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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

Current Flow and Pair Creation at Low Altitude in Rotation-Powered Pulsars' Force-Free Magnetospheres: Space Charge Limited Flow

We report the results of an investigation of particle acceleration and electron-positron plasma generation at low altitude in the polar magnetic flux tubes of rotation-powered pulsars, when the stellar surface is free to emit whatever charges and currents are demanded by the force-free magnetosphere. We apply a new 1D hybrid plasma simulation code to the dynamical problem, using Particle-in-Cell methods for the dynamics of the charged particles, including a determination of the collective electrostatic fluctuations in the plasma, combined with a Monte Carlo treatment of the high-energy gamma-rays that mediate the formation of the electron-positron pairs.We assume the electric current flowing through the pair creation zone is fixed by the much higher inductance magnetosphere, and adopt the results of force-free magnetosphere models to provide the currents which must be carried by the accelerator. The models are spatially one dimensional, and designed to explore the physics, although of practical relevance to young, high-voltage pulsars. We observe novel behaviour (a) When the current density j is less than the Goldreich-Julian value (0 < j/j(sub GJ) < 1), space charge limited acceleration of the current carrying beam is mild, with the full Goldreich-Julian charge density comprising the charge densities of the beam and a cloud of electrically trapped particles with the same sign of charge as the beam. The voltage drops are of the order of mc(sup 2)/e, and pair creation is absent. (b) When the current density exceeds the Goldreich-Julian value (j/j(sub GJ) > 1), the system develops high voltage drops (TV or greater), causing emission of curvature gamma-rays and intense bursts of pair creation. The bursts exhibit limit cycle behaviour, with characteristic time-scales somewhat longer than the relativistic fly-by time over distances comparable to the polar cap diameter (microseconds). (c) In return current regions, where j/j(sub GJ) < 0, the system develops similar bursts of pair creation. These discharges are similar to those encountered in previous calculations by Timokhin of pair creation when the surface has a high work function and cannot freely emit charge. In cases (b) and (c), the intermittently generated pairs allow the system to simultaneously carry the magnetospherically prescribed currents and adjust the charge density and average electric field to force-free conditions. We also elucidate the conditions for pair creating beam flow to be steady (stationary with small fluctuations in the rotating frame), finding that such steady flows can occupy only a small fraction of the current density parameter space exhibited by the force-free magnetospheric model. The generic polar flow dynamics and pair creation are strongly time dependent. The model has an essential difference from almost all previous quantitative studies, in that we sought the accelerating voltage (with pair creation, when the voltage drops are sufficiently large; without, when they are small) as a function of the applied current.

Timokhin, A. N.↗
NASA NTRS1974

Quantum descriptions of singularities leading to pair creation

A class of cosmological models is analyzed which provide a mathematically convenient (but idealized) description of a cosmological singularity that develops into a pair creation epoch and terminates in an adiabatic expansion with redshifting particle energies. This class of models was obtained by Gowdy (1971, 1974) as a set of exact solutions of the classical empty space Einstein equations describing inhomogeneous universes populated only by gravitational waves. It is shown that these models can be used to exhibit simplified models of quantized gravitational fields, and that a quantum description can be given arbitrarily near a cosmological singularity. Graviton pair creation occurs, and can be seen to convert anisotropic expansion rates into the energy of graviton pairs.

Misner, C. W.↗
NASA NTRS1985

Missing energies at pair creation

Wilson cloud chamber measurements of the separated spectra of positrons and electrons produced by gamma quanta of 6.14 MeV differ considerably from the theoretically predicted spectra by BETHE and HEITLER, but are in good agreement with those of a modified theory of pair creation.

El-Ela, A. A.↗
NASA NTRS2022

High-energy Photon Opacity in the Twisted Magnetospheres of Magnetars

Magnetars are neutron stars characterized by strong surface magnetic fields generally exceeding the quantum critical value of 44.1 TG. High-energy photons propagating in their magnetospheres can be attenuated by QED processes like photon splitting and magnetic pair creation. In this paper, we compute the opacities due to photon splitting and pair creation by photons emitted anywhere in the magnetosphere of a magnetar. Axisymmetric, twisted dipole field configurations embedded in the Schwarzschild metric are treated. The paper computes the maximum energies for photon transparency that permit propagation to infinity in curved spacetime. Special emphasis is given to cases where photons are generated along magnetic field loops and/or in polar regions; these cases directly relate to resonant inverse Compton scattering models for the hard X-ray emission from magnetars and Comptonized soft gamma-ray emission from giant flares. We find that increases in magnetospheric twists raise or lower photon opacities, depending on both the emission locale and the competition between field-line straightening and field strength enhancement. Consequently, given the implicit spectral transparency of hard X-ray bursts and persistent "tail" emission of magnetars, photon splitting considerations constrain their emission region locales and the twist angle of the magnetosphere; these constraints can be probed by future soft gamma-ray telescopes such as COSI and AMEGO. The inclusion of twists generally increases the opaque volume of pair creation by photons above its threshold, except when photons are emitted in polar regions and approximately parallel to the field.

radiation mechanisms: non-thermal↗
NASA NTRS1990

Massive soliton stars

The structure of nontopological solutions of Einstein field equations as proposed by Friedberg, Lee, and Pang (1987) is examined. This analysis incorporates finite temperature effects and pair creation. Quarks are assumed to be the only species that exist in interior of soliton stars. The possibility of primordial creation of soliton stars in the incomplete decay of the degenerate vacuum in early universe is explored. Because of dominance of pair creation inside soliton stars, the luminosity of soliton stars is not determined by its radiative transfer characteristics, and the surface temperature of soliton stars can be the same as its interior temperature. It is possible that soliton stars are intense X-ray radiators at large distances. Soliton stars are nearly 100 percent efficient energy converters, converting the rest energy of baryons entering the interior into radiation. It is possible that a sizable number of baryons may also be trapped inside soliton stars during early epochs of the universe. In addition, if soliton stars exist they could assume the role played by massive black holes in galactic centers.

Chiu, Hong-Yee↗
NASA NTRS1979

Some problems of pulsar physics

The theories of particle acceleration along polar field lines are reviewed, and the total energization of the charge separated plasma is summarized, when pair creation is absent. The application of these theories and plasma supply to pulsars is discussed, with attention given to the total amount of electron-positron plasma created and its momentum distribution. Various aspects of radiation emission and transport are analyzed, based on a polar current flow model with pair creation, and the phenomenon of marching subpulses is considered. The coronation beaming and the relativistically expanding current sheet models for pulsar emission are also outlined, and the paper concludes with a brief discussion of the relation between the theories of polar flow with pair plasma and the problem of the energization of the Crab Nebula.

Arons, J.↗
NASA NTRS1976

Pulsar extinction

The radiation properties of pulsars are reinvestigated in the context of the 'PCFB' model, according to which the radiation originates at the polar caps and the magnetic-field lines change from a closed to an open configuration at the 'force-balance' or 'corotation' radius. Major attention is given to the condition for electron-positron pair creation, which leads, in turn, to an extinction condition whereby any pulsar will cease to be a radio emitter after its period has increased beyond a certain value. This extinction condition is derived on the basis of a model where the magnetic field is the same as that of a point dipole located at the center of the star; effects of dipole distortion are also considered. A comparison of the results with observational data shows that most pulsars satisfy or nearly satisfy the pair-creation condition for undistorted dipoles and seem to satisfy the extinction condition. It is noted that pulsars which should be extinguished according to the undistorted-dipole model need not be if the magnetic field is sufficiently distorted at the polar caps.

Sturrock, P. A.↗
NASA NTRS1975

Pulsar extinction

Radio emission from pulsars, attributed to an instability associated with the creation of electron-positron pairs from gamma rays was investigated. The condition for pair creation therefore lead to an extinction condition. The relevant physical processes were analyzed in the context of a mathematical model, according to which radiation originated at the polar caps and magnetic field lines changed from a closed configuration to an open configuration at the force balance or corotation radius.

Sturrock, P. A.↗
NASA NTRS1977

Photon pair production in astrophysical transrelativistic plasmas

Photon pair-production processes in marginally relativistic (transrelativistic) accretion plasmas are investigated in both Planckian and non-Planckian cases. Pair production in a plasma with an equilibrium (Planck) spectrum is reviewed, and pair-concentration calculations are performed for three general non-Planckian situations most relevant to black-hole accretion scenarios: steady-state transrelativistic plasmas of relatively high density characterized by a pure bremsstrahlung spectrum, a comptonized bremsstrahlung spectrum, and an unsaturated Compton scattering spectrum. The results obtained indicate that for transrelativistic temperatures (600 million to 6 billion K) photon pair production is not generally a dominant process for a plasma with a pure bremsstrahlung spectrum, but becomes dominant for plasmas where comptonization is important. It is also shown that photon pair-creation processes in a transrelativistic bremsstrahlung-radiating plasma that is more than marginally optically thick to Compton scattering significantly alter the plasma's spectrum by forcing it to become black-body before it reaches relativistic temperatures. Pair production and instabilities in unsteady-state plasmas are briefly considered

Stoeger, W. R.↗
NASA NTRS1973

Magnetosphere structure and radiation mechanisms of pulsars

A presentation is made of the two main models of the radiation mechanism of pulsars: the light cylinder model and the polar cap model. They have to account for gamma radiation, radio and X-ray emission, and in the case of the Crab pulsar, optical radiation. It is hypothesized that pair creation is essential for radio emission. None of the models yields a theoretical pulse-width-period distribution or braking index that correlates well with observations. A revision of the polar cap model suggests that a glitch may be present in the magnetosphere, necessitating the presence of a large centrifugal force strong enough to tear open the magnetic field lines into a Y-type configuration. This model yields a braking index in line with observations.

Roberts, D. H.↗
NASA NTRS1977

Radiation mechanisms and magnetospheric structure of pulsars

Considerations made in developing a model of pulsars are explored. Observational data seems to support the argument that pulsar magnetospheres may contain large masses of plasma. The cascade process resulting from pair creation enables one to interpret the X-ray emission from the Crab and Vela pulsars as synchrotron radiation. On the other hand, the optical radiation from the Crab pulsar is best understood as coherent curvature radiation. Radio emission is interpreted as curvature radiation produced by charge bunches moving along magnetic-field lines. Certain tests of this model are proposed.

Sturrock, P. A.↗
NASA NTRS1980

Presupernova models and supernovae

The present status of theories of presupernova stellar evolution and the triggering mechanisms of supernova explosions are reviewed. The validity of the single-star approximation for stellar core evolution is considered, and the central density and temperature of the stellar core are discussed. Attention is then given to the results of numerical models of supernova explosions by carbon deflagration of an intermediate mass star, resulting in the total disruption of the star; the photodissociation of iron nuclei in a massive star, resulting in neutron star or black hole formation; and stellar core collapse triggered by electron capture in stars of mass ranging between those of the intermediate mass and massive stars, resulting in neutron star formation despite oxygen deflagration. Helium and carbon combustion and detonation in accreting white dwarfs and the gravitational collapse triggered by electron-pair creation in supermassive stars are also discussed, and problems requiring future investigation are indicated.

Sugimoto, D.↗
NASA NTRS1985

Muons in gamma showers

Muon production in gamma-induced air showers, accounting for all major processes. For muon energies in the GeV region the photoproduction is by far the most important process, while the contribution of micron + micron pair creation is not negligible for TeV muons. The total rate of muons in gamma showers is, however, very low.

Stanev, T.↗
NASA NTRS1987

Quantum synchrotron spectra from semirelativistic electrons in teragauss magnetic fields

Synchrotron spectra are calculated from quantum electrodynamic transition rates for thermal and power-law electron distributions. It is shown that quantum effects appear in thermal spectra when the photon energy is greater than the electron temperature, and in power-law spectra when the electron energy in units of the electron rest mass times the magnetic field strength in units of the critical field strength is of order unity. These spectra are compared with spectra calculated from the ultrarelativistic approximation for synchrotron emission. It is found that the approximation for the power-law spectra is good, and the approximation for thermal spectra produces the shape of the spectrum accurately but fails to give the correct normalization. Single photon pair creation masks the quantum effects for power-law distributions, so only modifications to thermal spectra are important for gamma-ray bursts.

Brainerd, J. J.↗
NASA NTRS1987

The pulsar contribution to galactic cosmic ray positrons

Measurements of high energy positrons in the cosmic rays appear to show an increase in the positron fraction above 10 GeV which is inconsistent with theoretical predictions of secondary positron production. We explore the possibility that observations of .1 - 1 GeV and very high energy (VHE) gamma-rays from the Crab and Vela pulsars could imply a significant primary positron contribution from galactic radio pulsars at energies above 10 GeV. Assuming that positrons are produced through magnetic pair creation in the cascades near the polar cap which may be the source of the observed gamma rays, we can estimate the flux and spectrum of the pulsar positron contribution. The pulsar positron component has a flatter spectrum than that expected from secondary cosmic ray production. The level of this contribution above 10 GeV is high enough to make pulsars viable sources of the high energy positron excess, and may also put interesting constraints on pulsar emission models.

Harding, Alice K.↗
NASA NTRS1994

Producing the universal spectrum of cosmological gamma-ray bursts with the Klein-Nishina cross section

A power-law spectrum attenuated through Compton scattering by an optically thick medium produces spectra that have a characteristic energy of several hundred keV. Add a redshift, and one finds that this model can qualitatively reproduce the color-color diagrams found for individual gamma-ray bursts. This model is easily tested through model fits to burst spectra and through comparisons of the parameters derived from model fits to the limits on parameters derived from the burst log N - log P(sub max) curve. The heavy attenuation makes the amount of energy released in the burst approximately equal to 10(exp 3) times larger than is inferred from the observed flux. The requirements of high optical depth and no photon-photon pair creation place a lower limit on the size of the scattering region. This size suggests that the attenuation occurs in giant molecular clouds in the cores of galaxies. This indicates that gamma-ray bursts are probably from supermassive black holes. If the Lorentz factor of the radiation source is large, the optical depth, and therefore the hardness ratio of a burst, can change over the duration of the burst.

Brainerd, J. J.↗