Particle motion and electromagnetic fields of rotating compact gravitating objects with gravitomagnetic charge

The exact solution for the electromagnetic field occuring when the Kerr–Taub–NUT compact object is immersed (i) in an originally uniform magnetic field aligned along the axis of axial symmetry (ii) in dipolar magnetic field generated by current loop has been investigated. Effective potential of motion of charged test particle around Kerr–Taub–NUT gravitational source immersed in magnetic field with different values of external magnetic field and NUT parameter has been also investigated. In both cases presence of NUT parameter and magnetic field shifts stable circular orbits in the direction of the central gravitating object. Finally we find analytical solutions of Maxwell equations in the external background spacetime of a slowly rotating magnetized NUT star. The star is considered isolated and in vacuum, with monopolar configuration model for the stellar magnetic field.     

太阳日冕物质抛射的磁流体力学性质

太阳是离地球最近的一颗恒星，太阳日冕物质抛射是太阳大气中最剧烈的一种活动现象．当日冕物质抛射爆发时，大量的等离子体物质从接近太阳日面的低日冕被抛出，瞬时释放出巨大的能量．当一部分这些物质和能量传播到地球附近时，可以造成短波通讯中断、卫星工作失常等破坏性现象．文章作者认为，是缠绕的太阳磁场提供了足够的能量，使这些日冕物质可以克服恒星的重力以及周边磁场的束缚抛射出来；而磁螺度在日冕中的不断积累，不仅为日冕物质抛射提供了能量基础，而且使爆发在一定程度上成为一种日冕演化的必然。     

A quantum phase space formulation of radiative transfer

A recent transport model for partially coherent light is reexamined. In its original formulation, this model involves a transport equation for the one-photon Wigner function with nonlocal source and loss terms. We show here that under suitable approximations this equation can be reformulated in a compact form involving the Moyal star product and the related symmetric and antisymmetric brackets. This formulation provides a general framework for the establishment of opacity models accounting for radiation coherence. An investigation of anomalous dispersion in magnetic fusion plasma conditions is reported as an illustration of the model.     

Energy dissipation and angular momentum transfer within a magnetically torqued accretion disc

We discuss transportation and redistribution of energy and angular momentum in the magnetic connection (MC) process and Blandford-Payne (BP) process. MC results in readjusting the interior viscous torque, and its effects are operative not only in but also beyond the MC region. The BP process is invoked to transfer the “excessive” angular momentum from an accretion disc. In addition, we derive a criterion for the interior viscous torque to resolve the puzzle of the overall equilibrium of angular momentum in disc accretion. It turns out that the efficiency of BP at extracting angular momentum and the intensity of the outflow are required to be greater than some critical values.

     

Temporal study of GRS 1915+105 with rms-flux relation: The importance of magnetic activities in the corona

The rms-flux relations for some observations of GRS 1915+105 are studied. The rms-flux relations of the light curves in only one state, state C or state A, can be described by the simple non-linear model provided Zhang; we thus interpret that such a linear relation reflects the relative importance of magnetic instability for X-ray emission in the corona of the system, compared to the thermal viscous instability for the X-ray emission in the accretion disk. The rms-flux relations for state B are very scattered, possibly because of the dominance of thermal viscous instability for the X-ray emission in the accretion disk. The complex rms-flux relations for the observations of transitions between two or three states are caused by the combination of the different rms-flux relations of these states. The underlying physical processes are the combination of magnetic topology in the corona and thermal viscous instability in the accretion disk.

     

A Toy Model for Magnetic Field Configurations in Black Hole Accretion Discs

We discuss the feature of the magnetic field configuration arising from double counter oriented electric currentrings in the accretion disc around a Kerr black hole （BH）. We discuss the relevant physical quantities corresponding to this configuration： （1） the power and torque transferred by the large-scale magnetic field, （2） the angular momentum and energy fluxes transferred from the BIt to the inner disc, （3） the radiation flux from the disc. In addition, we discuss the possibility that the closed magnetic field anchored at the disc probably evolves to the open magnetic field, which is helpful to produce the jet from the disc.     

A Multi-parameter Model for Radio Dichotomy of Active Galactic Nuclei and Jets

Based on the coexistence of the Blandford-Znajek and magnetic coupling processes in black hole (BH) accretion disc, a multi-parameter model for jet powers and radio loudness of active galactic nuclei (AGNs) is studied. It turns out that radio-loudness of AGNs could be governed by five parameters: (i) the BH spin, (ii) a power-law index of the variation of the magnetic field on the disc; (iii) a parameter determining the position of the inner edge of the disc, (iv) the ratio of the pressure of the magnetic field on the horizon to the ram pressure of the innermost parts of an accretion flow, and (v) the ratio of the angular velocity of the open field lines to that of the horizon. The observed dichotomy between radio-loud and radio-quiet AGNs is well interpreted by the effects of the above parameters. Furthermore, we discuss the derivative of radio loudness of AGNs with respect to each parameter separately. In addition, the effect of the screw instability on radio loudness of AGNs is discussed.     

X-Ray emission from two-phase accretion disks

Summary We consider a ?two phase? accretion disk consisting of an optically thick layer with temperatureT≃10⁴ K embedded in a hot thin corona (T≃10⁹ K). The main energy input occurs through magnetic heating of the electrons in the corona, while cooling is due to Compton losses of the hot electrons on the soft photons provided by the thick layer. We write the balance equations for the two phases. We show that a possible mode of variability yields steeper spectra for increasing soft-photon luminosity as observed in Seyfert galaxies and compute composite model spectra in the X-ray range, via Monte Carlo simulations. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Optically—thick accretion discs with advection

The structures of optically-thick accretion discs with radial advection have been investigated by the iteration and integration algorithms. The advective cooling term changes mostly the inner part of disc solution, and even results in an optically-thick advection-dominated accretion flow (ADAF). Three distinct branches－the outer Shakura－Sunyaev disc (SSD), the inner ADAF and the middle transition layer－are found for a super-Eddington disc. The SSD－ADAF transition radius can be estimated as 18(\dot{M}/\dot{M}_E)R_G where R_G is the Schwarzschild radius, \dot{M} is the mass accretion rate and \dot{M}_E is the Eddington accretion rate. SSD solutions calculated with the iteration and integration methods are identical, while ADAF solutions obtained by these two methods differ greatly. Detailed algorithms and their differences have been analysed. The iteration algorithm is not self-consistent, since it implies that the dimensionless advection factor ξ is invariant, but in the inner ADAF region the variation of ξ is not negligible. The integration algorithm is always effective for the whole region of an optically-thick disc if the accretion rate is no smaller than 10^-4\dot{M}_E. For optically-thin discs, the validity of these two algorithms is different. We suggest that the integration method be employed to calculate the global solution of a disc model without assuming ξ to be a constant. We also discuss its application to the emergent continuum spectrum in order to explain observational facts.     

RX J0720.4-3125 as a possible example of magnetic field decay in neutron stars

We consider the possible evolution of the rotation period and magnetic field of the X-ray source RX J0720.4-3125, assuming that this source is an isolated neutron star accreting from the interstellar medium. The magnetic field of the source is estimated to be 10⁶–10⁹ G (the most probable value is about 2·10⁸ G), and it is difficult to explain the observed rotational period 8.38 s without invoking the hypothesis of magnetic field decay. For calculations we used the model of ohmic dissipation of the field in the core of the neutron star. Estimates for the accretion rate (10^?14–10^?16 M_⊙/yr), velocity of the source through the interstellar medium (10–50 km/s), and neutron star age (2·10⁹–10¹⁰ yrs) are obtained.     

Observing Lense-Thirring precession in tidal disruption flares

When a star is tidally disrupted by a supermassive black hole (SMBH), the streams of liberated gas form an accretion disk after their return to pericenter. We demonstrate that Lense-Thirring precession in the spacetime around a rotating SMBH can produce significant time evolution of the disk angular momentum vector, due to both the periodic precession of the disk and the nonperiodic, differential precession of the bound debris streams. Jet precession and periodic modulation of disk luminosity are possible consequences. The persistence of the jetted x-ray emission in the Swift J164449.3+573451 flare suggests that the jet axis was aligned with the spin axis of the SMBH during this event.     

X-ray emission from non-Keplerian magnetically dominated accretion disks

Summary Most accretion disk models do assume Keplerian rotation as the ?natural? one. This is not so obvious and is somewhat aprioristic, as shown by some attempts to follow different approaches. In the present work we suggest a new approach to the problem, pointing out that the disk structure is deeply related to the balance between the magnetic energy produced by dynamo action and the rotational energy associated with the accreting flow. This balance, together with the conservation laws, allows us to derive analytically the angular-velocity field, the radial velocity field and the magnetic-energy distribution inside the disk. As far as the disk X-ray emission is concerned, we adopt the point of view that the buoyancy and emergence at the disk surface of the azimuthal magnetic field generated by differential rotation gives rise to looplike structures in a hot magnetically confined corona, which is not merely overimposed, as in other models, but ?naturally? fits to the disk structure. The magnetic-energy dissipation into the coronal medium, which sustains the X-ray luminosity, is due to twisting of the magnetic loops. we give an exact analytical expression for the integrated luminosityL _x in terms of the mass of the compact object, the accretion rate, the disk size and rotational velocity, and show that in the limitr _i≪r _e (wherer _i andr _e are the inner and the outer radius of the accretion disk, respectively) it agrees fairly well with the characteristic observed values of both galactic and extragalactic strong compact sources, within the reasonable range commonly accepted for the relevant parameters. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Equilibrium of a gravitating plasma in a dipolar magnetic field

The equilibria of plasma in a dipolar magnetic field under the gravitational influence of a massive body (a star or black hole) and a self gravitating plasma are considered. Analytical solutions are found that can be useful for understanding the physics of plasma flows in accretion disks and star formation.     

Fate of an accretion disc around a black hole when both the viscosity and dark energy is in effect

This paper deals with the viscous accretion flow of a modified Chaplygin gas towards a black hole as the central gravitating object. A modified Chaplygin gas is a particular type of dark energy model which mimics of radiation era to phantom era depending on the different values of its parameters. We compare the dark energy accretion with the flow of adiabatic gas. An accretion disc flowing around a black hole is an example of a transonic flow. To construct the model, we consider three components of the Navier–Stokes equation, the equation of continuity and the modified Chaplygin gas equation of state. As a transonic flow passes through the sonic point, the velocity gradient being apparently singular there, it gives rise to two flow branches: one in-falling, the accretion and the other outgoing, the wind. We show that the wind curve is stronger and the wind speed reaches that of light at a finite distance from the black hole when dark energy is considered. Besides, if we increase the viscosity, the accretion disc is shortened in radius. These two processes acting together make the system deviate much from the adiabatic accretion case. It shows a weakening process for the accretion procedure by the work of the viscous system influencing both the angular momentum transport and the repulsive force of the modified Chaplygin gas.     

Accreting neutron stars as probes of dense matter physics

There are over 100 accreting neutron stars in our galaxy, in which matter (typically H/He) is tidally transferred from a secondary companion to the neutron star. Accretion of this matter perturbs the thermal structure of the interior away from that of an isolated cooling neutron star. In this paper. we review how this accretion induces reactions in the crust of the neutron star that keep the interior hot. If the accretion is intermittent, then the heated surface layers are directly observable when accretion stops. This heating also affects the unstable ignition of light elements in the neutron star envelope. Observations of the neutron star cooling following an accretion outburst can in principle constrain the thermal properties of the crust and core.     

Isolated neutron stars in the galaxy: from magnetars to antimagnetars

Using the model with decaying magnetic fields it is possible to describe with one smooth (log-Gaussian) initial magnetic field distribution three types of isolated neutron stars: radiopulsar, magnetars, and cooling close-by compact objects. The same model is used here to make predictions for old accreting isolated neutron stars. It is shown that using the updated field distribution we predict a significant fraction of isolated neutron stars at the stage of accretion despite long subsonic propeller stage.     

棒-板电极直流负电晕放电特里切尔脉冲的微观过程分析

本文基于流体动力学理论改进出一种新的棒-板电极负电晕放电混合数值模型, 模型中加入了27种主要碰撞反应, 并考虑了光电离和二次电子发射过程. 对棒-板间距3.3 mm, 施加电压-5.0 kV情况下进行数值计算, 得到负电晕放电的特里切尔脉冲. 重点分析了单个特里切尔脉冲持续过程中5个关键时刻的微观特征量发展规律, 丰富并量化描述了特里切尔脉冲的微观过程, 主要结论如下: 随着放电时间的发展, 电场集中分布区域向阳极移动且幅值变小, 这对电子崩的发展非常不利. 大部分放电区域都是电中性的, 只有在阴极鞘和阳极鞘附近有带正电的等离子体特性, 带负电的离子云随着放电时间的发展缓慢向阳极发散式移动. 整个特里切尔脉冲持续过程中, 阴极鞘内电子密度几乎为0; 特里切尔脉冲前期, 阴极鞘附近电子密度迅速增加至最大值并保持基本不变; 随着放电时间的增加, 放电间隙内电子密度整体增加, 并且向阳极发展. 在特里切尔脉冲后期, 电子的产生主要来自于N₂和O₂的碰撞电离, 电子的消失则主要由N₂⁺的复合决定, O₄⁺和O₂^-分别是数量最多的正离子和负离子. 关键词： 负电晕 混合模型 特里切尔脉冲 微观特征量     

Sonic Point Instability in the Causally Limited Viscosity and Magnetized Accretion Disc

The stability of sonic point in an isothermal accretion disc is examined for the case in which the causally limited viscosity and magnetic fields coexist. The results show that the sonic point is a saddle and stable against small amplitude perturbations. Finally, we discuss these results.     

Viscous shear in general-relativistic astrophysical flows

Conclusion Introducing the concept of general co-moving frames (gcmf) in [4] we have argued that it may become useful in a number of hydrodynamical and mhd applications. In [5] using thegcmf technique we have constructed a fully covariant, general-relativistic theory of strongly magnetized collisionless plasma. The approach proved itself to be highly convenient-it allowed us to find new equations of state for such a medium.In the present paper we have considered viscous shear in generalrelativistic astrophysical flows as an another example of the effective usage of orthonormal tetrads method. Namely, we have specified general corotating frames (gcrf)-subclass ofgcmf corresponding to the flows being in purely rotational motion. By means of gcrf we have been able to find expressions for nonzero components of shear tensor and turbulent viscosity tensor for the innermost region of a black hole accretion disc.We think that the method may be useful when considering analogous problems with astrophysical flows of more complicated geometry and/or dynamics. In particular, the method may become efficient for jets in active galactic nuclei (agn) and quasars [13], general-relativistic winds of compact objects [14] and the innermost regions of candidates for galactic black hole accretion discs [15]. To be sure, in some of these problems we have to use a more general set ofgcmf instead ofgcrf. Such problems, however, are beyond the scope of this paper, where we have only outlined the main background of the method and demonstrated its productivity in a simple case of quasi-keplerian accretion flow in a general-relativistic standard accretion disc.     

The influence of frequency redistribution on the transfer of gyroresonant photons in the atmospheres of compact stars: Monte-Carlo analysis

We numerically model the frequency redistribution of gyroresonant photons in a plane-parallel semi-infinite plasma atmosphere of a compact star by means of the Monte-Carlo method. We calculate the mean frequency shift and the mean number of scatterings for photons leaving the atmosphere as functions of the optical depth at emission, plasma temperature, and magnetic field strength. The probability of the escape of cyclotron-radiation photons from the atmosphere as a function of their optical depth at birth is calculated. The influence of the frequency redistribution effects on the spectrum of emergent radiation is studied.