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At least 1,459 records · Page 81
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NASA NTRS1979

New hard X-ray sources observed with HEAO A-2

A search for new hard X-ray sources using data from the first complete view of the sky with the HEAO A-2 experiment has discovered 47 new sources, detected seven sources recently discovered with other experiments, and significantly reduced the size of the error boxes for six previously known sources. Intensities and error boxes are given for each of these sources; identifications are suggested when an error box contains an object similar to known X-ray sources. The new identifications consist of seven type 1 Seyfert galaxies, including two whose Seyfert characteristics were discovered as a result of their location in an X-ray error box; one intermediate Seyfert galaxy; three Abell clusters; five N galaxies; two bursting radio sources; and an additional three nearby galaxies with bright nuclei and narrow emission lines.

Marshall, F. E.↗
NASA NTRS1979

Confinement of hot, hard X-ray producing electrons in solar flares

Possible thermal models for solar, hard X-ray emission, consisting of small volumes in which the electrons are rapidly heated to 400 million K, are examined to determine under what conditions such models can be more efficient than nonthermal models. The analysis is restricted to one dimension by supposing that the hot electrons are confined in a flux tube and that conduction and convection act only along the magnetic field. A conduction front, thin in relation to other length scales involved, separates the hot electrons from the relatively cold plasma. The range of conduction-front velocity is studied by systematizing the present knowledge of heat conduction and applying it first to a simple model in which convection is neglected, and then to a detailed numerical model in which convection is included. Convection in which mass motion goes to zero at the conduction front does not lead to a change in the conduction-front velocity compared to conduction alone, but does constitute another nonradiative energy loss which reduces the source efficiency.

Smith, D. F.↗
NASA NTRS1979

Application of the trap-plus-precipitation hard X-ray burst model to the flare of August 4, 1972

The theory of Melrose and Brown (1976) concerning the X-ray spectrum resulting from combined precipitation and in-trap acceleration of nonthermal electrons is applied to a large solar flare on the basis of high-time-resolution hard X-ray data. The time behavior of the electron source function is inferred as a function of energy and mean ambient trap density. The results are interpreted in the context of a purely thick-target model, a static trap with continuous injection of fresh electrons, and a dynamic trap with continuous redistribution of electron energies. It is concluded that: (1) a minimum trap density exists below which the static-trap description cannot be valid; (2) a mean ambient trap density less than the cited minimum is possible if a dynamic trap is invoked; and (3) the electron source function at a given instant in such low-density dynamic traps may bear little or no resemblance to a power-law spectrum despite its production, via nonthermal bremsstrahlung, of a power-law X-ray spectrum.

Emslie, A. G.↗
NASA NTRS1981

Cleaving machine for hard crystals

Hard crystalline materials such as lithium fluoride (LiF) are cleaved in thin sections by semiautomatic machine. Yield of undistorted LiF crystals is almost 100 percent, even when cleaved section is only 1/32 inch thick. Machine contains spring-activated hammer that limits penetration of blade and controls shock that cleaves crystal. Fixture with spring-loaded clamps precisely locates and holds crystal, restraining it in ideal position for cleaving. Crystal then splays apart.

Benedicto, J. S. J.↗
NASA NTRS1979

CCEA second-generation wheat yield model for hard red wheat in North Dakota

The problem of estimating wheat yields for years with unusual planting dates and/or unusual phenological development is investigated. A yield model developed for hard red spring wheat in North Dakota using historical yields for crop reporting districts in conjunction with meteorological predictor variables based on weekly data is assessed.

Leduc, S. K.↗
NASA NTRS1979

Effect of hard X-rays on the emission lines of Seyfert galaxies and QSOs

Models of photoionized emission-line regions show that inclusion of a hard X-ray component in the ionizing continuum has relatively minor effects on most optical line intensities but helps to explain the strong O VI emission observed from QSOs. The optical-forbidden lines of NGC 4151 indicate roughly solar abundances ratios of N/O/Ne/Fe. The X-ray absorption arises largely in neutral portions of the emission-line clouds.

Shields, G. A.↗
NASA NTRS1979

Mean-spherical model for soft potentials - The hard core revealed as a perturbation

The mean-spherical approximation for fluids is extended to treat the case of dense systems interacting via soft potentials. The extension takes the form of a generalized statement concerning the behavior of the direct-correlation function c(r) and the radial-distribution function g(r). From a detailed analysis that views the hard-core portion of a potential as a perturbation on the whole, a specific model is proposed which possesses analytic solutions for both Coulomb and Yukawa potentials, in addition to certain other remarkable properties. A variational principle for the model leads to a relatively simple method for obtaining numerical solutions.

Rosenfeld, Y.↗
NASA NTRS1980

The hard X-ray burst spectrometer on the Solar Maximum Mission

The paper considers the primary scientific objectives of the Hard X-ray Burst Spectrometer to be flown on the Solar Maximum Mission. The objectives are: (1) to determine mechanisms which accelerate electrons to 20-100 keV in the first stage of a solar flare and to more than 1 MeV in the second stage of many flares, and (2) to characterize the spatial and temporal relation between electron acceleration, storage, and energy loss throughout a solar flare. In the first year after launch, it is expected that about 1000 flares will be observed above the instrument threshold.

Orwig, L. E.↗
NASA NTRS1980

First phase acceleration mechanisms and implications for hard X-ray burst models in solar flares

The paper considers the number of nonthermal electrons which must be accelerated in the impulsive phase of a flare, as well as possible acceleration mechanisms. Direct electric field acceleration in a plasma is considered, taking explicit account of magnetohydrodynamic processes; it is shown that significant heating is involved. Attention is also given to acceleration by electron plasma waves, which requires the waves to be driven to large amplitude by already streaming electrons or anisotropic ion-acoustic waves. It is concluded that at most 0.1% of the flare energy can be deposited into nonthermal streaming electrons with the proposed acceleration and energy conversion mechanisms. Therefore, hard X-ray production above 10 keV primarily by hot thermal electrons is the only choice compatible with existing primary energy release models.

Smith, D. F.↗
NASA NTRS1979

Degenerate dwarfs in future hard X-ray experiments

The hard X-ray characteristics of weak sources (dwarf novae and AM Her-like), and strong steady sources, are discussed. HEAO A2 light curves for the greater than 2 keV flux from SS Cygni are presented, along with the pulse height spectrum and inferred incident spectrum. With regard to the high luminosity degenerate dwarf candidates, it is noted that only for Sco X-1 and Cyg X-2 has even optical evidence for binary periodicity been seen. The attributes of suitable X-ray experiments are reviewed, concerning the simultaneous observation of low energy X-rays, a large area for studying short flares, low background, and the need for both broad band spectral response and high resolution.

Swank, J. H.↗
NASA NTRS1980

Thermal models for solar hard X-ray bursts

Thermal models for hard X-ray bursts consisting of a one-dimensional flux tube whose central electrons are heated to about 400 million K are examined. It is found that the evolution of a thermal X-ray source is a sensitive function of the electron-ion thermal coupling and the state of the plasma into which the source expands. When this coupling is weak, the heated electrons separate into a region of high temperature of about 400 million K and a region of lower temperature of about 100 million K, a process which leads to a power-law X-ray spectrum. In the case of strong coupling there is only one dominant temperature, about 200 million K, and the X-ray spectrum resembles a true thermal spectrum.

Smith, D. F.↗
NASA NTRS1980

Dynamic spectral characteristics of thermal models for solar hard X-ray bursts

It is proposed that the source for solar hard X-ray bursts may consist of a distribution of many small impulsively-heated kernels, each cooled by anomalous conduction, with lifetimes shorter than current burst data temporal resolution. The dynamic spectra of bursts are governed by the dynamic evolution of the kernel production process, such as magnetic-field dissipation in the tearing mode. An integral equation is formulated, the solution of which yields information on this kernel production process from dynamic burst spectra, for any kernel model.

Brown, J. C.↗
NASA NTRS1980

A high sensitivity determination of the hard X-ray spectrum of Sco X-1

The results of hard X-ray observations of Sco X-1 by HEAO 1 and OSO 7 are reported. The X-ray spectrum between 20 and 70 keV was found consistent with thermal bremsstrahlung from a hot plasma at a temperature of 5.15 plus or minus 0.05 keV. Observations with the instruments of HEAO 1 on Sept. 6-7, 1978, and with those of OSO 7 during 1972, have not confirmed reports that the bremsstrahlung spectrum is accompanied by an extra component at energies greater than 40 keV - the reported observations yield a two-standard deviation upper limit to the flux in this region of 6.4 x 10 to the -6th photons/sq cm/s/keV.

Rothschild, R. E.↗
NASA NTRS1981

Design and performance of the solar maximum mission Hard X-ray Burst Spectrometer

The Hard X-ray Burst Spectrometer acquires data on the temporal and energy distribution of solar X-rays in the energy region from 25 to 385 keV. The detector system is a CsI(Na) central detector, and an anti-coincidence shield with photomultiplier tubes optically coupled to the central and shield crystals. Additional detectors are included for calibration and South Atlantic Anomaly monitoring. A 15 channel pulse height analysis is performed over the energy range every 128 milliseconds. This instrument is capable of handling event rates up to 500 kHz and provides high rate data up to 100 kHz with low spectral distortion. Nine accumulated rates are telemetered every 8.192 seconds. A unique feature of the instrument is the ability to sample rates from the central detector or shield with a one millisecond minimum time resolution. Such samples are stored in a 32768 sample memory with a 40 percent pretrigger event history and a 60 percent posttrigger history.

Workman, L. G.↗
NASA NTRS1981

Large loop thermal models of solar hard X-ray bursts

Results for small loop thermal models of hard X-ray bursts are extended to large loops. In this model a magnetic arch with a coronal length of 45,000 km has the electrons near the top heated to temperatures above 1 billion K. The resulting conduction fronts which form are dominated by collisionless processes and travel down the arch to the transition region and chromosphere where they evaporate off part of the latter. This relatively cool material travels back up the loop and eventually quenches the source for energy injection times of order 10 sec. Most of the X-ray emission comes from the footpoints of the arch over most of the source lifetime and the spectrum is a power law with a typical spectral index of 3.0. Even though the efficiency gain in this model is only 2.8, it is much easier from the point of view of plasma physics to heat all the electrons in a plasma than to accelerate a substantial fraction of them.

Smith, D. F.↗
NASA NTRS1981

Fatigue crack growth in 7475-T651 aluminum alloy plate in hard vacuum and water vapor

Compact specimens of 25 mm thick aluminum alloy plate were subjected to constant amplitude fatigue testing at a load ratio of 0.2. Crack growth rates were determined at frequencies of 1 Hz and 10 Hz in hard vacuum and laboratory air, and in mixtures of water vapor and nitrogen at water vapor partial pressures ranging from 94 Pa to 3.8 kPa. A significant effect of water vapor on fatigue crack growth rates was observed at the lowest water vapor pressure tested. Crack rates changed little for pressures up to 1.03 kPa, but abruptly accelerated at higher pressures. At low stress intensity factor ranges, cracking rates at the lowest and highest water vapor pressure tested were, respectively, two and five times higher than rates in vacuum. Although a frequency was observed in laboratory air, cracking rates in water vapor and vacuum are insensitive to a ten-fold change in frequency. Surfaces of specimens tested in water vapor and vacuum exhibited different amounts of residual deformation. Reduced deformation on the fracture surfaces of the specimens tested in water vapor suggests embrittlement of the plastic zone ahead of the crack tip as a result of environmental interaction.

Dicus, D. L.↗
NASA NTRS1981

Hard X-ray astrophysics

Past hard X-ray and lower energy satellite instruments are reviewed and it is shown that observation above 20 keV and up to hundreds of keV can provide much valuable information on the astrophysics of cosmic sources. To calculate possible sensitivities of future arrays, the efficiencies of a one-atmosphere inch gas counter (the HEAO-1 A-2 xenon filled HED3) and a 3 mm phoswich scintillator (the HEAO-1 A-4 Na1 LED1) were compared. Above 15 keV, the scintillator was more efficient. In a similar comparison, the sensitivity of germanium detectors did not differ much from that of the scintillators, except at high energies where the sensitivity would remain flat and not rise with loss of efficiency. Questions to be addressed concerning the physics of active galaxies and the diffuse radiation background, black holes, radio pulsars, X-ray pulsars, and galactic clusters are examined.

Rothschild, R. E.↗
NASA NTRS1982

Microwave and hard X-ray observations of a solar flare with a time resolution of better than 100 MS

Simultaneous microwave and X-ray observations are presented for a solar flare detected on 1980 May 8 starting at 1937 UT. The X-ray observations were made with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission and covered the energy range from 28-490 keV with a time resolution of 10 ms. The microwave observations were made with the 5 and 45 foot antennas at the Itapetinga Radio Observatory at frequencies of 7 and 22 GHz, with time resolutions of 100 ms and 1 ms respectively. Detailed correlation analysis of the different time profiles of the event show that the major impulsive in the X-ray flux preceded the corresponding microwave peaks at 22 GHz by about 240ms. For this particular burst the 22 GHz peaks preceded the 7 GHz by about 1.5s. Observed delays of the microwave peaks are too large for a simple electron beam model but they can be reconciled with the speeds of shock waves in a thermal model.

Kaufmann, P.↗