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At least 505 records · Page 28
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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 NTRS1974

Interstellar scattering of the Vela pulsar

The frequency dependence of the parameters of interstellar scattering between 837 and 8085 MHz for the Vela pulsar are consistent with thin-screen models of strong scattering. The magnitudes of the parameters indicate an anomalous turbulence along the path when they are compared with results for other pulsars with comparable column densities of free electrons in the line of sight. This anomaly is due presumably to the Gum Nebula. The decorrelation frequency, appropriately defined, is related to the pulse broadening time by 2 pi as predicted theoretically.

Backer, D. C.↗
NASA NTRS1974

Is Jupiter's magnetosphere like a pulsar's or earth's?

The application of pulsar physics to determine the magnetic structure in the planet Jupiter outer magnetosphere is discussed. A variety of theoretical models are developed to illuminate broad areas of consistency and conflict between theory and experiment. Two possible models of Jupiter's magnetosphere, a pulsar-like radial outflow model and an earth-like convection model, are examined. A compilation of the simple order of magnitude estimates derivable from the various models is provided.

Kennel, C. F.↗
NASA NTRS1975

Optical radiation from the Crab pulsar

Possible mechanisms for producing the optical radiation from the Crab pulsar are proposed and discussed. There are severe difficulties in interpreting the radiation as being produced by an incoherent process, whether it be synchrotron radiation, inverse-Compton radiation, or curvature radiation. It is proposed, therefore, that radiation in the optical part of the spectrum is coherent. In the polar cap model, a small bunch of electrons and positrons forms near each primary electron as a result of the pair-production cascade process. Turbulent electric field components associated with large-scale bunching may lead to separation of the electrons and positrons in space and in energy, with the result that coherent radiation may be produced by the electrons, the positrons, or both. Calculations, which involve a number of simplifying assumptions, indicate that the optical radiation from the Crab pulsar can be understood in this way if the mass of the star is approximately 0.3 solar mass. Various consequences of this model, which may be subjected to observational test, are discussed.

Sturrock, P. A.↗
NASA NTRS1975

Clusters in the distribution of pulsars in period, pulse-width, and age

The question of whether pulsars form a single group or whether pulsars come in two or more different groups is discussed. It is proposed that such groups might be related to several factors such as the initial creation of the neutron star, or the orientation of the magnetic field axis with the spin axis. Various statistical models are examined.

Baker, K. B.↗
NASA NTRS1976

X-ray observations of the Vela pulsar: Statistics and spectrum

The Vela pulsar was observed in the range 2-60 keV by the GSFC proportional counter experiment onboard OSO-8 with temporal resolution sufficient to make possible a sensitive search for pulsed X-rays at the radio pulsar period. A statistical analysis yielded 8 per cent as the 3 sigma upper limit on the pulsed fraction. The energy spectrum is fit well by a structureless power law with number index 2.21 + or - 0.2 and absorption by a hydrogen column density of N sub H equals 2.9 + or - 2.0 times ten to the twenty-second power per sq.cm.

Pravdo, S. H.↗
NASA NTRS1976

Gamma-ray emission and nucleosynthesis of lithium by young pulsars

It is proposed that Li-7 is produced in the Galaxy primarily by alpha-alpha collisions surrounding newly born pulsars. About 10 percent of the pulsar energy losses are converted to medium-energy alpha-particles which collide in a dominantly He nebula. The problem of the origin of lithium would be solved by the scenario, and clear-cut tests by nuclear gamma-ray astronomy are described.

Clayton, D. D.↗
NASA NTRS1976

Radius of the Vela pulsar

Lasker (1976) has proposed to identify a B roughly equals 23.7 mag star as thermal radiation from the hot neutron star associated with the Vela pulsar. It is shown that this identification, if correct, together with existing X-ray surveys of the Vela supernova remnant, requires that the radius of this pulsar certainly exceed 45 km, and probably 95 km.

Greenstein, G.↗
NASA NTRS1976

X-ray observations of the Vela pulsar - Statistics and spectrum

The Vela pulsar (3U 0833-45) was observed in the range 2-60 keV by a proportional-counter experiment on board OSO-8 with temporal resolution sufficient to make possible a sensitive search for pulsed X-rays at the radio pulsar period. A statistical analysis yielded 8 percent as the 3-sigma upper limit on the pulsed fraction. The energy spectrum is fitted well by a structureless power law with number index 2.2 (+ or - 0.2) and absorption by a hydrogen column density of 2.9(+ or - 2.0) by 10 to the 22nd power per sq cm.

Pravdo, S. H.↗
NASA NTRS1977

The time structure of the gamma-ray emission from the Crab and Vela pulsars

High resolution data on the pulsed gamma-ray emission from the Crab and Vela pulsars are presented. The light curves of these two pulsars at gamma-ray energies show striking similarities. The measured pulsed intensity from Vela at energies greater than 50 MeV was found to be .000013 sq cm/sec. The energy spectrum is not consistent with a power law.

Source record↗
NASA NTRS1977

Small scale turbulence in the Crab Nebula: Evidence of lower hybrid parametric instabilities driven by the pulsar wave

Strong small scale turbulence is observed in the Crab Nebula from the temporal pulse broadening data. It is shown that the strong 30 Hz pulsar wave can parametrically excite instabilities near the lower hybrid frequency in the thermal plasma of the Crab Nebula with a characteristic wavelength of the order of the scale size a of the turbulence observed. These instabilities provided a coupling mechanism between the pulsar wave and the Nebula plasma.

Lee, L. C.↗
NASA NTRS1976

Meter-wavelength VLBI. IV - Temporal and spatial scattering of the Crab Nebula pulsar's radiation

The spatial and temporal phenomena observed in the Crab Nebula pulsar's radiation are reviewed, with emphasis on the recent VLBI observations of the apparent angular size and the ratio of pulsing to total power. Three models for scattering (single thin screen, two screen, and extended medium) are discussed and shown to be deficient, under the usual assumption of a uniform scattering medium, in explaining the observed combination of temporal and angular broadening. A new model is proposed, in which the temporal broadening is dominated by scattering in the small filament which lies in the nebula in front of the pulsar, while the extended interstellar medium causes the angular broadening. This model explains the otherwise discordant angular and temporal effects and also the lack of an apparent source-size expansion with pulse phase.

Vandenberg, N. R.↗
NASA NTRS1976

Pulsar average wave forms and hollow-cone beam models

Pulsar wave forms have been analyzed from observations conducted over a wide radio-frequency range to assess the wave-form morphologies and to measure wave-form widths. The results of the analysis compare favorably with the predictions of a model with a hollow-cone beam of fixed dimensions and with random orientation of both the observer and the cone axis with respect to the pulsar spin axis. A class of three-component wave forms is included in the model by adding a central pencil beam to the hollow-cone hypothesis. The consequences of a number of discrepancies between observations and quantitative predictions of the model are discussed.

Backer, D. C.↗
NASA NTRS1977

A search for thermal extreme ultraviolet radiation from nearby pulsars

Results are presented for extreme ultraviolet (100-1000 A) observations of radio pulsars. Using an EUV telescope carried aboard the Apollo-Soyuz mission, data were acquired on the nearby pulsars PSR 1133 + 16, 1451 - 68, and 1929 + 10. None of the objects was detected, with typical upper limits of 6 by 10 to the -26th power erg/sq cm/sec per Hz in the 170-620-A band set on flux incident at earth. The data are interpreted to set limits on the effective temperatures of the neutron stars, yielding values of less than 160,000 K in the best cases, and the limits compared with theoretical predictions.

Greenstein, G.↗
NASA NTRS1977

Binary X-ray pulsars

Recent progress in the study of the pulse profiles of the nine known binary X-ray pulsars is discussed. The sources considered include SMC X-1, Her X-1, Cen X-3, A0535+26, GX1+4, 3U 0900-40, A1118-61, GX301-2, and 3U 0352+30. Sample X-ray profiles for these sources are provided, and general features exhibited by the profiles are identified. It is noted that the apparent existence of a gap in the period distribution of X-ray pulsars, in the range from about 5 to 100 sec, seems to be genuine within obvious statistical limitations and is probably not an observational effect. Two explanations for the production of X-ray pulsations are examined: one requiring a temporal modulation of mass accretion onto the surface of a neutron star and another requiring the funnelling of matter along magnetic field lines to the neutron star's surface.

Rappaport, S.↗
NASA NTRS1977

Accretion torques in X-ray pulsars

An analysis of the accretion process in an X-ray pulsar, whereby angular momentum is transferred to the star and its rotation period is changed, is presented, and an expression for the fractional rate of change of the pulse period in terms of X-ray luminosity and other star parameters is derived. It is shown that observed characteristic spin-up time scales for seven X-ray pulsars strongly support the view that in every source (1) the pulse period reflects the rotation period of a compact object, (2) the accretion is mediated by a disk surrounding the compact object and rotating in the same sense, and (3) the compact object is a neutron star rather than a white dwarf.

Rappaport, S.↗
NASA NTRS1978

Particle acceleration in pulsar magnetospheres

The structure of pulsar magnetospheres and the acceleration mechanism for charged particles in the magnetosphere was studied using a pulsar model which required large acceleration of the particles near the surface of the star. A theorem was developed which showed that particle acceleration cannot be expected when the angle between the magnetic field lines and the rotation axis is constant (e.g. radial field lines). If this angle is not constant, however, acceleration must occur. The more realistic model of an axisymmetric neutron star with a strong dipole magnetic field aligned with the rotation axis was investigated. In this case, acceleration occurred at large distances from the surface of the star. The magnitude of the current can be determined using the model presented. In the case of nonaxisymmetric systems, the acceleration is expected to occur nearer to the surface of the star.

Baker, K. B.↗