arXiv subjects

Jaziel G. Coelho

Publications and source records attributed to Jaziel G. Coelho.

35 records · Page 2Linked to original sources

arXiv2020

Weighing massive neutron star with screening gravity: A look on PSR J0740+6620 and GW190814 secondary component

Neutron stars (NSs) are excellent natural laboratories to constrain gravity on strong field regime and nuclear matter in extreme conditions. Motivated by the recent discovery of a compact object with $2.59^{+0.08}_{-0.09} M_\odot$ in the binary merger GW190814, if this object was a NS, it serves as a strong constraint on the NS equation of state (EoS), ruling out several soft EoSs favored by GW170817 event. In this work, we revisit the question of the maximum mass of NSs considering a chameleon screening (thin-shell effect) on the NS mass-radius relation, where the microscopic physics inside the NS is given by realistic soft EoSs. We find that from appropriate and reasonable combination of modified gravity, rotation effects and realistic soft EoSs, that it is possible to achieve high masses and explain GW190814 secondary component, and in return also NSs like PSR J0740+6620 (the most NS massive confirmed to date). It is shown that gravity can play an important role in estimating maximum mass of NSs, and even with soft EoSs, it is possible to generate very high masses. Therefore, in this competition on the hydrostatic equilibrium between gravity and EoS, some soft EoSs, in principle, cannot be completely be ruled out without first taking into account gravitational effects.

astro-ph.HE↗

arXiv2020

Gravitational waves from SGRs and AXPs as fast-spinning white dwarfs

Gravitational waves (GWs) emission due to magnetic deformation mechanism is applied for Soft Gamma Repeaters (SGRs) and Anomalous X-Ray Pulsars(AXPs), described as fast-spinning and magnetized white dwarfs (WDs). The emission is caused by the asymmetry around the rotation axis of the star generated by its own intense magnetic field. Thus, for the first time in the literature, it is estimated the GWs counterpart for SGRs/AXPs described as WD pulsars. We find that some SGRs/AXPs can be observed by the space detectors BBO and DECIGO. In particular, 1E 1547.0-5408 and SGR 1806-20 could be detected in 1 year of observation, whereas SGR 1900+14, CXOU J171405.7-381031, Swift J1834.9-0846 and SGR 1627-41 could be observed with a 5-year observation time. We also found that SGRs/AXPs as highly magnetized neutron stars are far below the sensitivity curves of BBO and DECIGO. This result indicates that a possible detection of continuous GWs originated from these objects would corroborate the WD pulsar model.

astro-ph.SR↗

arXiv2020

A magnetic white-dwarf accretion model for the anomalous X-ray pulsar 4U 0142+61

The quiescent emission of the anomalous X-ray pulsar (AXP) 4U 0142+61 extends over a broad range of energy, from radio up to hard X-rays. In particular, this object is unique among soft gamma-ray repeaters (SGRs) and AXPs in presenting simultaneously mid-infrared emission and pulsed optical emission. In spite of the many propositions to explain this wide range of emission, it still lacks one that reproduces all the observations. Filling this gap, we present a model to reproduce the quiescent spectral energy distribution of 4U 0142+61 from mid-infrared up to hard X-rays using plausible physical components and parameters. We propose that the persistent emission comes from a magnetic accreting white dwarf (WD) surrounded by a debris disk. This model assumes that: (i) the hard X-rays are due to the bremsstrahlung emission from the post-shock region of the accretion column; (ii) the soft X-rays are originated by hot spots on the WD surface; and (iii) the optical and infrared emissions are caused by an optically thick dusty disk, the WD photosphere, and the tail of the postshock region emission. In this scenario, the fitted model parameters indicate that 4U 0142+61 harbors a fast-rotator magnetic near-Chandrasekhar WD, which is very hot and hence young. Such a WD can be the recent outcome of a merger of two less massive WDs. In this case, 4U 0142+61 can evolve to an SN Ia and hence can give hints of the origin of these important astrophysical events. Additionally, we also present a new estimate of 4U 0142+61 distance, 3.78 (errors: +0.12 / -0.18 kpc), based on the measured Hydrogen column density and new interstellar extinction 3D maps.

astro-ph.HE↗

arXiv2020

Gravitational waves from fast-spinning white dwarfs

Two mechanisms of gravitational waves (GWs) emission in fast-spinning white dwarfs (WDs) are investigated: accretion of matter and magnetic deformation. In both cases, the GW emission is generated by an asymmetry around the rotation axis of the star. However, in the first case, the asymmetry is due to the amount of accreted matter on the magnetic poles, while in the second case it is due to the intense magnetic field. We have estimated the GW amplitude and luminosity for three binary systems that have a fast-spinning magnetized WD, namely, AE Aquarii, AR Scorpii and RX J0648.0-4418. We find that, for the first mechanism, the systems AE Aquarii and RX J0648.0-4418 can be observed by the space detectors BBO and DECIGO if they have an amount of accreted mass of $δm \geq 10^{-5}M_{\odot }$. For the second mechanism, the three systems studied require that the WD have a magnetic field above $\sim 10^{9}$ G to emit GWs that can be detected by BBO. We also verified that, in both mechanisms, the gravitational luminosity has an irrelevant contribution to the spindown luminosity of these three systems. Therefore, other mechanisms of energy emission are needed to explain the spindown of these objects.

astro-ph.SR↗

arXiv2019

Evidence for a multipolar magnetic field in SGR J1745-2900 from X-ray light-curve analysis

SGR J1745-2900 was detected from its outburst activity in April 2013 and it was the first soft gamma repeater (SGR) detected near the center of the Galaxy (Sagittarius A$^*$). We use 3.5-year Chandra X-ray light-curve data to constrain some neutron star (NS) geometric parameters. We assume that the flux modulation comes from hot spots on the stellar surface. Our model includes the NS mass, radius, a maximum of three spots of any size, temperature and positions, and general relativistic effects. We find that the light-curve of SGR J1745-2900 could be described by either two or three hot spots. The ambiguity is due to the small amount of data, but our analysis suggests that one should not disregard the possibility of multi-spots (due to a multipolar magnetic field) in highly magnetized stars. For the case of three hot spots, we find that they should be large and have angular semi-apertures ranging from 16-67 degrees. The large size found for the spots points to a magnetic field with a nontrivial poloidal and toroidal structure (in accordance with magnetohydrodynamics investigations and NICER's recent findings for PSR J0030+0451) and is consistent with the small characteristic age of the star. Finally, we also discuss possible constraints on the mass and radius of SGR J1745-2900 and briefly envisage possible scenarios accounting for the 3.5-year evolution of SGR J1745-2900 hot spots.

astro-ph.SR↗

arXiv2019

Gravitational Waves From Pulsars Due To Their Magnetic Ellipticity

We discuss some aspects of de Araujo, Coelho & Costa (2016, 2017) concerning the role of a time dependent magnetic ellipticity on the pulsars' braking indices and on the putative gravitational waves (GWs) these objects can emit. Since only nine of more than 2000 known pulsars have accurately measured braking indices, it is of interest to extend this study to all known pulsars, in particular as regards GW generation. In summary, our results show a pessimistic prospect for the detection of GWs generated by these pulsars, even for advanced detectors such as aLIGO and AdVirgo, and the planned Einstein Telescope, would not be able to detect these pulsar, if the ellipticity has magnetic origin.

astro-ph.HE↗

arXiv2019

Born-Infeld magnetars: larger production of gravitational waves due to larger toroidal magnetic fields

We discuss some aspects of Pereira et al. (2018) concerning magnetars described by nonlinear theories of the electromagnetism and make the case for the Born-Infeld Lagrangian. We focus on the increase of toroidal magnetic fields in these systems with respect to ordinary magnetars and the subsequent increase of gravitational wave production. In summary, nonlinear theories of the electromagnetism would make it more likely for the detection of gravitational waves with future detectors, which could constrain nonlinear aspects of electrodynamics not entirely possible on Earth-based particle accelerators.

astro-ph.HE↗

arXiv2018

Born-Infeld magnetars: larger than classical toroidal magnetic fields and implications for gravitational-wave astronomy

Magnetars are neutron stars presenting bursts and outbursts of X- and soft-gamma rays that can be understood with the presence of very large magnetic fields. Thus, nonlinear electrodynamics should be taken into account for a more accurate description of such compact systems. We study that in the context of ideal magnetohydrodynamics and make a realization of our analysis to the case of the well-known Born-Infeld (BI) electromagnetism in order to come up with some of its astrophysical consequences. We focus here on toroidal magnetic fields as motivated by already known magnetars with low dipolar magnetic fields and their expected relevance in highly magnetized stars. We show that BI electrodynamics leads to larger toroidal magnetic fields when compared to Maxwell's electrodynamics. Hence, one should expect higher production of gravitational waves (GWs) and even more energetic giant flares from nonlinear stars. Given current constraints on BI's scale field, giant flare energetics and magnetic fields in magnetars, we also find that the maximum magnitude of magnetar ellipticities should be $10^{-6}-10^{-5}$. Besides, BI electrodynamics may lead to a maximum increase of order $10\%-20\%$ of the GW energy radiated from a magnetar when compared to Maxwell's, while much larger percentages may arise for other physically motivated scenarios. Thus, nonlinear theories of the electromagnetism might also be probed in the near future with the improvement of GW detectors.

astro-ph.SR↗

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↗

arXiv2017

Gravitational waves from pulsars in the context of magnetic ellipticity

In one of our previous articles we have considered the role of a time dependent magnetic ellipticity on pulsars' braking indices and on the putative gravitational waves these objects can emit. Since only nine of more than 2000 known pulsars have accurately measured braking indices, it is of interest to extend this study to all known pulsars, in particular to what concerns the gravitational waves generation. To do so, as shown in our previous article, we need to know some pulsars' observable quantities such as: periods and their time derivatives, and estimated distances to the Earth. Moreover, we also need to know the pulsars' masses and radii, for which, in here we are adopting current fiducial values. Our results show that the gravitational wave amplitude is at best $h \sim 10^{-28}$. This leads to a pessimistic prospect for the detection of gravitational waves generated by these pulsars, even for Advanced LIGO and Advanced Virgo, and the planned Einstein Telescope, whether the ellipticity has magnetic origin.

astro-ph.SR↗

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

Gravitational Waves from Pulsars and Their Braking Indices: The Role of a Time Dependent Magnetic Ellipticity

We study the role of time dependent magnetic ellipticities ($ε_{B}$) on the calculation of the braking index of pulsars. Moreover, we study the consequences of such a $ε_{B}$ on the amplitude of gravitational waves (GWs) generated by pulsars with measured braking indices. We show that, since the ellipticity generated by the magnetic dipole is extremely small, the corresponding amplitude of GWs is much smaller than the amplitude obtained via the spindown limit.

astro-ph.HE↗

arXiv2016

Gravitational waves from pulsars with measured braking index

We study the putative emission of gravitational waves (GWs) in particular for pulsars with measured braking index. We show that the appropriate combination of both GW emission and magnetic dipole brakes can naturally explain the measured braking index, when the surface magnetic field and the angle between the magnetic dipole and rotation axes are time dependent. Then we discuss the detectability of these very pulsars by aLIGO and the Einstein Telescope. We call attention to the realistic possibility that aLIGO can detect the GWs generated by at least some of these pulsars, such as Vela, for example.

astro-ph.HE↗

arXiv2016

Gravitational wave emission by the high braking index pulsar PSR J1640-4631

Recently, a braking index for the pulsar PSR J1640-4631 has been measured. With a braking index of $n = 3.15 \pm 0.03$, this pulsar has the highest braking index ever measured. As it is well known, a pure magnetic dipole brake yields $n = 3$, whereas a pure gravitational wave brake yields $n = 5$. Therefore, each of these mechanisms alone can not account for the braking index found for PSR J1640-4631. Here we show that such a braking index can be accounted for if the spindown model combines magnetic dipole and gravitational wave brakes. Then, we briefly discuss the detectability of this pulsar by aLIGO and the planned Einstein Telescope. In particular, we show that the amplitude of the gravitational wave that comes from our model is around a factor four lower than the amplitude modeled exclusively by gravitational wave energy loss. Another interesting outcome of our modeling is that we are able to obtain the ellipticity from the braking index and other pulsar parameters.

astro-ph.HE↗

arXiv2016

The influence of quantum vacuum friction on pulsars

We firstly revisit the energy loss mechanism known as quantum vacuum friction (QVF), clarifying some of its subtleties. Then we investigate the observables that could easily differentiate QVF from the classical magnetic dipole radiation for pulsars with braking indices (n) measured accurately. We show this is specially the case for the time evolution of a pulsar's magnetic dipole direction ($\dotϕ$) and surface magnetic field ($\dot{B}_0$). As it is well known in the context of the classic magnetic dipole radiation, $n<3$ would only be possible for positive $(\dot{B}_0/B_0 + \dotϕ/\tanϕ)$, which, for instance, leads to $\dot{B}_0>0$ ($\dotϕ>0$) when $ϕ$ ($B_0$) is constant. On the other hand, we show that QVF can result in very contrasting predictions with respect to the above ones. Finally, even in the case $\dot{B}_0$ in both aforesaid models for a pulsar has the same sign, for a given $ϕ$, we show that they give rise to different associated timescales, which could be another way to falsify QVF.

astro-ph.HE↗

arXiv2016

Magnetars and White Dwarf Pulsars

The Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are a class of pulsars understood as neutron stars (NSs) with super strong surface magnetic fields, namely $B\gtrsim10^{14}$ G, and for that reason are known as Magnetars. However, in the last years some SGRs/AXPs with low surface magnetic fields $B\sim(10^{12}-10^{13})$ G have been detected, challenging the Magnetar description. Moreover, some fast and very magnetic white dwarfs (WDs) have also been observed, and at least one showed X-Ray energy emission as an ordinary pulsar. Following this fact, an alternative model based on white dwarfs pulsars has been proposed to explain this special class of pulsars. In this model, AXPs and SGRs as dense and magnetized white dwarfs can have surface magnetic field $B\sim 10^{7}-10^{10}$ G and rotate very fast with frequencies $Ω\sim 1$ rad/s, consistent with the observed rotation periods $P\sim (2-12)$ s.

astro-ph.HE↗

arXiv2015

Stability of thin-shell interfaces inside compact stars

We use the thin-shell Darmois-Israel formalism to model and assess the stability of the interfaces separating phases, e.g. the core and the crust, within compact stars. We exemplify the relevance and non-triviality of this treatment in the simplest case of an incompressible star, in constant pressure phase transitions, and in the case of strange quark stars with crust.

gr-qc↗