100$ kHz). This remarkable signature of a "running vacuum" cosmology combined with the proposed high frequency gravitational wave detectors and measurements of the CMB polarization (B-modes) may provide a new window to confront more conventional models of inflation.

astro-ph.CO↗

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

Point Particle Binary System with Components of Different Masses in the Linear Regime of the Characteristic Formulation of General Relativity

A study of binary systems composed of two point particles with different masses in the linear regime of the characteristic formulation of general relativity in a Minkowski background is provided. The present paper generalizes a previous study find in the literature. The boundary conditions at the world tubes generated by the particle's orbits are explored, when the metric variables are decomposed in spin-weighted spherical harmonics. The power lost by the emission of gravitational waves is computed using the News Bondi's functions. The power found is nothing but the well-known result for the power obtained by Peters and Mathews in a different approach. This agreement shows the consistency of the approach considered here. The contribution to the gravitational radiation of several multipole terms is also shown.

gr-qc↗

arXiv2016

Gravitational radiation by point particle eccentric binary systems in the linearised characteristic formulation of general relativity

We study a binary system composed of point particles of unequal masses in eccentric orbits in the linear regime of the characteristic formulation of general relativity, generalising a previous study found in the literature in which a system of equal masses in circular orbits is considered. We also show that the boundary conditions on the time-like world tubes generated by the orbits of the particles can be extended beyond circular orbits. Concerning the power lost by the emission of gravitational waves, it is directly obtained from the Bondi's News function. It is worth stressing that our results are completely consistent, because we obtain the same result for the power derived by Peters and Mathews, in a different approach, in their seminal paper of 1963. In addition, the present study constitutes a powerful tool to construct extraction schemes in the characteristic formalism to obtain the gravitational radiation produced by binary systems during the inspiralling phase.

gr-qc↗

arXiv2015

Master equation solutions in the linear regime of characteristic formulation of general relativity

From the field equations in the linear regime of the characteristic formulation of general relativity, Bishop, for a Schwarzschild's background, and Mädler, for a Minkowski's background, were able to show that it is possible to derive a fourth order ordinary differential equation, called master equation, for the $J$ metric variable of the Bondi-Sachs metric. Once $β$, another Bondi-Sachs potential, is obtained from the field equations, and $J$ is obtained from the master equation, the other metric variables are solved integrating directly the rest of the field equations. In the past, the master equation was solved for the first multipolar terms, for both the Minkowski's and Schwarzschild's backgrounds. Also, Mädler recently reported a generalisation of the exact solutions to the linearised field equations when a Minkowski's background is considered, expressing the master equation family of solutions for the vacuum in terms of Bessel's functions of the first and the second kind. Here, we report new solutions to the master equation for any multipolar moment $l$, with and without matter sources in terms only of the first kind Bessel's functions for the Minkowski, and in terms of the Confluent Heun's functions (Generalised Hypergeometric) for radiative (nonradiative) case in the Schwarzschild's background. We particularize our families of solutions for the known cases for $l=2$ reported previously in the literature and find complete agreement, showing the robustness of our results.

gr-qc↗

arXiv2015

The Gravitational Wave Background From Coalescing Compact Binaries: A New Method

Gravitational waves are perturbations in the spacetime that propagate at the speed of light. The study of such phenomenon is interesting because many cosmological processes and astrophysical objects, such as binary systems, are potential sources of gravitational radiation and can have their emissions detected in the near future by the next generation of interferometric detectors. Concerning the astrophysical objects, an interesting case is when there are several sources emitting in such a way that there is a superposition of signals, resulting in a smooth spectrum which spans a wide range of frequencies, the so-called stochastic background. In this paper, we are concerned with the stochastic backgrounds generated by compact binaries (i.e. binary systems formed by neutron stars and black holes) in the coalescing phase. In particular, we obtain such backgrounds by employing a new method developed in our previous studies.

astro-ph.CO↗

arXiv2015

Stochastic Background of Gravitational Waves Generated by Compact Binary Systems

Binary Systems are the most studied sources of gravitational waves. The mechanisms of emission and the behavior of the orbital parameters are well known and can be written in analytic form in several cases. Besides, the strongest indication of the existence of gravitational waves has arisen from the observation of binary systems. On the other hand, when the detection of gravitational radiation becomes a reality, one of the observed pattern of the signals will be probably of stochastic background nature, which are characterized by a superposition of signals emitted by many sources around the universe. Our aim here is to develop an alternative method of calculating such backgrounds emitted by cosmological compact binary systems during their periodic or quasiperiodic phases. We use an analogy with a problem of Statistical Mechanics in order to perform this sum as well as taking into account the temporal variation of the orbital parameters of the systems. Such a kind of background is of particular importance since it could well form an important foreground for the planned gravitational wave interferometers DECI-Hertz Interferometer Gravitational wave Observatory (DECIGO), Big Bang Observer (BBO), Laser Interferometer Space Antenna (LISA) or Evolved LISA (eLISA), Advanced Laser Interferometer Gravitational-Wave Observatory (ALIGO) and Einstein Telescope (ET).

astro-ph.CO↗

arXiv2015

A New Method to Calculate the Stochastic Background of Gravitational Waves Generated by Compact Binaries

In the study of gravitational waves (GWs), the stochastic background generated by compact binary systems are among the most important kinds of signals. The reason for such an importance has to do with their probable detection by the interferometric detectors [such as the Advanced LIGO (ALIGO) and Einstein Telescope (ET)] in the near future. In this paper we are concerned with, in particular, the stochastic background of GWs generated by double neutron star (DNS) systems in circular orbits during their periodic and quasi--periodic phases. Our aim here is to describe a new method to calculate such spectra, which is based on an analogy with a problem of Statistical Mechanics. Besides, an important characteristic of our method is to consider the time evolution of the orbital parameters.

astro-ph.CO↗

arXiv2015

Stochastic Background of Gravitational Waves Generated by Eccentric Neutron Star Binaries

Binary systems emit gravitational waves in a well-known pattern; for binaries in circular orbits, the emitted radiation has a frequency that is twice the orbital frequency. Systems in eccentric orbits, however, emit gravitational radiation in the higher harmonics too. In this paper, we are concerned with the stochastic background of gravitational waves generated by double neutron star systems of cosmological origin in eccentric orbits. We consider in particular the long-lived systems, that is, those binaries for which the time to coalescence is longer than the Hubble time ($\sim 10$Gyr). Thus, we consider double neutron stars with orbital frequencies ranging from $10^{-8}$ to $2\times 10^{-6}$Hz. Although in the literature some papers consider the spectra generated by eccentric binaries, there is still space for alternative approaches for the calculation of the backgrounds. In this paper, we use a method that consists in summing the spectra that would be generated by each harmonic separately in order to obtain the total background. This method allows us to clearly obtain the influence of each harmonic on the spectra. In addition, we consider different distribution functions for the eccentricities in order to investigate their effects on the background of gravitational waves generated. At last, we briefly discuss the detectability of this background by space-based gravitational wave antennas and pulsar timing arrays.

astro-ph.CO↗

arXiv2013

Searching for Gravitational Waves with a Geostationary Interferometer

We analyze the sensitivities of a geostationary gravitational wave interferometer mission operating in the sub-Hertz band. Because of its smaller armlength, in the lower part of its accessible frequency band ($10^{-4} - 2 \times 10^{-2}$ Hz) our proposed Earth-orbiting detector will be less sensitive, by a factor of about seventy, than the Laser Interferometer Space Antenna (LISA) mission. In the higher part of its band instead ($2 \times 10^{-2} - 10$ Hz), our proposed interferometer will have the capability of observing super-massive black holes (SMBHs) with masses smaller than $\sim 10^{6}$ M$_{\odot}$. With good event rates for these systems, a geostationary interferometer will be able to accurately probe the astrophysical scenarios that account for their formation.

gr-qc↗