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1.
We introduce a new Rigid-Field Hydrodynamics approach to modelling the magnetospheres of massive stars in the limit of very strong magnetic fields. Treating the field lines as effectively rigid, we develop hydrodynamical equations describing the one-dimensional flow along each, subject to pressure, radiative, gravitational and centrifugal forces. We solve these equations numerically for a large ensemble of field lines to build up a three-dimensional time-dependent simulation of a model star with parameters similar to the archetypal Bp star σ Ori E. Since the flow along each field line can be solved independently of other field lines, the computational cost of this approach is a fraction of an equivalent magnetohydrodynamical treatment.
The simulations confirm many of the predictions of previous analytical and numerical studies. Collisions between wind streams from opposing magnetic hemispheres lead to strong shock heating. The post-shock plasma cools initially via X-ray emission, and eventually accumulates into a warped, rigidly rotating disc defined by the locus of minima of the effective (gravitational plus centrifugal) potential. However, a number of novel results also emerge. For field lines extending far from the star, the rapid area divergence enhances the radiative acceleration of the wind, resulting in high shock velocities (up to  ∼3000 km s−1  ) and hard X-rays. Moreover, the release of centrifugal potential energy continues to heat the wind plasma after the shocks, up to temperatures around twice those achieved at the shocks themselves. Finally, in some circumstances the cool plasma in the accumulating disc can oscillate about its equilibrium position, possibly due to radiative cooling instabilities in the adjacent post-shock regions.  相似文献   

2.
In this third paper in a series on stable magnetic equilibria in stars, I look at the stability of axisymmetric field configurations and, in particular, the relative strengths of the toroidal and poloidal components. Both toroidal and poloidal fields are unstable on their own, and stability is achieved by adding the two together in some ratio. I use Tayler's stability conditions for toroidal fields and other analytic tools to predict the range of stable ratios and then check these predictions by running numerical simulations. If the energy in the poloidal component as a fraction of the total magnetic energy is written as Ep / E , it is found that the stability condition is a ( E / U ) < Ep / E ≲ 0.8 where E /U is the ratio of magnetic to gravitational energy in the star and a is some dimensionless factor whose value is of order 10 in a main-sequence star and of order 103 in a neutron star. In other words, whilst the poloidal component cannot be significantly stronger than the toroidal, the toroidal field can be very much stronger than the poloidal–given that in realistic stars we expect E / U < 10−6. The implications of this result are discussed in various contexts such as the emission of gravitational waves by neutron stars, free precession and a 'hidden' energy source for magnetars.  相似文献   

3.
We study, via a Monte Carlo simulation, a population of isolated asymmetric neutron stars where the magnitude of the magnetic field is low enough so that the dynamical evolution is dominated by the emission of gravitational waves. A starting population, with age uniformly distributed back to 100 Myr (or 500 Myr) and endowed with a birth kick velocity, is evolved in the Galactic gravitational potential to the present time. In describing the initial spatial distribution, the Gould belt, with an enhanced neutron star formation rate, is taken into account. Different models for the initial period distribution are considered. The star ellipticity, measuring the amount of deformation, is drawn from an exponential distribution. We estimate the detectability of the emitted gravitational signals by the first and planned second generation of interferometric detectors. Results are parametrized by the fraction of the whole galactic neutron star population made up of these kinds of sources. Some possible mechanisms, which would make possible the existence of such a population, are discussed. A comparison of the gravitational spin-down with the braking due to a possible interaction of the neutron star with the interstellar medium is also presented.  相似文献   

4.
We study the formation of the absorption features, called the cyclotron–annihilation lines, in the γ-spectra of the neutron stars (pulsars), owing to the fundamental quantum-electrodynamic effect of the one–photon pair creation in magnetized vacuum. As a result, we substantiate a new method for the determination of the neutron star magnetic fields B based on measuring the interval between the main annihilation and the first cyclotron–annihilation absorption lines. It is found that these lines may be easily resolved, and, consequently, the method is surely applicable if the following conditions are satisfied. (i) A γ-source has to be compact enough and located near a star, but not close to its magnetic poles. For instance, it may be a disc in the plane of a star magnetic equator with latitudinal angular width less than     and radial extent up to 25 per cent of the star radius. (ii) The source is to produce detectable γ-radiation at large angles ≳60° to the local magnetic field. Being situated in a closed field line region and having a broad radiation pattern, such a source is not what is usually considered in the context of the polar cap and outer gap models of the pulsar γ-emission dealing with open field lines only. (iii) Magnetic field strength must lie in a certain narrow interval with the centre at  ∼(3–4) × 1012  G. Its width depends on the star orientation and disc radial extend and in the most favourable case is about 20–30 per cent of its lower boundary. Finally, the influence of the star rotation on this method employment is considered and new possibilities arising from forthcoming polarization observations are briefly discussed.  相似文献   

5.
We obtained self-similar solutions of relativistically expanding magnetic loops taking into account the azimuthal magnetic fields. We neglect stellar rotation and assume axisymmetry and a purely radial flow. As the magnetic loops expand, the initial dipole magnetic field is stretched into the radial direction. When the expansion speed approaches the light speed, the displacement current reduces the toroidal current and modifies the distribution of the plasma lifted up from the central star. Since these self-similar solutions describe the free expansion of the magnetic loops, i.e.  d v /d t = 0  , the equations of motion are similar to those of the static relativistic magnetohydrodynamics. This allows us to estimate the total energy stored in the magnetic loops by applying the virial theorem. This energy is comparable to that of the giant flares observed in magnetars.  相似文献   

6.
Three-dimensional numerical magnetohydrodynamic (MHD) simulations are performed to investigate how a magnetically confined mountain on an accreting neutron star relaxes resistively. No evidence is found for non-ideal MHD instabilities on a short time-scale, such as the resistive ballooning mode or the tearing mode. Instead, the mountain relaxes gradually as matter is transported across magnetic surfaces on the diffusion time-scale, which evaluates to  τI∼ 105–108 yr  (depending on the conductivity of the neutron star crust) for an accreted mass of   M a= 1.2 × 10−4 M⊙  . The magnetic dipole moment simultaneously re-emerges as the screening currents dissipate over  τI  . For non-axisymmetric mountains, ohmic dissipation tends to restore axisymmetry by magnetic reconnection at a filamentary neutral sheet in the equatorial plane. Ideal-MHD oscillations on the Alfvén time-scale, which can be excited by external influences, such as variations in the accretion torque, compress the magnetic field and hence decrease  τI  by one order of magnitude relative to its standard value (as computed for the static configuration). The implications of long-lived mountains for gravitational wave emission from low-mass X-ray binaries are briefly explored.  相似文献   

7.
We present the results of three-dimensional hydrodynamical simulations of the final stages of in-spiral in a black hole–neutron star binary, when the separation is comparable to the stellar radius. We use a Newtonian smooth particle hydrodynamics (SPH) code to model the evolution of the system, and take the neutron star to be a polytrope with a soft (adiabatic indices     and     equation of state and the black hole to be a Newtonian point mass. The only non-Newtonian effect we include is a gravitational radiation back reaction force, computed in the quadrupole approximation for point masses. We use irrotational binaries as initial conditions for our dynamical simulations, which are begun when the system is on the verge of initiating mass transfer and followed for approximately 23 ms. For all the cases studied we find that the star is disrupted on a dynamical time-scale, and forms a massive     accretion torus around the spinning (Kerr) black hole. The rotation axis is clear of baryons (less than 10−5 M⊙ within 10°) to an extent that would not preclude the formation of a relativistic fireball capable of powering a cosmological gamma-ray burst. Some mass (the specific amount is sensitive to the stiffness of the equation of state) may be dynamically ejected from the system during the coalescence and could undergo r-process nucleosynthesis. We calculate the waveforms, luminosities and energy spectra of the gravitational radiation signal, and show how they reflect the global outcome of the coalescence process.  相似文献   

8.
We have re-analysed the long-term optical light curve (LC) of the symbiotic star Z Andromedae, covering 112 yr of mostly visual observations. Two strictly periodic cycles and one quasi-periodic cycle can be identified in this LC. A   P 1 = 7550  d quasi-periodicity characterizes the repetition time of the outburst episodes of this symbiotic star. Six such events have been recorded so far. During quiescence states of the system, that is, in time-intervals between outbursts, the LC is clearly modulated by a stable coherent period of   P 2 = 759.1  d. This is the well-known orbital period of the Z Andromedae binary system that has been measured also spectroscopically. A third coherent period of   P 3 = 658.4  d is modulating the intense fluctuations in the optical brightness of the system during outbursts. We attribute the trigger of the outburst phenomenon and the clock that drives it, to a solar-type magnetic dynamo cycle that operates in the convection and the outer layers of the giant star of the system. We suggest that the intense surface activity of the giant star during maximum phases of its magnetic cycle is especially enhanced in one or two antipode regions, fixed in the atmosphere of the star and rotating with it. Such spots could be active regions around the North Pole and the South Pole of a general magnetic dipole field of the star. The P3 periodicity is half the beat of the binary orbital period of the system and the spin period of the giant. The latter is then either 482 or 1790 d. If only one pole is active on the surface of the giant, P3 is the beat period itself, and the spin period is 352 d. It could also be 5000 d if the giant is rotating in a retrograde direction. We briefly compare these findings in the LC of Z Andromedae to similar modulations that were identified in the LC of two other prototype symbiotics, BF Cyg and YY Her.  相似文献   

9.
The gravitational radiation from millisecond pulsars owing to glitches in their angular velocity is examined. It is assumed that the energy transferred from interior superfluid regions to the crust of a neutron star is converted into gravitational wave energy by damping oscillations of the matter in the star. The gravitational wave intensity and amplitude are calculated for fourteen millisecond pulsars. Gravitational radiation can explain the observed spin-down of millisecond pulsars and an estimate is given for the magnetic field at which the proposed mechanism predominates in the spin-down of these pulsars. __________ Translated from Astrofizika, Vol. 51, No. 3, pp. 479–486 (August 2008).  相似文献   

10.
Neutron stars contain persistent, ordered magnetic fields that are the strongest known in the Universe. However, their magnetic fluxes are similar to those in magnetic A and B stars and white dwarfs, suggesting that flux conservation during gravitational collapse may play an important role in establishing the field, although it might also be modified substantially by early convection, differential rotation, and magnetic instabilities. The equilibrium field configuration, established within hours (at most) of the formation of the star, is likely to be roughly axisymmetric, involving both poloidal and toroidal components. The stable stratification of the neutron star matter (due to its radial composition gradient) probably plays a crucial role in holding this magnetic structure inside the star. The field can evolve on long time scales by processes that overcome the stable stratification, such as weak interactions changing the relative abundances and ambipolar diffusion of charged particles with respect to neutrons. These processes become more effective for stronger magnetic fields, thus naturally explaining the magnetic energy dissipation expected in magnetars, at the same time as the longer-lived, weaker fields in classical and millisecond pulsars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

11.
We present a numerical study of the hydrodynamics in the final stages of inspiral of a black hole–neutron star binary, when the binary separation becomes comparable to the stellar radius. We use a Newtonian three-dimensional smooth particle hydrodynamics (SPH) code, and model the neutron star with a soft (adiabatic index Γ=5/3) polytropic equation of state, and the black hole as a Newtonian point mass that accretes matter via an absorbing boundary at the Schwarzschild radius. Our initial conditions correspond to tidally locked binaries in equilibrium, and we have explored configurations with different values of the mass ratio q M NS M BH, ranging from q =1 to 0.1. The dynamical evolution is followed for approximately 23 ms, and in every case studied here we find that the neutron star is tidally disrupted on a dynamical time-scale, forming a dense torus around the black hole that contains a few tenths of a solar mass. A nearly baryon-free axis is present in the system throughout the coalescence, and only modest beaming of a fireball that could give rise to a gamma-ray burst would be sufficient to avoid excessive baryon contamination. We find that some mass (of the order of 10−3–10−2 M⊙) may be dynamically ejected from the system, and could thus contribute substantially to the amount of observed r-process material in the galaxy. We calculate the gravitational radiation waveforms and luminosity emitted during the coalescence in the quadrupole approximation.  相似文献   

12.
We find numerical solutions of the coupled system of Einstein–Maxwell equations with a linear approach, in which the magnetic field acts as a perturbation of a spherical neutron star. In our study, magnetic fields having both poloidal and toroidal components are considered, and higher order multipoles are also included. We evaluate the deformations induced by different field configurations, paying special attention to those for which the star has a prolate shape. We also explore the dependence of the stellar deformation on the particular choice of the equation of state and on the mass of the star. Our results show that, for neutron stars with mass   M = 1.4 M⊙  and surface magnetic fields of the order of 1015 G, a quadrupole ellipticity of the order of 10−6 to 10−5 should be expected. Low-mass neutron stars are in principle subject to larger deformations (quadrupole ellipticities up to 10−3 in the most extreme case). The effect of quadrupolar magnetic fields is comparable to that of dipolar components. A magnetic field permeating the whole star is normally needed to obtain negative quadrupole ellipticities, while fields confined to the crust typically produce positive quadrupole ellipticities.  相似文献   

13.
Equilibrium models of differentially rotating nascent neutron stars are constructed, which represent the result of the accretion-induced collapse of rapidly rotating white dwarfs. The models are built in a two-step procedure: (1) a rapidly rotating pre-collapse white dwarf model is constructed; (2) a stationary axisymmetric neutron star having the same total mass and angular momentum distribution as the white dwarf is constructed. The resulting collapsed objects consist of a high-density central core of size roughly 20 km, surrounded by a massive accretion torus extending over 1000 km from the rotation axis. The ratio of the rotational kinetic energy to the gravitational potential energy of these neutron stars ranges from 0.13 to 0.26, suggesting that some of these objects may have a non-axisymmetric dynamical instability that could emit a significant amount of gravitational radiation.  相似文献   

14.
It is shown that the radius of curvature of magnetic field lines in the polar region of a rotating magnetized neutron star can be significantly less than the usual radius of curvature of the dipole magnetic field. The magnetic field in the polar cap is distorted by toroidal electric currents flowing in the neutron star crust. These currents close up the magnetospheric currents driven by the electron–positron plasma generation process in the pulsar magnetosphere. Owing to the decrease in the radius of curvature, electron–positron plasma generation becomes possible even for slowly rotating neutron stars, with   PB −2/312 < 10 s  , where P is the period of star rotation and   B 12= B /1012 G  is the magnitude of the magnetic field on the star surface.  相似文献   

15.
We analyse a light curve (LC) of the symbiotic star BF Cyg, covering 114 yr of its photometric history. The star had a major outburst around the year 1894. Since then the mean optical brightness of the system is in steady decline, reaching only in the last few years its pre-outburst value. Superposed on this general decline are some six less intense outbursts of 1–2 mag and duration of 2000–5000 d. We find a cycle of 6376 d, or possibly twice this period, in the occurrence of these outbursts. We suggest that the origin of the system outbursts is in some magnetic cycle in the outer layers of the giant star of the system, akin to the less intense 8000-d magnetic cycle of our Sun. We further find, that in addition to its well-known binary period of 757.3 d, BF Cyg possesses also another photometric period of 798.8 d. This could be the rotation period of the giant star of the system. If it is, the beat period of these two periodicities, 14 580 d, is the rotation period of a tidal wave on the surface of the giant. A fourth period of 4436 d, the beat period of the 14 580-d and the 6376-d cycles is possibly also present in the LC. We predict that BF Cyg will be at the peak of its next outburst around the month of May in the year 2007. The newly discovered 798.8-d period explains the disappearance of the orbital modulation at some epochs in the LC. The 757.3-d oscillations will be damped again around the year 2013.  相似文献   

16.
In this paper we model the gravitational wave emission of a freely precessing neutron star. The aim is to estimate likely source strengths, as a guide for gravitational wave astronomers searching for such signals. We model the star as a partly elastic, partly fluid body with quadrupolar deformations of its moment of inertia tensor. The angular amplitude of the free precession is limited by the finite breaking strain of the star's crust. The effect of internal dissipation on the star is important, with the precession angle being rapidly damped in the case of a star with an oblate deformation. We then go on to study detailed scenarios where free precession is created and/or maintained by some astrophysical mechanism. We consider the effects of accretion torques, electromagnetic torques, glitches and stellar encounters. We find that the mechanisms considered are either too weak to lead to a signal detectable by an Advanced LIGO interferometer, or occur too infrequently to give a reasonable event rate. We therefore conclude that, using our stellar model at least, free precession is not a good candidate for detection by the forthcoming laser interferometers.  相似文献   

17.
We discuss the observed orbital period modulations in close binaries, and focus on the mechanism proposed by Applegate relating the changes of the stellar internal rotation associated with a magnetic activity cycle with the variation of the gravitational quadrupole moment of the active component; the variation of this quadrupole moment in turn forces the orbital motion of the binary stars to follow the activity level of the active star. We generalize this approach by considering the details of this interaction, and develop some illustrative examples in which the problem can be easily solved in analytical form. Starting from such results, we consider the interplay between rotation and magnetic field generation in the framework of different types of dynamo models, which have been proposed to explain solar and stellar activity. We show how the observed orbital period modulation in active binaries may provide new constraints for discriminating between such models. In particular, we study the case of the prototype active binary RS Canum Venaticorum, and suggest that torsional oscillations — driven by a stellar magnetic dynamo — may account for the observed behaviour of this star. Further possible applications of the relationship between magnetic activity and orbital period modulation, related to the recent discovery of binary systems containing a radio pulsar and a convecting upper main-sequence or a late-type low-mass companion, are discussed.  相似文献   

18.
We investigate the stochastic gravitational wave background that results from neutron star birth throughout the Universe. The neutron star birth rate, as a function of redshift, is calculated using an observation-based model for the evolving star formation rate, together with an estimate of the rate of core-collapse supernovae in the nearby Universe and an estimate of the neutron star/black hole branching ratio. Using three sample waveforms, based on numerical models of stellar core collapse by Zwerger & Müller, the spectral flux density, spectral strain, spectral energy density and duty cycle of the background have been computed. Our results show, contrary to recent claims, that the spectrum of the stochastic background clearly reflects the different physics in the core-collapse models. For a star formation model that is corrected for dust extinction, the neutron star formation rate throughout the Universe is high enough to result in a nearly continuous background of gravitational waves, with spectral features that can be related to emission mechanisms.  相似文献   

19.
Soft gamma repeaters outside the Local Group   总被引：1，自引：0，他引：1  
We propose that the best sites to search for soft gamma repeaters (SGRs) outside the Local Group are galaxies with active massive-star formation. Different possibilities to observe SGR activity from these sites are discussed. In particular, we have searched for giant flares from the nearby galaxies (∼2–4 Mpc away) M82, M83, NGC 253 and 4945 in the Burst and Transient Source Experiment (BATSE) data. No candidate giant SGR flares were found. The absence of such detections implies that the rate of giant flares with energy release in the initial spike above  0.5 × 1044 erg  is less than 1/30 yr−1 in our Galaxy. However, hyperflares similar to that of 2004 December 27 can be observed from larger distances. Nevertheless, we do not see any significant excess of short GRBs from the Virgo galaxy cluster or from the galaxies Arp 299 and NGC 3256 (both with extremely high star formation rates). This implies that the Galactic rate of hyperflares with energy release  ∼1046 erg  is less than ∼10−3 yr−1. With this constraint the fraction of possible extragalactic SGR hyperflares among BATSE's short GRBs should not exceed a few per cent. We present the list of short GRBs coincident with the galaxies mentioned above, and discuss the possibility that some of them are SGR giant flares. We propose that the best target for the observations of extragalactic SGR flares with Swift is the Virgo cluster.  相似文献   

20.
We treat the phenomenon of a γ -ray burst as the non-linear collapse of a magnetic cavity surrounding a neutron star with extremely large magnetic field B ∼1015–1016 G due to the process of bubble shape instability in the resonant MHD field of an accreting plasma or on a neutron star surface. The QED effect of vacuum polarizability by a strong magnetic field is taken into a consideration. We develop an analogy with the phenomenon of sonoluminescence in which the gas bubble is located in a surrounding liquid with a driven sound intensity.  相似文献   

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