Narrow Results

The present paper is devoted to the study of relativistic fermions evolving in a periodic magnetic field frozen in the magnetar's crust. The Heun's Double Confluent functions being the solutions to the Dirac-type equation, their series expansion allows us to write down the expressions of the current density components. For typical values of the magnetic induction, b₀~10¹⁵G, so that the magnetic length is much smaller than the extension of the crust, we get a nontrivial quantization law for the particle momentum along Oz.

Fast radio bursts (FRBs) are millisecond bursts of radio radiation at frequencies of about 1 GHz, recently discovered in pulsar surveys. They have not yet been definitively identified with any other astronomical object or phenomenon. The bursts are strongly dispersed, indicating passage through a high column density of low density plasma. The most economical interpretation is that this is the intergalactic medium, indicating that FRB are at “cosmological” distances with redshifts in the range 0.3–1.3. Their inferred brightness temperatures are as high as 10${}^{37}$ K, implying coherent emission by “bunched” charges, as in radio pulsars. I review the astronomical sites, objects and emission processes that have been proposed as the origin of FRB, with particular attention to soft gamma repeaters (SGRs) and giant pulsar pulses.

This work deals with the behavior of fermions in a configuration supposed to exist in magnetar’s corona. For a static magnetic induction parallel to a time-harmonic electric field, the solution to the U(1)-gauge invariant Dirac equation is expressed in terms of Laguerre polynomials and Mathieu’s functions of complex parameter. Using the Fourier series valid before the branching point, we are computing the conserved current density components.

Magnetars are comparatively young neutron stars with ultra-strong surface magnetic field in the range $10^{14}\text{–}10^{16}\mathrm{\text{G}}$. The old neutron stars have surface magnetic field somewhat less $\sim$$10^8\mathrm{\text{G}}$ which clearly indicates the decay of field with time. One possible way of magnetic field decay is by ambipolar diffusion. We describe the general procedure to solve for the ambipolar velocity inside the star core without any approximation. With a realistic model of neutron star, we determine the ambipolar velocity configuration inside the neutron star core and hence find the ambipolar decay rate, associated timescales and the magnetic energy dissipated which is consistent with the magnetar observations.

We put together many observational data of SGRs and AXPs and analyze them with the main purpose to remove contradiction between the real age of these objects and their characteristic times of period change. This work indicates that SGRs and AXPs are neutron stars with magnetic fields up to ~3×10¹⁴G at birth, which is less than the possible value in the existing magnetar model. These neutron stars undergo star-quakes and reconnection of magnetic field occurs from time to time. As a result of these processes plasma is ejected from the NS and propeller mechanism starts to work. Due to the propeller effect increases and τ decreases. Indeed, high values are observed in SGRs and in half of the AXPs. Then, for a long time NS looses its activity, its decreases, τ increases and rapid cooling begins. Each NS stage (AXP, SGR, dim) may occur once or several times until the spin period of the neutron star becomes P>10–12 s. At the transition period to the SGR and AXP stages, the magnetic field component perpendicular to the rotation axis may increase up to 2–3 times. Observational data and mainly the data of AXP1E1048-5937 and DRQNS RX J1308.8+2127 support this idea.

We use axisymmetric magnetohydrodynamic simulations to investigate the spinning-down of magnetars rotating in the propeller regime and moving supersonically through the interstellar medium. The simulations indicate that magnetars spin down rapidly due to this interaction, and faster than for the case of a non-moving star. We discuss this model with respect to soft gamma repeaters (SGRs) and the isolated neutron star candidates.

We briefly review main observational properties of fast radio bursts (FRBs) and discuss two most popular hypothesis for the explanation of these enigmatic intense millisecond radio flashes. FRBs most probably originate on extragalactic distances, and their rate on the sky is about a few thousand per day with fluences above $\sim$1Jyms (or with fluxes larger than few tenths of Jy). Two leading scenarios describing these events include strong flares of magnetars and supergiant pulses of young radio pulsars with large rotational energy losses, correspondingly. At the moment, it is impossible to choose between these models. However, new telescopes can help to solve the puzzle of FRBs in near future.

We study numerically a combined gravitational and nonlinear magnetic lensing effect on electromagnetic flux. A magnetar with a dipole magnetic field and background gravitational field is considered to deflect the light rays which passed through its magnetosphere. We assume a square wave front with sides $(150\times 150)\times 10^3$m as a grid with the $10^3$m dynamic step. At the nodes of this grid, the rays enter perpendicularly into the cubic area, which covers the main magnetic lensing region with a magnetar at the center. On the basis of general relativity (GR) and nonlinear vacuum electrodynamics, the distribution of rays by the deflection angle in the combined field of the magnetar was obtained. On the basis of the analysis of the obtained data, it is possible to assert that the magnetic field distorts the result of gravitational lensing. Therefore, the magnetar is regarded as a gravitational-magnetic lensing object, wherein the magnetic field induces axial distortion within $10^{-6\circ }$. These results are expected to be detectable by modern instruments.

We show that, by studying the arrival times of radio pulses from highly-magnetized transient beamed sources, it may be possible to detect light pseudo-scalar particles, such as axions and axion-like particles, whose existence could have considerable implications for the strong-CP problem of QCD as well as the dark matter problem in cosmology. Specifically, such light bosons may be detected with a much greater sensitivity, over a broad particle mass range, than is currently achievable by terrestrial experiments, and using indirect astrophysical considerations. The observable effect was discussed in Chelouche & Guendelman (2009), and is akin to the Stern-Gerlach experiment: the splitting of a photon beam naturally arises when finite coupling exists between the electro-magnetic field and the axion field. The splitting angle of the light beams linearly depends on the photon wavelength, the size of the magnetized region, and the magnetic field gradient in the transverse direction to the propagation direction of the photons. If radio emission in radio-loud magnetars is beamed and originates in regions with strong magnetic field gradients, then splitting of individual pulses may be detectable. We quantify the effect for a simplified model for magnetars, and search for radio beam splitting in the 2GHz radio light curves of the radio loud magnetar XTEJ1810-197.

In this work, we discuss white dwarf pulsars found recently making also reference of the possibility of some SGRs/AXPs being part of this class of pulsars. We also study the properties of very massive compact ultra magnetized white dwarfs that could be the progenitors candidates of super luminous type Ia supernovae, and also a previous stage of these white dwarf pulsars before the magnetic field decay.The structure of this ultra magnetized white dwarfs is obtained by solving the Einstein-Maxwell equations with a poloidal magnetic field in a fully general relativistic approach. The stellar interior is composed of a regular crystal lattice made of carbon ions immersed in a degenerate relativistic electron gas. We find that magnetized white dwarfs violate the standard Chandrasekhar mass limit significantly. We obtain a maximum white dwarf mass of around $2.12M_{\odot }$ with an equatorial radius $R\sim 1596$ Km, a central magnetic field of $B_c=1.74\times 10^{14}$ G and $B_s=3.6\times 10^{13}$ G at the stellar surface.

In this work, we study the effects of different magnetic field configurations in neutron stars described by a many-body forces formalism (MBF model). The MBF model is a relativistic mean field formalism that takes into account many-body forces by means of a meson field dependence of the nuclear interaction coupling constants. We choose the best parametrization of the model that reproduces nuclear matter properties at saturation and also describes massive neutron stars. We assume matter to be in beta-equilibrium, charge neutral and at zero temperature. Magnetic fields are taken into account both in the equation of state and in the structure of the stars by the self-consistent solution of the Einstein-Maxwell equations. We assume a poloidal magnetic field distribution and calculate its effects on neutron stars, showing its influence on the gravitational mass and deformation of the stars.

Neutron stars manifest themselves as different classes of astrophysical sources that are associated to distinct phenomenology. Here we focus our attention on magnetars (or strongly magnetized neutron stars) that are associated to Soft Gamma Repeaters and Anomalous X-ray Pulsars. The magnetic field on surface of these objects, reaches values greater than $10^{15}$ G. Under intense magnetic fields, relativistic effects begin to be decisive for the definition of the structure and evolution of these objects. We are tempted to question ourselves to how strengths fields affect the structure of neutron star. In this work, our objective is study and compare two solutions of Einstein-Maxwell equations: the Bonnor solution, which is an analytical solution that describe the exterior spacetime for a massive compact object which has a magnetic field that is characterize as a dipole field and a complete solution that describe the interior and exterior spacetime for the same source found by numerical methods). For this, we describe the geodesic equations generated by such solutions. Our results show that the orbits generated by the Bonnor solution are the same as described by numerical solution. Also, show that the inclusion of magnetic fields with values up to $10^{17}$G in the center of the star does not modify sharply the particle orbits described around this star, so the use of Schwarzschild solution for the description of these orbits is a reasonable approximation.

It appears that studying data from the catalogue of Gamma-Ray Bursts (GRBs) can be used to examine the birefringence phenomenon in the magnetosphere of the magnetars. By analysing the data from the McGill Online Magnetar and HEASARC Fermi Burst Catalogues, in this work we studied the angular distances between GRBs and magnetars in projection, we built their distribution map by the end of 2020, and the relative lag time periods of lights coming from GRBs and magnetars. It is confirmed that there are 29 galactic magnetars and their candidates, while the other two are located out of the Milkyway. The maximum separation angle for GRB and Magnetar projectiles was 3.76 degrees (4U0142+61 and GRB110818860), while minimum angular resolution was 0.54 degrees (SGR 1627-41 and GRB090829672). Currently, we discuss the relationship of GRB light intensity by their lag time as it would come after bending in the magnetosphere of the magnetar.

The soft gamma repeaters (SGRs) are one manifestation of magnetars. There is now a solid body of multi-wavelength observations of SGRs spanning several decades. There are also a number of fundamental unanswered questions, which will require better theory, more sensitive observations, and many years to answer.

In 2013 April a new magnetar, SGR1745−2900, was discovered as it entered an outburst, at an angular separation of only 2.4 arcsec from the supermassive black hole at the centre of the Milky Way, Sagittarius A*. The new source was detected both in the radio and X-ray bands, with a peak X-ray luminosity L_X ∼ 5 × 10³⁵ erg s⁻¹, and it has a spin-down magnetic field of ∼ 2 × 10¹⁴ G. Here we report on the long-term Chandra (twenty-six observations) and XMM–Newton (eight observations) X-ray monitoring campaign of SGR 1745−2900 from the onset of the outburst in 2013 April until 2014 September. This magnetar shows a slow flux decay compared to other magnetars and a rather inefficient surface cooling. Starquake-induced crustal cooling models alone have difficulty in explaining the high luminosity of the source for the first ∼200 d of its outburst. Additional heating of the star surface from currents flowing in a twisted magnetic bundle is probably playing an important role in the outburst evolution.

We consider anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs) as rotationally powered pulsar-like white dwarfs. For the spin-down power of the white dwarf we use the traditional dipole braking formula as well as two other more recent and realistic models provided by numerical studies of the magnetosphere of a compact object including the wind braking torque. We show that, for the inferred values of the structure parameters and magnetic field obtained from the observed rotation period and spin-down rates, these white dwarfs are located above the so-called death-line. This result allows us to explain the blackbody component observed in the soft X-ray band of SGRs and AXPs via the polar caps’ heating by the bombardment of back flowing pair-created particles. For the sake of example, we show our results for the case of 1E 2259+586.

We study torsional Alfvén oscillations of magnetars, i.e., neutron stars with a strong magnetic field. We consider the poloidal and toroidal components of the magnetic field and a wide range of equilibrium stellar models. Our results can explain both the lower and the higher observed frequencies in SGR 1806-20 and SGR 1900+14.

Using a fully general relativistic implementation of ideal magnetohydrodynamics with no assumed symmetries in three spatial dimensions, the dynamics of magnetized, rigidly rotating neutron stars are studied. Beginning with fully consistent initial data constructed with Magstar, part of the Lorene project, we study the dynamics and stability of rotating, magnetized polytropic stars as models of neutron stars. Evolutions suggest that some of these rotating, magnetized stars may be minimally unstable occurring at the threshold of black hole formation.

We describe a new mechanism for the production of ultra high-energy neutrinos based upon meson synchrotron emission. The acceleration of ultra-relativistic protons and nuclei in the presence of strong magnetic fields (H ~ 10¹⁵ G) in such environments as Magnetars, or GRB central engines could be a viable site for strong meson synchrotron emission. We show that charged scalar mesons like π^± (along with π⁰’s), vector mesons like ρ, and even heavier mesons like D_S, J/Ψ and ϒ, could be emitted with high intensity (~ 10³ times the photon intensity) through strong couplings to ultra-relativistic nucleons. We estimate the flux of energetic neutrinos originating from the decay of ultra-high-energy neutrons produced in a SGR magnetar environment by proton synchrotron emission of p → π⁺ + n. We deduce the event rate in energetic neutrino detectors and show that a nearby strong SGR burst might be detectable. We also analyze the possibility that the synchrotron emission of massive mesons like the ϒ might produce a burst of three avors of ultra high-energy cosmic neutrinos with Eν ≥ 10¹² eV and evaluate the spectra of νe, νμ and ντ from this process.

We investigate numerically the spinning-down of magnetars rotating at the propeller stage and propagating supersonically through the interstellar medium. Fast moving magnetars lose angular momentum due to direct interaction of the rotating magnetospheres with the non-rotating interstellar medium. Simulations have shown that magnetars spin-down very fast due to such interaction, much faster compared to non-moving ones.