Rotation Profiles of Solar-like Stars with Magnetic Fields

We investigate the rotation profile of solar-like stars with magnetic fields. A diffu- sion coefficient of magnetic angular momentum transport is deduced. Rotating stellar models with different mass incorporating the coefficient are computed to give the rotation profiles. The total angular momentum of a solar model with only hydrodynamic instabilities is about 13 times larger than that of the Sun at the age of the Sun, and this model can not reproduce quasi-solid rotation in the radiative region. However, the solar model with magnetic fields not only can reproduce an almost uniform rotation in the radiative region, but also a total angular momentum that is consistent with the helioseismic result at the 3 tr level at the age of the Sun. The rotation of solar-like stars with magnetic fields is almost uniform in the radiative region, but for models of 1.2-1.5 MG, there is an obvious transition region between the convective core and the radiative region, where angular velocity has a sharp radial gradient, which is different from the rotation profile of the Sun and of massive stars with magnetic fields. The change of angular velocity in the transition region increases with increasing age and mass.

Frame Dragging Effect on Properties of Rotating Neutron Stars with Strong Magnetic Field

The general relativistic frame dragging effect on the properties,such as the moments of inertia and the radiiof gyration of fast rotating neutron stars with a uniform strong magnetic field,is calculated accurate to the first orderin the uniform angular velocity.The results show that compared with the corresponding non-rotating static sphericalsymmetric neutron star with a weaker magnetic field,a fast rotating neutron star(millisecond pulsar)with a strongermagnetic field has a relative smaller moment of inertia and radius of gyration.

Research on the Magnetic Field of NGC 7793 P13 and Other Confirmed Pulsating Ultraluminous X-Ray Sources

A pulsating ultraluminous X-ray source(PULX)is a new kind of pulsar(PSR)whose characteristics are different from all known neutron stars.The magnetic field of PULX is suspected to be the main reason to support its supper Eddington luminosity of PULX.NGC 7793 P13,which is the second confirmed PULX,can be easily studied due to its nearby position and isolation from other sources in its host galaxy.In this paper,we calculate its magnetic field to be∼1.0×10^(12) G based on the continued observations from 2016 to 2020.The magnetic field evolution of NGC 7793 P13 is analyzed,which shows that the source has spent about 10^(4) yr for the field decaying from the simulated initial strength 4.0×10^(14) G to the present value.In case of an assumed constant accretion and the limitation of the companion mass,it will be a recycled PSR whose magnetic field is ∼10^(9) G and spin period is a few hundred milliseconds.We estimate the field strength of the other confirmed PULXs and find main range is 10^(13)-10^(14) G.Their positions of the magnetic field and spin period are around or below the magnetars.This is because these PULXs are in the binary systems and are with the spin-up rate that are 2-3 orders higher than the normal binary pulsars.We suggest that PULXs are the accreting magnetars whose multi-pole strong magnetic field can support the supper Eddington luminosity.They would be helpful for studying the evolution of the magnetars,the formation of the binary PSRs above the Eddington spin-up line,and the millisecond PSRs with the magnetic field stronger than ∼10^(9) G.

Effect of magnetic field decay on the chemical heating of cooling neutron stars

The effect of magnetic field decay on the chemical heating and thermal evolution of neutron stars is discussed in this paper. Our main goal is to study how the chemical heating mechanism and thermal evolution are changed by the field decay and how the magnetic field decay is modified by the thermal evolution. We compare stars cooling with chemical heating with one without chemical heating and find that the decay of the magnetic field is delayed significantly by the chemical heating. We find that the effect of chemical heating has been suppressed through the decaying magnetic field by the spin-down of the stars at a later stage. Compared with typical chemical heating, we find the decay of the magnetic field can even cause the surface temperature to turn down at an older age. When we discuss the cooling of neutron stars, we should consider the coupling effect of the magnetic field and the rotational evolution of neutron stars on the heating mechanisms.

Unifying neutron star sub-populations in the supernova fallback accretion model

We employ the supernova fallback disk model to simulate the spin evolution of isolated young neutron stars(NSs). We consider the submergence of the NS magnetic fields during the supercritical accretion stage and its succeeding reemergence. It is shown that the evolution of the spin periods and the magnetic fields in this model is able to account for the relatively weak magnetic fields of central compact objects and the measured braking indices of young pulsars. For a range of initial parameters, evolutionary links can be established among various kinds of NS sub-populations including magnetars, central compact objects and young pulsars. Thus, the diversity of young NSs could be unified in the framework of the supernova fallback accretion model.

Pulsating magneto-dipole radiation of a quaking neutron star powered by energy of Alfvn seismic vibrations

We compute the characteristic parameters of the magneto-dipole radiation of a neutron star undergoing torsional seismic vibrations under the action of Lorentz restoring force about an axis of a dipolar magnetic field experiencing decay. After a brief outline of the general theoretical background of the model of a vibration-powered neutron star, we present numerical estimates of basic vibration and radiation characteristics, such as frequency, lifetime and luminosity, and investigate their time dependence on magnetic field decay. The presented analysis suggests that a gradual decrease in frequencies of pulsating high-energy emission detected from a handful of currently monitored AXP/SGR-like X-ray sources can be explained as being produced by the vibration-powered magneto-dipole radiation of quaking magnetars.

The Remnant of Neutron Star-White Dwarf Merger and the Repeating Fast Radio Bursts

Fast radio bursts (FRBs) at cosmological distances still hold concealed physical origins. Previously Liu (2018) proposes a scenario that the collision between a neutron star (NS) and a white dwarf (WD) can be one of the progenitors of non-repeating FRBs and notices that the repeating FRBs can also be explained if a magnetar formed after such NS-WD merger. In this paper, we investigate this channel of magnetar formation in more detail. We propose that the NS-WD post-merger, after cooling and angular momentum redistribution, may collapse to either a black hole or a new NS or even remains as a hybrid WDNS, depending on the total mass of the NS and WD. In particular, the newly formed NS can be a magnetar if the core of the WD collapsed into the NS while large quantities of degenerate electrons of the WD compressed to the outer layers of the new NS. A strong magnetic field can be formed by the electrons and positive charges with different angular velocities induced by the differential rotation of the newborn magnetar. Such a magnetar can power the repeating FRBs by the magnetic reconnections due to the crustal movements or starquakes.

Short-living Supermassive Magnetar Model for the Early X-ray Flares Following Short GRBs

We suggest a short-lived supermassive magnetar model to account for the X-ray flares following short γ-ray bursts. In this model the central engine of the short γ-ray bursts is a supermassive millisecond magnetar, formed in coalescence of double neutron stars. The X-ray flares are powered by the dipole radiation of the magnetar. When the magnetar has lost a significant part of its angular momentum, it collapses to a black hole and the X-ray flares cease abruptly.

A possible origin of the Galactic Center magnetar SGR 1745–2900

Since there is a large population of massive O/B stars and putative neutron stars （NSs） located in the vicinity of the Galactic Center （GC）, intermediate-mass X-ray binaries （IMXBs） constituted by an NS and a B-type star probably exist there. We investigate the evolutions of accreting NSs in IMXBs （similar to M82 X-2） with a - 5.2 M companion and orbital period 2.53 d. By adopting a mildly super-Eddington rate M = 6 × 10-8 M yr-1 for the early Case B Roche-lobe overflow （RLOF） accretion, we find that only in accreting NSs with quite elastic crusts （slippage factor s = 0.05） can the toroidal magnetic fields be amplified within 1 Myr, which is assumed to be the longest duration of the RLOF. These IMXBs will evolve into NS＋white dwarf （WD） binaries if they are dynamically stable. However, before the formation of NS＋WD binaries, the high stellar density in the GC will probably lead to frequent encounters between the NS＋evolved star binaries （in post-early Case B mass transfer phase） and NSs or exchange encounters with other stars, which may produce single NSs. These NSs will evolve into magnetars when the amplified poloidal magnetic fields diffuse out to the NS surfaces. Consequently, our results provide a possible expianation for the origin of the GC magnetar SGR 1745-2900. Moreover, the accreting NSs with s 〉 0.05 will evolve into millisecond pulsars （MSPs）. Therefore, our model reveals that the GC magnetars and MSPs could both originate from a special kind of IMXB.

The force-free dipole magnetosphere in nonlinear electrodynamics

Quantum electrodynamics(QED)effects may be included in physical processes of magnetar and pulsar magnetospheres with strong magnetic fields.Involving the quantum corrections,Maxwell electrodynamics is modified to nonlinear electrodynamics.In this work,we study the force-free magnetosphere in nonlinear electrodynamics in a general framework.The pulsar equation describing a steady and axisymmetric magnetosphere is derived,which now admits solutions with corrections.We derive the first-order nonlinear corrections to the near-zone dipole magnetosphere in some popular nonlinear effective theories.The field lines of the corrected dipole tend to converge on the rotational axis so that the fields in the polar region are stronger compared to the pure dipole case.

High Energy Emission from Magnetar Due to Giant Flare

We propose a theoretical model for magnetar giant flare to explain the flaring activity on 2004 December 27 from SGR1806-20 comprehensively. A global rearrangement is expected by the magnetic reconnection that requires explaining the giant SGR flares. In this paper we propose two regions of flares: preflare on the surface of magnetar and main burst at a distance of light cylinder radius. Acquiring the maximum potential drop on the magnetar surface, adopting space charge limited flow model, and using magnetic field B ≈ 1015 G, the luminosities of flare energies release for the preflare phase and main burst phase are found to be in the order of 1041 erg·sˉ1 and 1044 erg·sˉ1 respectively, conforming to magnetar burst energy and flare temperature is determined by considering black body radiation.

脉冲星减速研究与电磁和引力波辐射

脉冲星具有极强的磁场,由于其磁轴与自转轴并不完全重合,这使脉冲星存在电磁辐射.脉冲星靠消耗自转能来弥补辐射出去的能量,从而导致自旋速度的逐渐放慢.根据理论推导,对蟹状星云脉冲星(Crab)的初始周期进行预测,并推导出在未来任意时刻,在电磁辐射的影响下,其脉冲星自旋周期的演化规律.由于脉冲星的质量分布可能存在四极矩,引力波辐射也会使得脉冲星的旋转速度减小,因此进一步分析Crab脉冲星在引力波影响下自旋周期的变化.最后将两种辐射机制模型结合,对脉冲星在两种辐射机制共同作用下自旋周期的演化进行分析.

磁星外壳层核素的电子俘获的研究

利用朗道能级量子化近似和核壳层模型,研究了超强磁场对^(53,54,55)Fe的电子俘获的影响.结果表明在超强磁场下的电子俘获率比在弱场近似下的电子俘获率增加了约2个数量级.在弱场近似和磁场为B=4.414×10^(15)Gs条件下计算了每个核素的电子丰度变化率和总的电子丰度变化率,它们一般相差3个量级以上.这些结论对将来的磁星演化的研究起到重要的作用.

较差转动中子星放大磁场的数值计算与分析

存在较大较差转动角速度的中子星会由于磁冻结效应扭曲内部的径向磁场线,放大出一个超过1013T的环向磁场.这个环向磁场的磁浮力大于与其耦合的粒子的重力,能够携带物质冲出中子星外壳,形成喷发.基于此效应提出了较差转动中子星作为伽马射线暴(GRB)和伽马射线暴早期余辉中X射线耀发能源的两个模型,在不同的初始条件下做了数值计算和分析.根据计算得到的环向磁场的放大速度以及初始条件,指出了这两个模型存在的缺陷,最后提出较差转动中子星可作为快速射电暴(FRB)的能量来源.

Pulsar braking:magnetodipole vs.wind

Pulsars are good clocks in the universe. One fundamental question is that why they are good clocks? This is related to the braking mechanism of pulsars. Nowadays pulsar timing is done with unprecedented accuracy. More pulsars have braking indices measured.The period derivative of intermittent pulsars and magnetars can vary by a factor of several. However, during pulsar studies, the magnetic dipole braking in vacuum is still often assumed. It is shown that the fundamental assumption of magnetic dipole braking(vacuum condition) does not exist and it is not consistent with the observations. The physical torque must consider the presence of the pulsar magnetosphere. Among various efforts, the wind braking model can explain many observations of pulsars and magnetars in a unified way. It is also consistent with the up-to-date observations. It is time for a paradigm shift in pulsar studies: from magnetic dipole braking to wind braking. As one alternative to the magnetospheric model, the fallback disk model is also discussed.