Dipole Magnetic Field Research Articles

Timing analysis of Swift J0243.6+6124 with NICER and Fermi/GBM during the decay phase of the 2017–2018 outburst

ABSTRACT We present a timing and noise analysis of the Be/X-ray binary system Swift J0243.6+6124 during its 2017–2018 super-Eddington outburst using NICER/XTI observations. We apply a synthetic pulse timing analysis to enrich the Fermi/GBM spin frequency history of the source with the new measurements from NICER/XTI. We show that the pulse profiles switch from double-peaked to single-peaked when the X-ray luminosity drops below ∼7 × 1036 erg s−1. We suggest that this transitional luminosity is associated with the transition from a pencil beam pattern to a hybrid beam pattern when the Coulomb interactions become ineffective to decelerate the accretion flow, which implies a dipolar magnetic field strength of ∼5 × 1012 G. We also obtained the power density spectra (PDS) of the spin frequency derivative fluctuations. The red noise component of the PDS is found to be steeper (ω−3.36) than the other transient accreting sources. We find significantly high noise strength estimates above the super-Eddington luminosity levels, which may arise from the torque fluctuations due to interactions with the quadrupole fields at such levels.

Magnetic-field-controlled growth of magnetoelastic phase domains in FeRh

Magnetic phase transition materials are relevant building blocks for developing green technologies such as magnetocaloric devices for solid-state refrigeration. Their integration into applications requires a good understanding and controllability of their properties at the micro- and nanoscale. Here, we present an optical microscopy study of the phase domains in FeRh across its antiferromagnetic–ferromagnetic phase transition. By tracking the phase-dependent optical reflectivity, we establish that phase domains have typical sizes of a few microns for relatively thick epitaxial films (200 nm), thus enabling visualization of domain nucleation, growth, and percolation processes in great detail. Phase domain growth preferentially occurs along the principal crystallographic axes of FeRh, which is a consequence of the elastic adaptation to both the substrate-induced stress and laterally heterogeneous strain distributions arising from the different unit cell volumes of the two coexisting phases. Furthermore, we demonstrate a magnetic-field-controlled directional growth of phase domains during both heating and cooling, which is predominantly linked to the local effect of magnetic dipolar fields created by the alignment of magnetic moments in the emerging (disappearing) FM phase fraction during heating (cooling). These findings highlight the importance of the magnetoelastic character of phase domains for enabling the local control of micro- and nanoscale phase separation patterns using magnetic fields or elastic stresses.

The rocket effect mechanism in neutron stars in supernova remnants

ABSTRACT While the dipole magnetic field axis of neutron stars is usually postulated to cross the star’s centre, it may be displaced from this location, as it has been recently indicated in the millisecond pulsar J0030+0451. Under these conditions, the electromagnetic rocket effect may be activated, where the magnetic field exerts a net force, accelerating the star. This post-natal kick mechanism relies on asymmetric electromagnetic radiation from an off-centre dipole and may be relevant to the high spatial velocities of pulsars $\sim 10^{3}$ km s−1. Here, we explore its impact in young pulsars associated with supernova remnants, and we compare the observational data on characteristic quantities, such as the braking index and proper motion, with results obtained from the rocket effect. Using a Markov Chain Monte Carlo analysis, we explore the required conditions, for the initial spin periods and the distance between the magnetic axis and the star’s centre, so that the velocity kick due to the rocket effect approaches the present velocity. We find that the electromagnetic rocket effect can account for typical pulsar transverse velocities assuming an initial spin period of 3.8 $\rm {ms}$ and a dipole field whose distance from the centre of the star is approximately 7 $\rm {km}$ . We also explore the influence of the rocket effect on the braking index of a neutron star, and we find that for the sample studied this impact is minimal. Finally, we apply the rocket effect model on the pulsars J0030+0451 and J0538+2817, which are likely candidates for this mechanism.

Торможение аккреционного диска магнитным полем аккретора в приближении коротации

We analyze the process of interaction of the viscous keplerian accretion disk with the dipole magnetic field of the accretor within the corotation approximation. We estimate the radial dependence of the gaseous pressure in such non-magnetized accretion disk. We calculate the equilibrium radius at which the gaseous pressure in the disk is balanced by the pressure of the dipole magnetic field of the accreting star. We show that this radius depends on the disk viscosity and is significantly smaller than the canonical Alfvén radius realized in the process of spherical accretion.

Thermal case study and generated vortices by dipole magnetic field in hybridized nanofluid flowing: Alternating direction implicit solution

A vortex is a location in a fluid where the flow revolves around an axis line, which may be flat or curved. Researchers must look at vortices whenever and wherever they are found since they are present in natural systems. The purpose of this investigation is to present an understanding of how the nanofluid flow (driven by an external mechanism) interacts with the magnetic field produced by a nearby paced magnetic source. While most of the scientific literature is enriched with investigations dealing with the problems assuming a uniform magnetic field occupying the flow field, the current analysis is devoted to studying the complex interaction of the spatially varying magneto force with the nanofluid flow. We have noticed that the magnetic field has caused the evolution of new vortices (which are very important while analyzing any flow model due to their importance in interpreting fluid mixing and mass transport phenomena) in the flow field, thus adding much more significance to our work. The single-phase model (SPM) has been used to describe the hybrid nanofluid whereas the numerical solution to the governing differential equations has been obtained by employing an algorithm based on the central difference discretization and the alternating direction implicit method. The governing factors for the current examination are the Reynolds number, the magneto variable, and the volumetric concentration of the nanoparticles, with the ranges, considered being 1≤Re≤150, 0≤Mn≤150 and 0≤ϕ≤0.2, respectively. The analysis reveals that the Nusselt number (Nu) steepens noticeably at the enclosure's upper left corner. 20% increment in nanoparticle volume fraction may result in up to 72% boost in the Nusselt quantity whilst causing an up to 33% decrease in skin friction. Furthermore, compared to copper, iron oxide nanoparticles are shown to be more dependent on drag force. The flow field is dominated by two almost similar main vortices while there is no magnetic source, but as the magnetic source gets stronger, the bottom vortex gets stronger. It enlarges and constricts the top vortex, breaking it down while creating two new, weaker vortices. As it produces three vortices at comparatively lower values of the pertinent parameters, the Reynolds number functions as a catalyst for the action of the magneto force variable. Finally, the Nusselt quantity and frictional factor both significantly alter once the nanostructure is incorporated into the base fluid.

Electromagnetically driven anticyclonic rotation in spherical Couette flow

The electromagnetically driven anticyclonic flow in the wide gap of a rotating concentric spheres system is studied experimentally and numerically in the laminar regime. The working fluid is an electrolyte contained in the spherical gap. The outer sphere rotates at constant angular speed, and the inner sphere is at rest. The electromagnetic stirring is generated due to the interaction of a direct current, which is injected radially through ring-shaped electrodes located at the equatorial zone of each sphere, and a dipolar magnetic field produced by a permanent magnet located inside the inner sphere. Experimental velocity fields in the equatorial plane were obtained with particle image velocimetry. Additionally, full three-dimensional numerical simulations were performed. The high shearing at the equatorial region promotes an instability that can be perceived as a triggering mechanism of four tornado-like vortical structures tilted and entangled in the polar direction. The instability structure rotates in either the cyclonic or the anticyclonic direction, depending on the flow parameters, namely, the angular velocity of the outer sphere and the applied electric current. To the best knowledge of the authors, this is the first time the phenomenon under study is reported with electromagnetic forcing. The numerical results agree quantitatively with the experimental observations.

Maximum entropy states of collisionless positron–electron plasma in a dipole magnetic field

We are developing a positron–electron plasma trap based on a dipole magnetic field generated by a levitated superconducting magnet to investigate the physics of magnetized plasmas with mass symmetry as well as antimatter components. Such laboratory magnetosphere is deemed essential for the understanding of pair plasmas in astrophysical environments, such as magnetars and blackholes, and represents a novel technology with potential applications in antimatter confinement and the development of coherent gamma-ray lasers. The design of the device requires a preemptive analysis of the achievable self-organized steady states. In this study, we construct a theoretical model describing maximum entropy states of a collisionless positron–electron plasma confined by a dipole magnetic field and demonstrate efficient confinement of both species under a wide range of physical parameters by analyzing the effect of the three adiabatic invariants on the phase space distribution function. The theory is verified by numerical evaluation of spatial density, electrostatic potential, and toroidal rotation velocity for each species in correspondence with the maximum entropy state.

The magnetic field and stellar wind of the mature late-F star χ Draconis A

ABSTRACT F stars lie in the transition region between cool stars with dynamo generated fields and hot star fossil fields and offer an interesting window into the generation of magnetic fields in shallow convection zones. In this paper, we investigate the magnetic field of the mature F7V primary of the bright χ Draconis system. χ Dra was observed in circularly polarized light at four epochs from 2014 through to 2019 using the NARVAL spectropolarimeter at the Téléscope Bernard Lyot. Using the technique of least-squares deconvolution, we created high signal-to-noise line profiles from which we were able to measure the radial velocity of both the primary and secondary and use these to improve the orbital parameters of the system. Additionally, we used the technique of Zeeman Doppler imaging to reconstruct the large-scale magnetic field geometry of the primary at the four epochs. The magnetic maps show that χ Dra A has a predominately dipolar poloidal magnetic field at all epochs with the magnetic axis well aligned with the stellar rotation axis. The large-scale features of the magnetic field appear to be relatively stable over the ∼5 yr time base of our observations, with no evidence of any polarity reversals. We used the magnetic field maps to model the wind from χ Dra A at all epochs, showing that the mass-loss from the stellar wind of χ Dra A is 3–6 times the current solar value, while the angular momentum loss from the wind is around 3–4 times the solar value.

Scattering of light in thin magnetized envelopes – II

ABSTRACT We consider optically thin spherical stellar envelopes with dipole magnetic field. The source of magnetic dipole field is assumed point like and emits the non-polarized monochromatic radiation. This paper carry on the previous paper of authors, where the general theory is presented and the circular polarization of scattered radiation is calculated for small values of magnetic fields corresponding to parameter x ≪ 1 (x = ωB/ω ≃ 0.933 × 10−8λ(μm)B(G)). The Stokes parameters of scattered radiation mutually transform into each other due to interference. Here we obtained the explicit formulas describing the transformation of all Stokes parameters for any value of parameter x. The qualitative discussion of these formulas is presented. These formulas show that for x ∼ 1 the transformation leads to large variations of all Stokes parameters. The linear and circular polarizations acquire oscillating dependence on the propagation path. For x ≫ 1 the single scattered radiation has very small circular polarization of the order ∼1/x ≪ 1 and large linear polarization. In this case the transformation gives rise to additional value of observed circular polarization, which is proportional to initial single scattered value. The observed linear polarization acquires small inclination angle relative to the plane ‘line of sight–direction of magnetic dipole’ due to existence of small circular polarization. The Stokes parameters of single scattered radiation from all envelope without taking into account the interference effects are given for x ≫ 1.

On the non-minimal coupling of magnetic fields with gravity in Schwarzschild spacetime

In this work we study the effects of non-minimal coupling between electromagnetism and gravity, which are motivated by quantum effects such as vacuum polarization. We investigate the modification of both asymptotically dipole and uniform magnetic fields around a Schwarzschild black hole that come as the result of non-minimal coupling. While in both cases the magnetic field gets enhanced or suppressed with respect to the case of minimal coupling, depending on the sign of non-minimal coupling parameter, in the case of a background uniform magnetic field the direction of the magnetic field also alters in the vicinity of the black hole horizon. We have discussed the possible astrophysical and cosmological sources for which the vacuum polarization may be at play, while also discussing the observational effects, in particular the possibility of synchrotron radiation from the vicinity of a black hole. We conclude that such observations could be used to constrain the value of the non-minimal coupling parameter.

Diving into the magnetosphere of the Of?p star HD 108

ABSTRACT We analyse optical and X-ray spectroscopy of the Of?p star HD 108, known for its strong dipolar magnetic field and its optical line profile variability with a time-scale of 54 ± 3 yrs, interpreted as the stellar rotation period. Optical emission lines have now recovered from their minimum emission state reached in 2007–2008. The variations of the equivalent width of the Hα emission provide constraints on the inclination of the rotation axis (i) and the obliquity of the magnetic axis (β). The best agreement between model and observations is found for (i, β) pairs with i + β ≃ 85° and i ∈ [30°, 55°]. The Balmer emission lines display stochastic variability at the ∼5 per cent level on time-scales of a few days. TESS photometry unveils transient modulations on similar time-scales in addition to prominent red noise variations. A Chandra X-ray observation of December 2021, when the star was at a higher emission level, indicates a slight increase of the flux and a spectral hardening compared to the August 2002 XMM-Newton observation, taken near minimum emission state. Magnetohydrodynamic simulations are used to compute synthetic X-ray spectra. With our current best estimate of the $\dot{M}_{B=0}$ mass-loss rate, the simulated X-ray luminosity and spectral energy distribution agree very well with the observations. Finally, the radial velocities vary on a period of 8.5 yr with a peak-to-peak amplitude of 10–11 km s−1, suggesting orbital motion with an unseen companion of at least 4 M⊙.

Influence mechanism of magnetic field direction on magnetic drag of reentry vehicle and better magnetic field direction

The magnetic field condition has a great influence on the magnetic control drag of reentry vehicle, so it is an important research topic to select appropriate magnetic field condition to control the drag force of vehicle. This paper analyzes the magnetic control drag by solving the hypersonic magnetohydrodynamic equations and combining the magnetic field working conditions, studies the influence of the axisymmetric magnetic field direction on the drag, supplements the mechanism of Lorentz force, pressure and conductivity on the axisymmetric magnetic control drag, and compares the drag value and drag composition ratio under different magnetic field directions. The problem is solved numerically for a particular model of a dipole magnetic field. Under different dipole magnetic fields, the magnitude and distribution shape of Lorentz force are different. There are several dividing lines where the Lorentz force is 0, and the Lorentz force directions on both sides are opposite, and this line divides the Lorentz force field into multiple parts. In addition, it is found that the change of the angle between the axis of the aircraft and the direction of the magnetic field will cause the change of the aircraft resistance, which will reduce or increase it.

Characteristics of Electron Precipitation Directly Driven by Poloidal ULF Waves

AbstractA mechanism recently proposed for magnetospheric electron loss into the atmosphere is the precipitation directly driven by ultralow‐frequency (ULF) waves. In this study, we quantitatively analyze the properties of ULF wave‐induced precipitation by simulating the electron bounce and drift motion in poloidal‐mode waves excited in a dipole magnetic field. Our results reveal that precipitation occurs only when electrons encounter a westward‐directed wave electric field in the magnetosphere, which leads to cross‐field energy enhancements and reduces their mirror heights. The simulations also demonstrate longer duration electron precipitation at the drift‐resonance energy. We calculate the temporal variations of the energy spectrum for precipitating electrons and the total precipitating energy fluxes. These results improve our understanding of ULF wave‐induced electron precipitation as well as provide a point of comparison for observations from balloons or ground‐based instruments.

Investigation of Field-Free Switching of 2-D Material-Based Spin–Orbit Torque Magnetic Tunnel Junction

Spin Hall-assisted magnetization switching in a three-terminal magnetic tunnel junction (MTJ) has attracted much attention due to its high-speed magnetization switching, which enables low-power memory and logic applications. In this study, 2-D materials with high spin–orbit coupling (SOC) effects and high charge-to-spin conversion efficiency are used to assess the performance characteristics of spin–orbit torque MTJ (SOT-MTJ). External field-free switching in SOT-MTJ is accomplished by employing a standard MTJ structure with a bias magnetic layer on top of the structure that projects a dipolar magnetic field onto a free layer (FL). In addition, the Dzyaloshinskii–Moriya interaction (DMI), an asymmetric exchange interaction, is considered while evaluating the critical current density. We were able to demonstrate that the required critical current density decreases by 99.5%, while the switching speed increases by 86.67% in the proposed external field-free 2-D material-based SOT-MTJ. We have also shown the significance of DMI in the field-free switching of the magnetization without the requirement for additional mechanisms when the device shrank down to the subnanometer regime.

Crustal Magnetic Fields Do Not Lead to Large Magnetic-field Amplifications in Binary Neutron Star Mergers

The amplification of magnetic fields plays an important role in explaining numerous astrophysical phenomena associated with binary neutron star mergers, such as mass ejection and the powering of short gamma-ray bursts. Magnetic fields in isolated neutron stars are often assumed to be confined to a small region near the stellar surface, while they are normally taken to fill the whole star in numerical modeling of mergers. By performing high-resolution, global, and high-order general-relativistic magnetohydrodynamic simulations, we investigate the impact of a purely crustal magnetic field and contrast it with the standard configuration consisting of a dipolar magnetic field with the same magnetic energy but filling the whole star. While the crust configurations are very effective in generating strong magnetic fields during the Kelvin–Helmholtz-instability stage, they fail to achieve the same level of magnetic-field amplification of the full-star configurations. This is due to the lack of magnetized material in the neutron-star interiors to be used for further turbulent amplification and to the surface losses of highly magnetized matter in the crust configurations. Hence, the final magnetic energies in the two configurations differ by more than 1 order of magnitude. We briefly discuss the impact of these results on astrophysical observables and how they can be employed to deduce the magnetic topology in merging binaries.

Beam Pattern Evolution of Accreting X-Ray Pulsar 1A 0535+262 during Its 2020 Giant Outburst

We report on pulse profile decomposition analysis of bright transient X-ray pulsar 1A 0535+262 using broadband Insight-HXMT observations during a giant outburst of the source in 2020. We show that the observed pulse profile shape can be described in terms of a combination of two symmetric single-pole contributions for a wide range of energies and luminosities for a fixed geometry defining the basic geometry of the pulsar. This corresponds to a slightly distorted dipole magnetic field, i.e., one pole has to be offset by ∼12° from the antipodal position of the other pole. We reconstruct the intrinsic beam patterns of the pulsar assuming that the geometry is recovered from the decomposition analysis, and we find evidence for a transition between “pencil” and “fan” beams in energy ranges above the cyclotron line energy, which can be interpreted as a transition from sub- to supercritical accretion regimes associated with the onset of an accretion column. At lower energies, however, the beam pattern appears to be more complex and contains a substantial “fan” beam and an additional “pencil” beam component at all luminosities. The latter is not related to the accretion rate and is stronger in the fading phase of the outburst. We finally discuss the results in the context of other observational and theoretical findings earlier reported for the source in the literature.

Decreasing Magnetic-Field Inhomogeneity by Inlaying the RF Coil Into a Tubular Bracket

The homogeneous magnetic field is highly desired for achieving high-resolution nuclear magnetic resonance (NMR) spectra. Inevitably, however, nonzero magnetic susceptibility (non-ZMS) structures near the detection region, such as radio-frequency (RF) coils and brackets among others, accentuate the magnetic field inhomogeneity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> ) and broaden NMR spectral lines. In this study, we discover that when cylindrical sensors are perpendicular to the applied magnetostatic field, inlaying the RF coil into a tubular bracket that coaxially overlays the tube can reduce <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> within the detection region of this RF coil. This inlay depth can be optimized by enforcing magnetic dipole fields induced by the RF coil to equal those of the materials replaced by this RF coil. Based on this mechanism, we have developed an inlaid-coil sensor for a lab-built compact NMR spectrometer, which is merely additionally installed with a tubular Teflon bracket. Simulated results demonstrate that, as this depth approaches the optimal theoretical value, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> within the detection region gradually decreases, supporting our identification of such a mechanism. Furthermore, experimental results reveal that the inlaid-coil sensor can acquire higher-resolution NMR spectra than the bracketless sensor can, validating the compensation mode based on this mechanism. This approach can achieve magnetostatic-field compatibility among different materials inside cylindrical sensors perpendicular to the imposed magnetostatic field. Hopefully, our study can help provide a guide for the magnetostatic-field design of complex cylindrical sensors in emerging NMR technologies such as microsystems, hyperpolarization, and in situ detection.

Numerical Studies of the Impact of Electromagnetic Field of Radiation on Valine.

We present the results of numerical calculations of the effect of an electromagnetic field of radiation on valine, and compare them to experimental results available in the literature. We specifically focus on the effects of a magnetic field of radiation, by introducing modified basis sets, which incorporate correction coefficients to the s-, p- or only the p-orbitals, following the method of anisotropic Gaussian-type orbitals. By comparing the bond length, angle, dihedral angles, and condense-to-atom-all electrons, obtained without and with the inclusion of dipole electric and magnetic fields, we concluded that, while the charge redistribution occurs due to the electric field influence, the changes in the dipole momentum projection onto the y- and z- axes are caused by the magnetic field. At the same time, the values of the dihedral angles could vary by up to 4 degrees, due to the magnetic field effects. We further show that taking into account the magnetic field in the fragmentation processes provides better fitting of the experimentally obtained spectra: thus, numerical calculations which include magnetic field effects can serve as a tool for better predictions, as well as for analysis of the experimental outcomes.

Linking the interiors and surfaces of magnetic stars

ABSTRACTStrong magnetic fields are observed in a substantial fraction of upper main sequence stars and white dwarfs. Many such stars are observed to exhibit photometric modulations as the magnetic poles rotate in and out of view, which could be a consequence of magnetic perturbations to the star’s thermal structure. The magnetic pressure is typically larger than the gas pressure at the star’s photosphere, but much smaller than the gas pressure in the star’s interior, so the expected surface flux perturbations are not clear. We compute magnetically perturbed stellar structures of young $3 \, \mathrm{M}_\odot$ stars that are in both hydrostatic and thermal equilibrium, and which contain both poloidal and toroidal components of a dipolar magnetic field as expected for stable fossil fields. This provides semi-analytical models of such fields in baroclinic stably stratified regions. The star’s internal pressure, temperature, and flux perturbations can have a range of magnitudes, though we argue the most likely configurations exhibit flux perturbations much smaller than the ratio of surface magnetic pressure to surface gas pressure, but much larger than the ratio of surface magnetic pressure to central gas pressure. The magnetic pole is hotter than the equator in our models, but a cooler magnetic pole is possible depending on the magnetic field configuration. The expected flux variations for observed field strengths are δL/L ≲ 10−6, much smaller than those observed in magnetic stars, suggesting that observed perturbations stem from changes to the emergent spectrum rather than changes to the bolometric flux.

X-ray non-detection of PSR J0250 + 5854

ABSTRACT We conducted a deep XMM–Newton observing campaign on the 23.5-s radio pulsar PSR J0250 + 5854 in order to better understand the connection between long-period, radio-emitting neutron stars and their high-energy-emitting counterparts. No X-ray emission was detected resulting in an upper limit in the bolometric luminosity of PSR J0250 + 5854 of &amp;lt;1031 erg s−1 for an assumed blackbody with a temperature of 85 eV, typical of an X-ray Dim Isolated Neutron Star (XDINS). We compared the upper limit in the bolometric luminosity of PSR J0250 + 5854 with the known population of XDINSs and found that the upper limit is lower than the bolometric luminosity of all but one XDINS. We also compared PSR J0250 + 5854 with SGR 0418+5729, the magnetar with low dipole magnetic field strength, where the upper limit suggests that if PSR J0250 + 5854 has a thermal hotspot like SGR 0418 + 5729, it would have a blackbody temperature of &amp;lt;200 eV, compared to 320 eV of the magnetar.