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At least 145 records · Page 8

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NASA NTRS1981

PKS 2155-304 relativistically beamed synchrotron radiation from BL LAC object

The newly discovered BL Lacertae object, PKS 2155-304, was observed with the medium and high intensity energy detectors of the HEAO-1 A2 experiment. The variability by a factor of two in less than a day reported by Snyder, et al (1979) is confirmed. Two spectra, obtained a year apart, while the satellite was in scanning mode, are well fit by simple power laws with energy spectral index alpha sub 1 equals approximately 1.4. A third spectrum, of higher statistical quality, obtained while the satellite was pointed at its source, has has two components. An acceptable fit was obtained using a two power law model, with indices alpha sub 1 equals 2.0 (+1.2, -0.6) and alpha sub 2 equals -1.5 (+1.5, -2.3). An interpretation of the overall spectrum from radio through X-rays in terms of a synchrotron self-Compton model gives a good description of the data if allowance is made for relativistic beaming. Thus, from a consideration of the spectrum, combined with an estimate of the size of the source, the presence of jets is inferred without their observation.

Urry, C. M.↗

NASA NTRS1981

Synchrotron brightness distribution of turbulent radio jets

Radio jets are considered as turbulent mixing regions and it is proposed that the essential small scale viscous dissipation in these jets is by emission of MHD waves and by their subsequent strong damping due, at least partly, to gyro-resonant acceleration of supra-thermal particles. A formula relating the synchrotron surface brightness of a radio jet to the turbulent power input is deduced from physical postulates, and is tested against the data for NGC315 and 3C31 (NGC383). The predicted brightness depends essentially on the collimation behavior of the jet, and, to a lesser extent, on the CH picture of a 'high' nozzle with accelerating flow. The conditions for forming a large scale jet at a high nozzle from a much smaller scale jet are discussed. The effect of entrainment on the prediction is discussed with the use of similarity solutions. Although entrainment is inevitably associated with the turbulent jet, it may or may not be a dominant factor depending on the ambient density profile.

Henriksen, R. N.↗

NASA NTRS1980

The position angle of Jupiter's linearly polarized synchrotron emission Observations extending over 16 years

Parkes, Owens Valley and Goldstone measurements are presented, showing the variation with central meridian longitude of the position angle of Jupiter's linearly polarized synchrotron emission at wavelengths of 21, 13, 11 and 6 cm; the observations span a total time interval greater than one Jovian orbital period. The form of the position angle versus longitude curve shows a slight dependence on wavelength and epoch. The epoch dependence appears to reflect Jupiter's changing aspect with respect to the earth. The observations yield a Jovian rotation period agreeing to within about 0.02 s with that derived from the decametric burst measurements, implying a highly stable inner magnetic field configuration.

Komesaroff, M. M.↗

NASA NTRS1981

Synchrotron cooling and annihilation of an e/+/-e/-/ plasma - The radiation mechanism for the 5 March, 1979 transient

Positron-electron pair radiation is examined as a mechanism that could be responsible for the impulsive phase emission of the 5 March, 1979 transient. Synchrotron cooling and subsequent annihilation of the pairs can account for the energy spectrum, the very high brightness, and the 0.4 MeV feature observed from this transient, whose source is likely to be a neutron star in the supernova remnant N49 in the Large Magellanic Cloud. In this model, the observed radiation is produced in the skin layer of a hot, radiation-dominated pair atmosphere, probably confined to the vicinity of the neutron star by a strong magnetic field. In this layer, about 10 to the 12th generations of pairs are formed (by photon-photon collisions), cooled and annihilated during the 0.15 s duration of the impulsive phase.

Ramaty, R.↗

NASA NTRS1981

A comparison of the radio data and model calculations of Jupiter's synchrotron radiation. I - The high energy electron distribution in Jupiter's inner magnetosphere. II - East-west asymmetry in the radiation belts as a function of Jovian longitude

A comparison has been made between detailed model calculations of Jupiter's synchrotron radiation and the radio data at wavelengths of 6, 21, and 50 cm. The calculations were performed for a Jovian longitude of 200 deg and were based on the multipole field configurations as derived from the Pioneer data. The electron distribution in the inner magnetosphere was derived as a function of energy, pitch angle, and spatial coordinates. In addition, the hot region or east-west asymmetry in the radiation belts is investigated. It is suggested that this asymmetry is due to the combined effect of an overabundance of electrons at jovicentric longitudes of 240-360 deg and the existence of a dusk-to-dawn directed electric field over the inner magnetosphere generated by the wind system in the upper atmosphere.

De Pater, I.↗

NASA NTRS1981

Synchrotron emissivity from mildly relativistic particles

A simple approximate method for the evaluation of the spectrum and directivity of the synchrotron radiation from an ensemble of particles in a given magnetic field is considered, taking into account the results of this method for the total emissivity at high harmonics. The general formula is presented along with the details of the derivation of the emissivity for an arbitrary energy spectrum and pitch angle distribution of particles. The emissivities for some commonly encountered particle distributions are provided and some empirical formulae valid for a large range of the parameters are given. The results from the analytic expressions are compared with those obtained by detailed numerical calculations. For isotropic pitch angle distributions, spectra from the simple analytic expressions agree with the numerical results to a high degree of accuracy even at lower harmonics.

Petrosian, V.↗

NASA NTRS1982

PKS 2155-304 - Relativistically beamed synchrotron radiation from a BL Lacertae object

The previously reported variability by a factor of two in less than a day is confirmed for the newly discovered BL Lac object PKS 2155-304, by means of the HEAO 1 A-2 experiment's medium- and high-energy detectors. An interpretation of the overall spectrum from radio through X-rays in terms of a synchrotron self-Compton model yields a good description of the data if relativistic beaming is allowed for. The presence of jets is inferred without their direct observation through consideration of the spectrum, combined with an estimate of the size of the source.

Urry, C. M.↗

NASA NTRS1982

X-ray diffraction from high pressure Ge using synchrotron radiation

The high pressure structural phase transition in Ge has been studied using the energy dispersive X-ray diffraction technique and a synchrotron radiation source. Ge was observed to transform to the beta-Sn tetragonal structure in agreement with the earlier results of Jamieson, but the phase transition began at 80 + or - 5 kilobars, a somewhat lower value than generally reported. These experimental diffraction results are compared with the recent self-consistent pseudopotential calculations of Yin and Cohen (1981) and with the observed transition pressure for shock wave loaded Ge.

Baublitz, M., Jr.↗

NASA NTRS1983

X-ray studies of BL Lacertae objects with the Einstein Observatory - Confrontation with the synchrotron self-Compton predictions

X-ray flux densities measured with the Einstein Observatory are presented for a set of LB Lac objects with available VLBI angular sizes. The static, single-component synchrotron self-Compton formalism is used to predict the X-ray flux densities. Ratios of 10 to the -6th to the 10 to the -8th are found for the observed to predicted flux densities and it is concluded that such a model, at least in its simplest form, does not explain the X-ray emission from these objects as a class. The hypothesis that the compact source is in relativistic motion allows a simple resolution of the discrepancy. The results agree qualitatively with a broad distribution of bulk Lorentz factors Gamma with a fairly low typical value of Gamma approximately equal to and with a quasi-isotropic distribution of the direction of ejection relative to our line of sight.

Madejski, G. M.↗

NASA NTRS1984

Simple collimator for use with diamond-anvil cells in a synchrotron beam

A simple double-slit system is described which allows a synchrotron beam to be collimated onto a small sample (approximately 150-micron diameter) in a diamond-anvil cell using remote control. The apparatus can be constructed easily and inexpensively, and allows the shutter positions to be monitored accurately using readily available electronic equipment. A desirable feature of the collimator is the relatively small time needed for adjustment.

Spain, I. L.↗

NASA NTRS1985

Synchrotron Emission from Chaotic Stellar Winds

A model is presented for the radio emission from hot stars. Electrons are accelerated to relativistic energies by shocks in the wind near the star and emit radio radiation through the synchrotron mechanism. The particle spectrum and radio spectrum for this model are derived. The model accounts for many of the observed characteristics of some recently discovered stars which have peculiar radio emission.

White, R. L.↗

NASA NTRS1985

Onion-shell model for cosmic ray electrons and radio synchrotron emission in supernova remnants

The spectrum of cosmic ray electrons, accelerated in the shock front of a supernova remnant (SNR), is calculated in the test-particle approximation using an onion-shell model. Particle diffusion within the evolving remnant is explicity taken into account. The particle spectrum becomes steeper with increasing radius as well as SNR age. Simple models of the magnetic field distribution allow a prediction of the intensity and spectrum of radio synchrotron emission and their radial variation. The agreement with existing observations is satisfactory in several SNR's but fails in other cases. Radiative cooling may be an important effect, especially in SNR's exploding in a dense interstellar medium.

Beck, R.↗

NASA NTRS1985

Induced emission of radiation near 2(omega sub e) by a synchrotron-maser instability

In the literature, the emission of radiation at 2(omega sub e), where omega sub e denotes the electron plasma frequency, is usually explained as having been produced by the nonlinear interaction of two Langmuir waves via a backscattering process. Since the emission is frequently observed in solar radio bursts, the mechanism has attracted considerable theoretical interest. In the present paper a model is proposed based on a synchrotron-maser instability excited by a hollow beam of moderately relativistic electrons in a plasma, in which the plasma frequency is much higher than the gyrofrequency. An important conclusion is that, as a result of this instability, unpolarized electromagnetic waves with frequencies near 2(omega sub e) may be amplified.

Wu, C. S.↗

NASA NTRS1985

Studies of the Si/SiO2 interface using synchrotron radiation

Synchrotron radiation photoemission spectroscopy (SRPS) in the 1-4 KeV photon energy range is a useful tool for interface characterization. Results are presented of a series of studies of the near-interface region of Si/SiO2 which confirm that a bond strain gradient exists in the oxide as a result of lattice mismatch. These experiments include measurement of photoemission lineshape changes as a function of photon energy, corresponding changes in the electron escape depth near the interface, and surface extended X-ray absorption fine structure (SEXAFS) measurements directly indicating the shortening of the Si-Si second nearest neighbor distance in the near-interface region of the oxide.

Hecht, M. H.↗

NASA NTRS1986

New synchrotron luminosity distance limit for the 1979 March 5 gamma-ray event

New synchrotron luminosity estimates are provided for the March 5, 1979 gamma-ray event, based on the lack of low-frequency self-absorption cutoff. This leads to distance limits much lower than 55 kpc for a neutron-star emission area even in the most conservative case. The source is likely Galactic. If in addition the thermal self-Compton interpretation is invoked for the hard tail above 300 keV in the burst spectrum, then the source distance lies in the range 0.1-2 kpc for extreme ranges of the neutron-star emission area. This result strongly impacts current observational programs for this source.

Liang, E. P.↗

NASA NTRS1986

The synchrotron-maser theory of type II solar radio emission processes - The physical model and generation mechanism

A theory is proposed to explain the generation mechanism of type II solar radio bursts. It is suggested that the shock wave formed at the leading edge of a coronal transient can accelerate electrons. Because of the nature of the acceleration process, the energized electrons can possess a 'hollow-beam' type distribution function. When the electron beam propagates along the ambient magnetic field to lower altitudes and attains larger pitch angles, a synchrotron-maser instability can set in. This instability leads to the amplification of unpolarized or weakly polarized radiation. The present discussion incorporates a model which describes the ambient magnetic field and background plasma by means of MHD simulation. The potential emission regions may be located approximately, according to the time-dependent MHD simulation. Since the average local plasma frequency in the source region can be evaluated from the MHD model, the frequent drift associated with the radiation may be estimated. The result seems to be in good agreement with that derived from observations.

Wu, C. S.↗

NASA NTRS1986

Synchrotron and inverse Compton emission from expanding sources in jets - Application to SS 433

The formalism for an expanding source is extended to include continuous injection of electrons and the production of X-rays by inverse Compton scattering of external photons and synchrotron-self-Compton scattering. The model is applied to the expanding nonthermal sources entrained in the jets of SS 433. These sources are found capable of producing both the flaring radio component and the X-ray fluxes only if the relativistic electrons and the magnetic field are far out of equipartition. If the expanding source in the jets is in equipartition, the X-rays may originate from a stationary source within the binary system, which is self-absorbed at radio frequencies and produces X-rays by inverse Compton scattering of optical photons from the binary.

Band, David L.↗

NASA NTRS1986

Trace element analyses of spheres from the melt zone of the Greenland ice cap using synchrotron X ray fluorescence

Synchrotron X-ray fluorescence spectra of unpolished iron and chondritic spheres extracted from sediments collected on the melt zone of the Greenland ice cap allow the analysis of Ni, Cu, Zn, Ga, Ge, Pb, and Se with minimum detection limits on the order of several parts per million. All detected elements are depleted relative to chondritic abundance with the exception of Pb, which shows enrichments up to a factor of 500. An apparent anticorrelation between the Ni-content and trace element concentration was observed in both types of spherules. The fractionation patterns of the iron and chondritic spheres are not complementary and consequently the two iron spheres examined in this study are unlikely to result from ejection of globules of Fe/Ni from parent chondritic micrometeoroids.

Chevallier, P.↗