On pair production in intense electromagnetic fields occurring in astrophysical situations

We show that the pair production rate in a strong magnetic field is substantially altered when an electric field is also included. We illustrate and emphasize this significant alteration by considering a few special cases. In the vicinity of the polar cap of a rotating magnetized neutron star it is currently though thatboth steady electric and magnetic fields must be present. The results presented here then indicate that some considerable degree of caution must be exercised in applying pair production rates calculated under the assumption of zero electric field to the problems of pulsar emission and the generation and maintence of pulsar magnetospheres. In general such rates are very different from the rate computed allowing for the existence of an electric field.     

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.     

The nonlinear dispersion relation and solutions in the magnetosphere of a pulsar

Assuming some hydrogen atoms are distributed in the magnetosphere of a pulsar, the gas we are dealing with is a mixture of plasma and hydrogen atoms. Because the induced electrical field in the plasma surrounding the pulsar is very strong, due to the rotation of the pulsar associated with a strong magnetic field, the electric polarization of an atom will include the nonlinear term of the electric field. We obtain the nonlinear dispersion relation for the mixed gas from the Maxwell equations and derive the nonlinear Schrödinger equation, which has solitons as its solution under a certain condition. The curvature radiation of solitons moving along the magnetic field lines is a plausible way to explain the strong radio emission which comes from a pulsar, particularly some field lines existing near the light cylinder with radii of curvature smaller than the radius of the pulsar.     

Pulsar natal kick via neutrino-triggered magnetorotational asymmetry

The sterile neutrino mechanisms for pulsar natal kicks are reanalyzed. The necessary strength of the magnetic field generated in a supernova explosion is shown to have been underestimated significantly in previous works. An alternative mechanism for pulsar natal kicks involving only standard neutrinos in which the outgoing neutrino flux in a supernova explosionwith the generation of a strong toroidal magnetic field leads to a magnetic field redistribution in the ??upper?? and ??lower?? hemispheres of the supernova envelope is discussed. The emerging magnetic field pressure asymmetry can produce a pulsar natal kick.     

Populating the Galaxy with pulsars – I. Stellar and binary evolution

The computation of theoretical pulsar populations has been a major component of pulsar studies since the 1970s. However, the majority of pulsar population synthesis has only regarded isolated pulsar evolution. Those that have examined pulsar evolution within binary systems tend to either treat binary evolution poorly or evolve the pulsar population in an ad hoc manner. Thus, no complete and direct comparison with observations of the pulsar population within the Galactic disc has been possible to date. Described here is the first component of what will be a complete synthetic pulsar population survey code. This component is used to evolve both isolated and binary pulsars. Synthetic observational surveys can then be performed on this population for a variety of radio telescopes. The final tool used for completing this work will be a code comprised of three components: stellar/binary evolution, Galactic kinematics and survey selection effects. Results provided here support the need for further (apparent) pulsar magnetic field decay during accretion, while they conversely suggest the need for a re-evaluation of the assumed typical millisecond pulsar formation process. Results also focus on reproducing the observed     diagram for Galactic pulsars and how this precludes short time-scales for standard pulsar exponential magnetic field decay. Finally, comparisons of bulk pulsar population characteristics are made to observations displaying the predictive power of this code, while we also show that under standard binary evolutionary assumption binary pulsars may accrete much mass.     

Scale transformations, tree-level perturbation theory and the cosmological matter bispectrum

Soon after the discovery of radio pulsars in 1967, the pulsars are identified as strongly magnetic (typically 10¹² G) rapidly rotating (∼10²− 0.1 Hz) neutron stars. However, the mechanism of particle acceleration in the pulsar magnetosphere has been a longstanding problem. The central problem is why the rotation power manifests itself in both gamma-ray beams and a highly relativistic wind of electron–positron plasmas, which excites surrounding nebulae observed in X-ray. Here we show with a three-dimensional particle simulation for the global axisymmetric magnetosphere that a steady outflow of electron–positron pairs is formed with associated pair sources, which are the gamma-ray emitting regions within the light cylinder. The magnetic field is assumed to be a dipole, and to be consistent, the pair creation rate is taken to be small, so that the model might be applicable to old pulsars such as Geminga. The pair sources are charge-deficient regions around the null surface, and we identify them as the outer gap. The wind mechanism is the electromagnetic induction which brings about fast azimuthal motion and eventually trans-field drift by radiation drag in the close vicinity of the light cylinder and beyond. The wind causes loss of particles from the system. This maintains charge deficiency in the outer gap and pair creation. The model is thus in a steady state, balancing loss and supply of particles. Our simulation implies how the wind coexists with the gamma-ray emitting regions in the pulsar magnetosphere.     

High Magnetic Field Pulsars and Magnetars: A Unified Picture

We propose a unified picture of high magnetic field radio pulsars and magnetars by arguing that they are all rotating high-field neutron stars but that their magnetic axes have different orientations with respect to their rotation axes. In strong magnetic fields where photon splitting suppresses pair creation near the surface, the high-field pulsars can have active inner accelerators while the anomalous X-ray pulsars cannot. This can account for the very different observed emission characteristics of the anomalous X-ray pulsar 1E 2259+586 and the high-field radio pulsar PSR J1814-1744. A predicted consequence of this picture is that radio pulsars having surface magnetic fields greater than about 2x1014 G should not exist.     

Red supergiants in the LMC — II. Spectrophotometry and model atmospheres

We have analysed polarization data for a large number of isolated pulsars to investigate the evolution of pulsar radio beams. Assuming that a circular beam is directed along the axis of a dipolar magnetic field, we demonstrate that the distribution of magnetic inclination angles for the parent population of all pulsars is not flat but highly concentrated towards small inclination angles and that, consequently, the average beaming fraction is only ∼ 10 per cent. Furthermore, we find that there is a tendency for the beam axis to align with the rotational axis on a time-scale of ∼ 10⁷ yr. This has interesting consequences for statistical studies of the pulsar population. Finally, the luminosity of pulsars is shown to be independent of the impact parameter, which indicates that pulsar beams are sharp-edged and have a relatively flat integrated intensity distribution.     

Toroidal Magnetic Field Generation in the Magnetosphere of Crab Pulsar

Plasma mechanism for the generation of toroidal magnetic field in the magnetosphere of Crab pulsar is presented. The mechanism is based on the development of parametric type instability in the relativistic electron-positron plasma of the pulsar magnetosphere. As a result of plasma corotation with pulsar and its magnetic field, the effect of plasma radial braking takes place and the time dependence of plasma particle radial velocity is harmonic. This triggers the development of parametric type instability in the relativistic plasma of the pulsar magnetosphere. The energy for this process is drawn from the slowing down of pulsar rotation.     

On the analogy between the zebra patterns in radio emission from the sun and the crab pulsar

We investigate the close analogy between the solar radio emission with a quasi-harmonic spectrum structure and one of the microwave emission components of the Crab pulsar in the form of the so-called zebra pattern. The radio emission mechanism of this component can be provided by instability at double plasma resonance and can be realized in extraordinary (for a radio pulsar) conditions, namely in a nonrelativistic plasma with a relatively weak magnetic field. We point out possible models of the emission source in the form of a magnetic trap or a neutral current sheet with a transverse magnetic field localized in the corotating region of the pulsar magnetosphere far from the neutron star surface.     

A method for judging decay or growth of the magnetic field of pulsar

This paper provides a method for judging growth or decay of the magnetic field of pulsar by using pulse period P, or frequency ν, and its first and second derivatives $ \dot P,\ddot P $ or $ \dot v,\ddot v $ . The author uses this method to judge the growth or decay of the magnetic field of Crab pulsar. The judged result for Crab pulsar is that the magnetic field of Crab pulsar is growing now, but it is not decaying. The result corresponds with the actual case of Crab pulsar.     

Theory of the radio emission of pulsars

A consistent theory of excitation, stabilization, and propagation of electromagnetic oscillations in a relativistic one-dimensional electron-positron plasma flowing along curved magnetic field lines is presented. It is shown that in such a medium which is typical of the magnetosphere of a neutron star there exist unstable natural modes of oscillations. Nonlinear saturation of the instability leads to an effective energy conversion into transverse oscillations capable of leaving the magnetosphere of a pulsar. The polarization spectrum and the directivity pattern of generated radiation are determined. A comparison with observations has shown that the theory makes it possible to explain practically all the basic characteristics of observed pulsar radio emission.     

On the radio-frequency emission from the Geminga pulsar

The famous neutron star Geminga was until quite lately the only pulsar undetected in the radio regime, though observed as a strong pulsating γ- and X-ray source. Three independent groups from the Pushchino Radio Astronomy Observatory (Russia) reported recently the detection of pulsed radio emission from Geminga at 102.5 MHz, i.e., the first detection of the radio pulsar PSR J0633 + 1746 by Kuz'min &38; Losovskii, Malofeev &38; Malov and Shitov &38; Pugachev. This pulsar exhibits the weakest radio luminosity known. Its average pulse profile appears to be very wide, filling an entire 360° pulse window according to Kuz'min &38; Losovskii.   We present a model explaining the peculiarities of the Geminga radio pulsar, based on the assumption that it is an almost aligned rotator. The electromagnetic waves generated in the inner magnetosphere reach the region within the light cylinder with a weak magnetic field (at distances of a few light cylinder radii), where they are strongly damped due to the cyclotron resonance with particles of magnetospheric electron–positron plasma. The lowest frequencies that can escape are determined by the value of the magnetic field in the region where the line of sight passes through the light cylinder. The specific viewing geometry of an almost aligned rotator implies that the observer's line of sight probes the emission region near the bundle of the last open field lines. This explains the unusually weak emission from Geminga's low-frequency radio pulsar.     

On the creation of magnetic monopoles in pulsars

The production of pairs of magnetic monopoles-antimonopoles should be expected in the interactions of the high energy particles accelerated by pulsars. In the frame of the Sturrock model, the interactions of the very high energy protons emitted from the polar caps with the secondary electrons can be a source of magnetic monopoles. It may be the dominating process in very young pulsars such as the Crab pulsar. In the polar gap model of Ruderman and Sutherland, magnetic monopoles can be created by the electrons accelerated across the cap and interacting with the neutron star crust or by the negatons and positrons interacting head-on inside the sparks.Half of these monopoles are accelerated towards the interstellar medium by the pulsar magnetic field and the others are likely to be trapped inside the neutron star crust. This leads to a decrease in the pulsar magnetic field which would imply that the characteristic age may not give the true age of the pulsar This can be related to the discrepancy between and the real age of the Crab pulsar and the kinematical ages obtained from the measurement of the proper motion of some pulsars. Furthermore, the trapping of magnetic monopoles close under the surface of the neutron star perturbates the pulsar electrodynamics. To have such observable effects, it is shown that the cross-sections for the magnetic monopoles production can be several orders of magnitude smaller than the upper limits so far derived from cosmic rays or accelerator data.The possibility that the magnetic monopoles, accelerated outwards, are responsible for the highest energy extensive air showers, is considered.The production of an avalanche of secondary monopoles, due to acceleration by the magnetic field in the neutron star crust, is possible and the consequences of this process are considered.     

Modelling interaction of relativistic and non-relativistic winds in binary system PSR B1259−63/SS2883 – I. Hydrodynamical limit

In this paper, we present a detailed hydrodynamical study of the properties of the flow produced by the collision of a pulsar wind with the surrounding in a binary system. This work is the first attempt to simulate interaction of the ultrarelativistic flow (pulsar wind) with the non-relativistic stellar wind. Obtained results show that the wind collision could result in the formation of an 'unclosed' (at spatial scales comparable to the binary system size) pulsar wind termination shock even when the stellar wind ram pressure exceeds significantly the pulsar wind kinetic pressure. Moreover, the post-shock flow propagates in a rather narrow region, with very high bulk Lorentz factor (γ∼ 100). This flow acceleration is related to adiabatic losses which are purely hydrodynamical effects. Interestingly, in this particular case, no magnetic field is required for formation of the ultrarelativistic bulk outflow. The obtained results provide a new interpretation for the orbital variability of radio, X-ray and gamma-ray signals detected from binary pulsar system PSR B1259−63/SS2883.     

The production of electron-positron bunches in a pulsar magnetosphere and their thermal radioemission

It is shown that the high brightness temperature of pulsar radioemission, its sharp directivity and some general features of pulsar radiospectra can be obtained as a corollary of the general statistical regularities of motion of electron-positron bunches in a strong magnetic field. The production mechanism of the bunches is considered and their energy and sizes are estimated. In the presented pulsar model, the energy of accelerated electrons is estimated.     

On the electric field screening by electron–positron pairs in a pulsar magnetosphere

We present a steady one-dimensional model for a pulsar polar cap accelerator, where the field-aligned electric field and flow are solved self-consistently with a given current density. It is assumed that no particles return to the star. It is known that the space-charge-limited flow is accelerated to energies high enough to create electron–positron pairs if the assumed current density is high enough. We find that when pairs are created in such a space-charge-limited flow, the accelerating electric field is screened out within a short distance after pair creation, if the pair particle flux is larger than a critical value. We also find that a space charge density wave is excited in the screening region.
We find that a pair flux larger than the critical value M _c=10³–10⁵ must be reached in a layer with thickness equal to the braking distance for the decelerating component. Therefore, the required multiplicity – the number of pairs created by one primary particle – is too large to be realized in the actual pulsar magnetosphere. We suggest that in order to obtain a localized potential drop along the polar cap magnetic flux, one needs to take into account additional effects such as wave–particle interaction or quasi-periodic pair creation.     

The micro-glitch in PSR B1821−24: a case for a strange pulsar?

The single glitch observed in PSR B1821−24, a millisecond pulsar in M28, is unusual on two counts. First, the magnitude of this glitch is at least an order of magnitude smaller  (Δν/ν∼ 10⁻¹¹)  than the smallest glitch observed to date. Secondly, all other glitching pulsars have strong magnetic fields with   B ≳ 10¹¹ G  and are young, whereas PSR B1821−24 is an old recycled pulsar with a field strength of  2.25 × 10⁹ G  . We have earlier suggested that some of the recycled pulsars could actually be strange quark stars. In this work, we argue that the crustal properties of such a strange pulsar are just right to give rise to a glitch of this magnitude, explaining the scarcity of larger glitches in millisecond pulsars.     

Tearing instability in the pulsar magnetosphere

Equations governing the coupling of the scalar and vector potentials for a resistive electron-positron plasma in a strong magnetic field are derived. It is shown that in the presence of magnetic shear, a tearing instability may occur. The latter can lead to magnetic field line reconnection and the formation of magnetic islands which could affect the dynamics of the pulsar magnetosphere.     

The role of multipolar magnetic fields in pulsar magnetospheres

We explore the role of complex multipolar magnetic fields in determining physical processes near the surface of rotation powered pulsars. We model the actual magnetic field as the sum of global dipolar and star-centred multipolar fields. In configurations involving axisymmetric and uniform multipolar fields, 'neutral points' and 'neutral lines' exist close to the stellar surface. Also, the curvature radii of magnetic field lines near the stellar surface can never be smaller than the stellar radius, even for very high-order multipoles. Consequently, such configurations are unable to provide an efficient pair-creation process above pulsar polar caps, necessary for plasma mechanisms of generation of pulsar radiation. In configurations involving axisymmetric and non-uniform multipoles, the periphery of the pulsar polar cap becomes fragmented into symmetrically distributed narrow subregions where curvature radii of complex magnetic field lines are less than the radius of the star. The pair-production process is only possible just above these 'favourable' subregions. As a result, the pair plasma flow is confined within narrow filaments regularly distributed around the margin of the open magnetic flux tube. Such a magnetic topology allows us to model the system of 20 isolated subbeams observed in PSR B0943+10 by Deshpande & Rankin. We suggest a physical mechanism for the generation of pulsar radio emission in the ensemble of finite subbeams, based on specific instabilities. We propose an explanation for the subpulse drift phenomenon observed in some long-period pulsars.