Neutron star electromagnetic activity produces pairs that fill their magnetosphere represented to the zeroth order by the force-free approximation. Neither dissipation nor acceleration nor radiation from charged particles is expected from this simplified model. So far, only centred dipole magnetic fields have been studied in this limit. In this paper, we explore the consequences of a rotating off-centred dipole on the force-free magnetosphere, showing the new magnetic field geometry, its spin-down luminosity as well as the electromagnetic kick and torque felt by the neutron star. Solutions are obtained by time-dependent numerical simulations of the force-free regime using our pseudo-spectral code written in spherical coordinates. Our results are also compared to known analytical expressions found for the off-centred vacuum dipole by an expansion to lowest order in the parameter $ε= d/R$, where $d$ is the displacement of the dipole from the stellar centre and $R$ the neutron star radius. The presence of a force-free plasma enhances the spin-down luminosity as well as the electromagnetic kick and torque with respect to a centred force-free dipole. The impact on isolated and binary neutron stars is revised in light of these new results.
Neutron stars are strongly magnetized rotating compact objects. Therefore they also produce huge electric fields accelerating particles to ultra-relativistic energies. The simplest magnetic topology is a dipole traditionally located at the stellar centre. In this paper, we reinvestigate the consequences of an off-centred rotating magnetic dipole, showing accurate magnetic field line geometries, the associated spin-down luminosity as well as the corresponding electromagnetic kick and torque imprinted to the neutron star. Results are obtained by time dependent numerical simulations of Maxwell eq…
In this poster we present the structure of an axisymmetric, force-free magnetosphere of a twisted, aligned rotating dipole within a corotating plasma-filled magnetosphere. We explore various profiles for the twist. We find that as the current increases more field lines cross the light cylinder leading to more efficient spin-down. Moreover, we notice that the twist cannot be increased indefinitely and after a finite twist of about $π/2$ the field becomes approximately radial. This could have implications for torque variations of magnetars related to outbursts.
We interpret recent observations of high-to-low frequency drifting features in the spectra of the repeating FRBs as evidence of sharply changing plasma properties in the emission region, presumably the neutron stars magnetospheres. The drifts are then FRBs' analogues of radius-to-frequency mapping in pulsars and Solar type-III radio burst (but not in a sense of a particular emission mechanism). The drifts rates of $\sim 100$ MHz ms$^{-1}$ at frequencies $\sim$ GHz translate to physical size of $\sim {c ω}/{\dotω} \sim$ few $\times 10^8$ cm, matching the hypothesis of the FRB origin in the magn…
Neutron stars are compact objects rotating at high speed, up to a substantial fraction of the speed of light (up to 20\% for millisecond pulsars) and possessing ultra-strong electromagnetic fields (close to and sometimes above the quantum critical field of \numprint{4.4e9}~\SIunits{\tesla}). Moreover, due to copious $e^\pm$ pair creation within the magnetosphere, the relativistic plasma surrounding the star is forced into corotation up to the light cylinder where the corotation speed reaches the speed of light. The neutron star electromagnetic activity is powered by its rotation which becomes …
A magnetar's magnetosphere gradually evolves by the injection of energy and helicity from the interior. Axisymmetric static solutions for a relativistic force-free magnetosphere with a power-law current model are numerically obtained. They provide information about the configurations in which the stored energy is large. The energy along a sequence of equilibria increases and becomes sufficient to open the magnetic field. A magnetic flux rope, in which a large amount of toroidal field is confined, is formed in the vicinity of the star, for states exceeding the open field energy. These states ar…
This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of magnetospheres of isolated neutron stars. For a summary, we refer to the paper.
I argue that the problem of electromagnetically driven electron-positron cascades in magnetospheres of neutron stars must be addressed starting from first principles. I describe a general numerical algorithm for doing self-consistent kinetic simulations of electron-positron cascades - wherein particle acceleration, pair creation and screening of the electric field are calculated simultaneously - and apply it to model the Ruderman and Sutherland cascade in one dimension. I find that pair creation is quite regular and quasi-periodic. In each cycle a blob of ultrarelativistic electron-positron pl…
We re-estimate the surface magnetic fields of neutron stars (NSs) in Be X-ray binaries (BeXBs) with different models of torque, improved beyond Klus et al. (2014). In particular, a new torque model (Dai \& Li 2006) is applied to three models of magnetosphere radius. Unlike the previous models, the new torque model does not lead to divergent results for any fastness parameter. The inferred surface magnetic fields of these NSs for the two compressed magnetosphere models are much higher than that for the uncompressed magnetosphere model. The new torque model using the compressed-magnetosphere rad…
Abridged. Fast rotating and self-gravitating astrophysical objects suffer strong deformations from centrifugal forces. If moreover they are magnetized, they generate an electromagnetic wave that is perturbed accordingly. When stellar objects are also surrounded by an ideal plasma, a magnetosphere is formed. We study the electromagnetic configuration of a force-free magnetosphere encompassing an ideal spheroidal rotating conductor as an inner boundary. We put special emphasize to millisecond period neutron star magnetospheres, those showing a significant oblate shape. Force-free solutions are c…
We study the propagation of a strong, low frequency, linearly polarized fast magnetosonic wave inside the magnetosphere of a neutron star. The relative strength $δB/B$ of the wave grows as a function of radius before it reaches the light cylinder, and what starts as a small perturbation can grow to become nonlinear before it escapes the magnetosphere. Using first-principles Particle-in-Cell (PIC) simulations, we study in detail the evolution of the wave as it becomes nonlinear. We find that an initially sinusoidal wave becomes strongly distorted as $δB/B$ approaches order unity. The wave steep…
Eight pulsars have low braking indices, which challenge the magnetic dipole braking of pulsars. 222 pulsars and 15 magnetars have abnormal distribution of frequency second derivatives, which also make contradiction with classical understanding. How neutron star magnetospheric activities affect these two phenomena are investigated by using the wind braking model of pulsars. It is based on the observational evidence that pulsar timing is correlated with emission and both aspects reflect the magnetospheric activities. Fluctuations are unavoidable for a physical neutron star magnetosphere. Young p…
We present numerical simulations of electron-positron pair cascades in the magnetospheres of magnetic neutron stars for a wide range of surface fields (B_p = 10^{12}--10^{15} G), rotation periods (0.1--10 s), and field geometries. This has been motivated by the discovery in recent years of a number of radio pulsars with inferred magnetic fields comparable to those of magnetars. Evolving the cascade generated by a primary electron or positron after it has been accelerated in the inner gap of the magnetosphere, we follow the spatial development of the cascade until the secondary photons and pair…
We discuss constraints that the observed brightness temperatures impose on coherent processes in pulsars and Fast Radio Bursts (FRBs), and in particular on the hypothesis of coherent curvature emission by bunches. We estimate the peak brightness temperature that a bunch of charge $Ze$ can produce via synchrotron and/or curvature emission as $k_B T \sim (Z e)^2/λ$, where $λ$ is the typical emitted wavelength. We demonstrate that the bunch's electrostatic energy required to produce observed brightness temperature is prohibitively high, of the order of the total {\it bulk } energy. We compare cor…
The X-ray spectra observed in the persistent emission of magnetars are evidence for the existence of a magnetosphere. The high-energy part of the spectra is explained by resonant cyclotron upscattering of soft thermal photons in a twisted magnetosphere, which has motivated an increasing number of efforts to improve and generalize existing magnetosphere models. We want to build more general configurations of twisted, force-free magnetospheres as a first step to understanding the role played by the magnetic field geometry in the observed spectra. First we reviewed and extended previous analytica…
We perform first-principles relativistic particle-in-cell simulations of aligned pulsar magnetosphere. We allow free escape of particles from the surface of a neutron star and continuously populate the magnetosphere with neutral pair plasma to imitate pair production. As pair plasma supply increases, we observe the transition from a charge-separated electrosphere solution with trapped plasma and no spin-down to a solution close to the ideal force-free magnetosphere with electromagnetically-dominated pulsar wind. We calculate the magnetospheric structure, current distribution and spin-down powe…
The dependence of the spin frequency derivative $\dotν$ of accreting neutron stars with a strong magnetic field (X-ray pulsars) on the mass accretion rate (bolometric luminosity, $L_{bol}$) has been investigated for eight transient pulsars in binary systems with Be stars. Using data from the Fermi/GBM and Swift/BAT telescopes, we have shown that for seven of the eight systems the dependence $\dotν$ can be fitted by the model of angular momentum transfer through an accretion disk, which predicts the relation $\dotν\sim L^{6/7}_{bol}$. Hysteresis in the dependence $\dotν(L_{bol})$ has been confi…
The close vicinity of neutron stars remains poorly constrained by observations. Although plenty of data are available for the peculiar class of pulsars we are still unable to deduce the underlying plasma distribution in their magnetosphere. In the present paper, we try to unravel the magnetospheric structure starting from basic physics principles and reasonable assumptions about the magnetosphere. Beginning with the monopole force-free case, we compute accurate general-relativistic solutions for the electromagnetic field around a slowly rotating magnetized neutron star. Moreover, here we addre…
We construct hydromagnetic neutron star equilibria which allow for a non-zero electric current distribution in the exterior. The novelty of our models is that the neutron star's interior field is in equilibrium with its magnetosphere, thus bridging the gap between previous work in this area which either solves for the interior assuming a vacuum exterior or solves for the magnetosphere without modelling the star itself. We consider only non-rotating stars in this work, so our solutions are most immediately applicable to slowly-rotating systems such as magnetars. Nonetheless, we demonstrate that…
Pulsed, Polarized X-ray Emission from Neutron Star Surfaces: the Effects of Vacuum Birefringence in the Magnetosphere
Rice University（莱斯大学）；Washington University in St. Louis（圣路易斯华盛顿大学）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；George Washington University（乔治华盛顿大学）；NASA Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）；University of Maryland Baltimore County（马里兰大学巴尔的摩县分校）；Houston City College（休斯顿城市学院）
We find a class of twisted and differentially rotating neutron star magnetospheres that do not have a light cylinder, generate no wind and thus do not spin-down. The magnetosphere is composed of embedded differentially rotating flux surfaces, with the angular velocity decreasing as $Ω\propto 1/r$ (equivalently, becoming smaller at the foot-points closer to the axis of rotation). For each given North-South self-similar twist profile there is a set of self-similar angular velocity profiles (limited from above) with a "smooth", dipolar-like magnetic field structure extending to infinity. For spin…
The phenomenology of the emission of pulsars and magnetars depends dramatically on the structure and properties of their magnetic field. In particular it is believed that the outbursting and flaring activity observed in AXPs and SRGs is strongly related to their internal magnetic field. Recent observations have moreover shown that charges are present in their magnetospheres supporting the idea that their magnetic field is tightly twisted in the vicinity of the star. In principle these objects offer a unique opportunity to investigate physics in a regime beyond what can be obtained in the labor…
Over the last four decades, persistent and flaring emission of magnetars observed by various telescopes has provided us with a suite of light curves and spectra in soft and hard X-rays, with no emission yet detected above around 1 MeV. Attenuation of such high-energy photons by magnetic pair creation and photon splitting is expected to be active in the magnetospheres of magnetars, possibly accounting for the paucity of gamma-rays in their signals. This paper explores polarization-dependent opacities for these two QED processes in static vacuum dipole magnetospheres of highly-magnetized neutron…
We extend our previous study of equilibrium solutions of non-rotating force-free magnetospheres of neutron stars. We show that multiple solutions exist for the same sets of parameters, implying that the solutions are degenerate. We are able to obtain configurations with disconnected field lines, however, in nearly all cases these correspond to degenerate higher energy solutions. We carry out a wide parametric search in order to understand the properties of the solutions. We confirm our previous results that the lower energy solutions have up to $\sim 25\%$ more energy than the vacuum case, hel…
We conduct force-free simulations of a single neutron star undergoing orbital motion in flat spacetime, mimicking the trajectory of the star about the center of mass on a compact binary system. Our attention is focused on the kinetic energy being extracted from the orbit by the acceleration of the magnetic dipole moment of the neutron star, and particularly, on how this energy gets distributed within its surrounding magnetosphere. A detailed study of the resulting magnetospheric configurations in our setting is presented, incorporating as well the effects due to neutron star spin and the misal…
Soft Gamma-Ray Repeaters and Anomalous X-Ray Pulsars are extreme manifestations of the most magnetized neutron stars: magnetars. The phenomenology of their emission and spectral properties strongly support the idea that the magnetospheres of these astrophysical objects are tightly twisted in the vicinity of the star. Previous studies on equilibrium configurations have so far focused on either the internal or the external magnetic field configuration, without considering a real coupling between the two fields. Here we investigate numerical equilibrium models of magnetized neutron stars endowed …
Relativistic effects of compact objects onto electromagnetic fields in their vicinity are investigated using the test-field approximation. In particular, we study the possible emergence of magnetic null points which are astrophysically relevant for the processes of magnetic reconnection. While the magnetic reconnection occurs in the presence of plasma and may lead to violent mass ejection, we show here that strong gravitation of the supermassive black hole may actively support the process by suitably entangling the field lines even in the electro-vacuum description. In this contribution we fur…
HETE J1900.1--2455 is a peculiar accreting millisecond X-ray pulsar (AMXP) because it has shown intermittent pulsations after 22 days from the beginning of its outburst. The origin of intermittent pulses in accreting systems remains to be understood. To better investigate the phenomenon of intermittent pulsations here we present an analysis of 7 years of X-ray data collected with the Rossi X-Ray Timing Explorer and focus on the aperiodic variability. We show that the power spectral components follow the same frequency correlations as the non-pulsating atoll sources. We also study the known kHz…
The stationary axisymmetric force-free magnetosphere of a pulsar is considered. We present an exact dipolar solution of the pulsar equation, construct the magnetospheric model on its basis and examine its observational support. The new model has toroidal rather than common cylindrical geometry, in line with that of the plasma outflow observed directly as the pulsar wind nebula at much larger spatial scale. In its new configuration, the axisymmetric magnetosphere consumes the neutron star rotational energy much more efficiently, implying re-estimation of the stellar magnetic field, $B_{\mathrm …
Instabilities in a neutron star can generate Alfvén waves in its magnetosphere. Propagation along the curved magnetic field lines strongly shears the wave, boosting its electric current $j_{\rm A}$. We derive an analytic expression for the evolution of the wave vector $\boldsymbol{k}$ and the growth of $j_{\rm A}$. In the strongly sheared regime, $j_{\rm A}$ may exceed the maximum current $j_{0}$ that can be supported by the background $e^{\pm}$ plasma. We investigate these "charge-starved" waves, first using a simplified two-fluid analytic model, then with first-principles kinetic simulations…
Most of ultraluminous X-ray sources are thought to be objects accreting above their Eddington limits. In the recently identified class of ultraluminous X-ray pulsars, accretor is a neutron star and thus has a fairly small mass with a small Eddington limit. The accretion disc structure around such an object affects important observables such as equilibrium period, period derivative and the size of the magnetosphere. We propose a model of a nearly-standard accretion disc interacting with the magnetosphere only in a thin layer near the inner disc rim. Our calculations show that the size of the ma…
We study the dependence of the electromagnetic luminosity --produced by interactions of force-free magnetospheres-- on dipole inclinations in binary neutron star systems. We show that this interaction extracts kinetic energy from the system and powers a Poynting flux with a strong dependence on the dipole orientations. This dependence can be linked to the reconnection and redistribution of magnetic field as the stars interact. Although the details of the Poynting luminosity are very much dependent on the orientation, all the cases considered here nevertheless radiate a large Poynting flux. Thi…
The detection of gravitational waves from neutron star merger events has opened up a new field of multi-messenger astronomy linking gravitational waves events to short-gamma ray bursts and kilonova afterglows. A further - yet to be discovered - electromagnetic counterpart is precursor emission produced by the non-trivial interaction of the magnetospheres of the two neutron stars prior to merger. By performing special-relativistic force-free simulations of orbiting neutron stars we discuss the effect of different magnetic field orientations and show how the emission can be significantly enhance…
We show that axion dark matter (DM) may be detectable through narrow radio lines emitted from neutron stars. The neutron star magnetosphere hosts a strong magnetic field and a plasma frequency that increases towards the neutron star surface. As the axions pass through the magnetosphere, they can resonantly convert into radio photons in a narrow region around the radius at which the plasma frequency equals the axion mass. The bandwidth of the signal is set by the small DM velocity dispersion far away from the neutron star. We solve the axion-photon mixing equations, including a full treatment o…
Magnetars exhibit a variety of transient high-energy phenomena in the form of bursts, outbursts, and giant flares. It is a common belief that these events originate in the sudden release of magnetic energy due to the rearrangement of a twisted magnetic field. We present global models of a 3D Force-Free (FF) non-linear twisted magnetar magnetosphere. We solve the FF equations following the Grad-Rubin approach in a compactified spherical domain. Appropriate boundary conditions are imposed at the surface of the star for the current distribution and the magnetic field. Our implementation is tested…
We numerically investigate the dynamics of a supernova fallback accretion confronting with a relativistic wind from a newborn neutron star (NS). The time evolution of the accretion shock in the radial direction is basically characterized by the encounter radius of the flow $r_\mathrm{enc}$ and a dimensionless parameter $ζ\equiv L/\dot M_\mathrm{fb}c^2$, where $L$ is the NS wind luminosity and $\dot M_\mathrm{fb}$ is the fallback mass accretion rate. We find that the critical condition for the fallback matter to reach near the NS surface can be simply described as $ζ< ζ_\mathrm{min} \equiv GM_*…
Plasma magnetosphere surrounding rotating magnetized neutron star in the braneworld has been studied. For the simplicity of calculations Goldreich-Julian charge density is analyzed for the aligned neutron star with zero inclination between magnetic field and rotation axis. From the system of Maxwell equations in spacetime of slowly rotating star in braneworld, second-order differential equation for electrostatic potential is derived. Analytical solution of this equation indicates the general relativistic modification of an accelerating electric field and charge density along the open field lin…
The mechanism by which the radiation received from obliquely rotating neutron stars is generated remains an open question half a century after the discovery of pulsars. In contrast, considerable progress has recently been made in determining the structure of the magnetosphere that surrounds these objects: numerical computations based on the force-free, magnetohydrodynamic and particle-in-cell formalisms have now established that the magnetosphere of an oblique rotator entails a current sheet outside its light cylinder whose rotating distribution pattern moves with linear speeds exceeding the s…
Resonant cyclotron scattering (RCS) in pulsar magnetospheres is considered. The photon diffusion equation (Kompaneets equation) for RCS is derived. The photon system is modeled three dimensionally. Numerical calculations show that there exist not only up scattering but also down scattering of RCS, depending on the parameter space. RCS's possible applications to the spectra energy distributions of magnetar candidates and radio quiet isolated neutron stars (INSs) are point out. The optical/UV excess of INSs may caused by the down scattering of RCS. The calculations for RX J1856.5-3754 and RX J07…
We investigate the conditions for radio emission in rotating and oscillating magnetars, by focusing on the main physical processes determining the position of their death-lines in the P-\dot{P} diagram, i.e. of those lines that separate the regions where the neutron star may be radio-loud or radio-quiet. After using the general relativistic expression for the electromagnetic scalar potential in the magnetar magnetosphere, we find that larger compactness parameters of the star as well as larger inclination angles between the rotation axis and the magnetic moment produce death-lines well above t…
Magnetar magnetospheres gradually become twisted due to shearing by footpoint motion. The axisymmetric solutions for the force-free field with a power-law current model are calculated by taking into account general relativistic effects. Here we show how the magnetic energy and helicity are accumulated along a sequence of equilibria. In a strongly twisted case, a magnetic flux rope, in which a large amount of toroidal field is confined, detaches in the vicinity of the star. It is found this kind of magnetic field line structure is easily produced due to strong relativistic effects compared with…
In a previous paper, we derived the quantum states of a Dirac particle in a circular, intense magnetic field in the limit of low momentum perpendicular to the field with the purpose of giving a quantum description of the trajectory of an electron, or a positron, in a typical pulsar or magnetar magnetosphere. Here we continue this work by computing the radiation resulting from transitions between these states. This leads to derive from first principles a quantum theory of the so-called curvature and synchro-curvature radiations relevant for rotating neutron-star magnetospheres. We find that, wi…
The magnetar magnetosphere is gradually twisted by shearing from footpoint motion, and stored magnetic energy also increases at the same time. When a state exceeds a threshold, flares/outbursts manifest themselves as a result of a catastrophic transition. Axisymmetric static solutions for a relativistic force-free magnetosphere with dipole--quadrupole mixed fields at the surface have been calculated. The quadrupole component represents a kind of magnetic-field irregularity at a small scale. Locally twisted models are constructed by limiting current flow regions, where the small part originates…
Magnetar flares excite strong Alfvén waves in the magnetosphere of the neutron star. The wave energy can (1) dissipate in the magnetosphere, (2) convert to "fast modes" and possibly escape, and (3) penetrate the neutron star crust and dissipate there. We examine and compare the three options. Particularly challenging are nonlinear interactions between strong waves, which develop a cascade to small dissipative scales. This process can be studied in the framework of force-free electrodynamics (FFE). We perform three-dimensional FFE simulations to investigate Alfvén wave dissipation, how long it …
The remarkable null pulse coincident with the 2016 glitch in Vela rotation indicates a dynamical event involving the crust and the magnetosphere of the neutron star. We propose that a crustal quake associated with the glitch strongly disturbed the Vela magnetosphere and thus interrupted its radio emission. We present the first global numerical simulations of a neutron starquake. Our code resolves the elasto-dynamics of the entire crust and follows the evolution of Alfvén waves excited in the magnetosphere. We observe Rayleigh surface waves propagating away from the epicenter of the quake, arou…
Many millisecond pulsars are thought to be old neutron stars spun up (`recycled') during an earlier accretion phase. They typically have relatively weak ($\lesssim 10^{9} \text{ G}$) dipole field strengths, consistent with accretion-induced magnetic burial. Recent data from the Neutron Star Interior Composition Explorer indicate that hot spots atop the recycled pulsar PSR J0030$+$0451 are not antipodal, so that the magnetic field cannot be that of a centered dipole. In this paper it is shown that multipolarity is naturally expected in the burial scenario because of equatorial field line compre…
We consider the motion of charged particles in the vacuum magnetospheres of rotating neutron stars with a strong surface magnetic field, B>10^(12) G. The electrons and positrons falling into the magnetosphere or produced in it are shown to be captured by the force-free surface EB=0. Using the Dirac-Lorentz equation, we investigate the dynamics of particle capture and subsequent motion near the force-free surface. The particle energy far from the force-free surface has been found to be determined by the balance between the power of the forces of an accelerating electric field and the intensity …
X-ray photons emitted from the surface or atmosphere of a magnetized neutron star is highly polarized. However, the observed polarization may be modified due to photon propagation through the star's magnetosphere. For photon frequencies much larger than the typical radio frequency, vacuum birefringence due to strong-field quantum electrodynamics dominates over the plasma effect. We study the evolution of photon polarization in the magnetized QED vacuum of a neutron star magnetosphere, paying particular attention to the propagation effect across the quasi-tangential (QT) point, where the photon…
We explore the structure of a dipole-type vacuum field of a slowly rotating magnetic star near the horizon of a supermassive black hole, where the structure of field lines becomes highly distorted by effects of strong gravity. Such a situation may occur near a neutron star in the final stages of a plunging trajectory into a galactic center. We solve Maxwell's equations in the Rindler approximation for the rotating conducting source of dipolar magnetic field arbitrarily inclined with respect to the axis of rotation. For the fixed inclination angle we calculate the field including the radiative …
The magnetospheres around neutron stars should be very particular because of their strong magnetic field and rapid rotation. A study of the pulsar magnetospheres is of a crucial importance since it is the key issue to understand how energy inflow to the exterior is produced. In this paper, I discuss magnetohydrodynamic processes in the pulsar magnetosphere. I consider in detail the properties of magnetohydrodynamic waves that can exist in the magnetosphere and their instabilities.These instabilities lead to formation of magnetic structures and can be responsible for a short-term variability of…
