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At least 163 records · Page 9
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NASA NTRS1983

Millisecond time variations in hard X-ray solar flares

The results of a search for fast spikes in 2830 hard X-ray solar flares as observed with the hard X-ray burst spectrometer on the Solar Maximum Mission (SMM) are presented. Hundreds of fast spikes with durations of less than 1 sec have been detected at time resolutions of 128 msec and 10 msec. Fast spikes have been detected with rise and decay times as short as 20 msec and with widths as short as 45 msec. They are the fastest hard X-ray variations yet seen from the sun. The observations of such fast variations place new constraints on the physical nature of the source, and these observations and constraints are discussed in terms of nonthermal and thermal models of flares.

Kiplinger, A. L.↗
NASA NTRS1983

Models of the hard X-ray spectrum of AM Herculis and implications for the accretion rate

Phenomenological fits to the hard X-ray spectrum of AM Herculis left unexplained the high equivalent width (0.8 + or - 0.1 keV) of Fe K alpha emission. A purely thermal origin implies a much steeper spectrum than was observed. With Monte Carlo calculations, scattering and fluorescent line production in a cold or partially ionized accretion column of hard X-rays emitted at the base were investigated. The strength of the iron emission and the flat spectral continuum can be explained by the effects of fluorescence and absorption within the accretion column and the surface of the white dwarf on a thermal X-ray spectrum. Thomson optical depths across the column in the range 0.2 to 0.7 are acceptable. The accretion rate and gravitational power can be deduced from the optical depth across the column, if the column size is known, and, together with the observed hard X-ray and polarized light luminosities, imply a lower limit for the luminosity in the UV to soft X-ray range, for which the observations give model-dependent values. Estimates of the column size differ by a factor of 40. Small spot sizes and low luminosities would be consistent with the soft component being the expected reprocessed bremsstrahlung and cyclotron radiation, although the constraint of matching the spectrum confines one to solutions with fluxes exceeding 20% the Eddington limits.

Swank, J. H.↗
NASA NTRS1983

Observations of a hard X-ray component in the spectrum of Nova Ophiuchi

The spectrum and time variation of Nova Ophiuchi (H1705-25) in the 10-200 keV range as measured by the UCSD/MIT instruments aboard HEAO 1 during the period 1977 August 25 to September 28 are reported. The composite curve is best fitted by a kT = 2 keV thin thermal bremsstrahlung model below 10 keV and a separate hard X-ray component fitted equally well by a power-law component with photon index 2.19 + or - 0.06 or a kT = 32.1 + or - 2.4 keV thermal bremsstrahlung model. This is the first observation of a hard tail in the spectrum of a transient X-ray source with sufficient statistical significance to allow a detailed study of its spectral and temporal variability. It is found that the intensity variations of the high-energy X-rays are consistent with the variability at lower energies (3-6 keV), but no hard X-ray spectral index variability is found on time scales from 2 days to 2 weeks. The results can be interpreted as due to accretion onto a neutron star (or possibly onto a black hole) that may also be surrounded by an extended corona.

Wilson, C. K.↗
NASA NTRS1983

The damage equivalence of electrons, protons, alphas and gamma rays in rad-hard MOS devices

This paper reports on a study of damage equivalence in rad-hard MOS devices with 100,000 rads (SiO2) capability. Damage sensitivities for electrons of 1, 2, 3, 5, and 7 MeV, protons of 1, 3, 7, 22, and 40 MeV, 3.4-MeV alphas, and Co-60 gammas were measured and compared. Results indicated that qualitatively the same charge recombination effects occurred in hard oxide devices for doses of 100,000 rads (SiO2) as in soft oxide parts for doses of 1 to 4 krads (SiO2). Consequently, damage equivalency or non-equivalency depended on radiation type and energy. However, recovery effects, both during and after irradiation, controlled relative damage sensitivity and its dependency on total dose, dose rate, supply bias, gate bias, radiation type, and energy. Correction factors can be derived from these data or from similar tests of other hard oxide type, so as to properly evaluate the combined effects of the total space environment.

Stassinopoulos, E. G.↗
NASA NTRS1984

Models of the hard X-ray spectrum of AM Herculis and implications for the accretion rate

Phenomenological fits to the hard X-ray spectrum of AM Hercules have failed to explain the high equivalent width (0.8 + or - 0.1 keV) of Fe K-alpha emission. The scattering and fluorescent line production in a cold or partially ionized accretion column of hard X-rays emitted at the base are investigated using the Monte Carlo method. It is shown that the Fe emission equivalent width observed in the spectrum of AM Her can be explained without resorting to an overabundance of Fe in the accreting material. For sufficient optical depth across the accretion column, equivalent widths of K-alpha emission even larger than the observed values can be obtained. The accretion rate and gravitational power can be deduced from the optical depth across the column, if the column size is known, and, together with the observed hard X-ray and polarized light luminosities, imply a lower limit for the luminosity in the UV to soft X-ray range, for which the observations give model-dependent values.

Swank, J. H.↗
NASA NTRS1984

The energy relation between hard X-ray and O V emission in solar flares

The relationship between energy emitted in hard X-rays and the ultraviolet during the impulsive phase of solar flares provides an important diagnostic for understanding the energy flow from nonthermal to thermal. Many flares were observed from the Solar Maximum Mission satellite simultaneously in hard X-rays and the O V line at 1371 A formed at 250,000 K, providing information relevant to this problem. Previous work has shown that short time scale peaks in emission of these two types of radiation coincide in time to within 1 s. In this work the energy relation between the two types of emission is investigated and it is found that for any given flare there is a definite relation between hard X-ray and O V emissions throughout the flare, but from one flare to the next this relation varies markedly. These differences are attributed to the initial conditions in the flaring loops and some exploratory model calculations are presented to support this hypothesis.

Poland, A. I.↗
NASA NTRS1984

Solar hard X-ray microflares

Using balloon-borne instrumentation of very high sensitivity, approximately 25 hard X-ray bursts with peak fluxes of above 7 x 10 to the -3rd/(sq cm s keV) at 20 keV have been detected, in 141 minutes of observation of the sun on June 27, 1980. These hard X-ray microflares last from a few seconds to several tens of seconds and have power-law energy spectra. They are generally accompanied by small soft X-ray bursts, but H-alpha flares and solar radio bursts are reported for only a few of these hard X-ray bursts. The integral number of events varies approximately as the inverse of the peak flux, down to the limits of the measurements. These observations suggest that even very small transient releases of energy by the sun may be primarily nonthermal in character. It is speculated that the energy released in accelerated electrons for these microflares, averaged over time, may contribute significantly to the heating of the active corona.

Lin, R. P.↗
NASA NTRS1985

Temporal evolution of an energetic electron population in an inhomogeneous medium: Application to solar hard X-ray bursts

Energetic electrons accelerated during solar flares can be studied through the hard X-ray emission they produce when interacting with the solar ambient atmosphere. In the case of the non thermal hard X-ray emission, the instanteous X-ray flux emitted at one point of the atmosphere is related to the instantaneous fast electron spectrum at that point. A hard X-ray source model then requires the understanding of the evolution in space and time of the fast particle distribution. The physical processes involved here are energy losses due to Coulomb collisions and pitch angle scattering due to both collisions and magnetic field gradients.

Vilmer, N.↗
NASA NTRS1985

Lapex: A Phoswich balloon experiment for hard X-ray astronomy

Satellite and balloon observations have shown that several classes of celestial objects are hard ( 15 keV) energy band with a sensitivity of approx 10 mCrab has been performed with the UCSD/MIT instrument (A4) on board the HEAO 1 satellite. About 70 X-ray sources were detected, including galactic and extragalactic objects. Hard X-ray emission has been detected in the Galaxy from X-ray pulsars. Extragalactic sources of hard X-ray emission include clusters of galaxies, QSOs, BL Lac objects, Seyfert galaxies. The essential characteristics of the Large Area Phoswich Experiment (LAPEX) for crowded sky field observations are described. It has: (1) a broad energy band of operation (20-300 keV); (2) a 3 sigma sensitivity of about 1 mCrab in 10,000 s of live observing time; and (3) imaging capabilities with an angular resolution of about 20'.

Frontera, F.↗
NASA NTRS1985

'Diamondlike' carbon films - Optical absorption, dielectric properties, and hardness dependence on deposition parameters

An RF plasma deposition system was used to prepare amorphous 'diamondlike' carbon films. The source gases for the RF system include methane, ethylene, propane, and propylene, and the parameters varied were power, dc substrate bias, and postdeposition anneal temperature. Films were deposited on various substrates. The main diagnostics were optical absorption in the visible and in the infrared, admittance as a function of frequency, hardness, and Auger and ESCA spectroscopy. Band gap is found to depend strongly on RF power level and band gaps up to 2.7 eV and hardness up to 7 Mohs were found. There appears to be an inverse relationship between hardness and optical band gap.

Natarajan, V.↗
NASA NTRS1985

Great microwave bursts and hard X-rays from solar flares

In the present study of the microwave and hard X-ray characteristics of 13 solar flares emitting microwave fluxes greater than 500 solar flux units, simultaneous 3-35 GHz and hard X-ray observations were conducted in the 30-500 keV energy range. An analysis is conducted to determine whether the same distribution of energetic electrons can explain both emissions; tests for any correlations between them yield results suggesting that optically thick microwave emission, near the peak frequency, originates in the same electron population that produces the hard X-rays. A single temperature model and a multitemperature model were tested for consistency with the coincident X-ray and microwave spectra at microwave burst maximum; neither model, however, attempts to explain the high frequency component of the microwave spectrum.

Wiehl, H. J.↗
NASA NTRS1986

Interplanetary energetic particles, coronal flares and hard X-ray microflares

Solar electron phenomena which can produce low levels of hard X-ray emission at the Sun are reviewed. Small approx. 2 to 100 keV solar electrons events, the most common type of impulsive solar particle emission, appear to originate in flare-like bursts high in the corona. These events often are accompanied by approx. 1 MeV/nucleon to the 3rd power He-rich particle events in which lower energy ions are apparently accelerated as well. Long-lived (many days) streams associated with large flares or interplanetary shocks dominate the interplanetary electron fluxes, but even at the quietest times a significant outflow of non-thermal approx. 2 keV electrons occurs. These electron phenomena are accompanied by coronal and interplanetary radio emission. High sensitivity hard X-ray measurements show that microflares, bursts with peak keV fluxes 10 to 100 times smaller than observed in small solar flares, may occur as often as once every 5 min near solar maximum. The proposed Pinhole/Occulter Facility hard X-ray instrumentation provides the increase in sensitivity required to image these phenomena for the first time.

Lin, R. P.↗
NASA NTRS1986

The possible importance of synchrotron/inverse Compton losses to explain fast MM-wave and hard X-ray emission of a solar event

The solar burst of 21 May 1984 presented a number of unique features. The time profile consisted of seven major structures (seconds), with a turnover frequency or approx. 90 GHz, well correlated in time to hard X-ray emission. Each structure consisted of multiple fast pulses (.1 seconds), which were analyzed in detail. A proportionality between the repetition rate of the pulses and the burst fluxes at 90 GHz and or approx. 100 keV hard X-rays, and an inverse proportionality between repetition rates and hard X-rays power law indices have been found. A synchrotron/inverse Compton model has been applied to explain the emission of the fast burst structures, which appear to be possible for the first three or four structures.

Correia, E.↗
NASA NTRS1985

Solar hard X-ray bursts

The major results from the Solar Maximum Mission (SMM) are presented as they relate to our understanding of the energy release and particle transportation processes that lead to the high-energy X-ray aspects of solar flares. Evidence is reviewed for a 152-158 day periodicity in various aspects of solar activity, including the rate of occurence of hard X-ray and gamma-ray flares. The statistical properties of over 7000 hard X-ray flares detected with the Hard X-Ray Burst Spectrometer are presented, including the spectrum of peak rates and the distribution of the photon number spectrum. A flare classification scheme introduced by Tanaka is used, and characteristics of the different types (types A, B, and C) are noted. A model based on the association of type C flares and coronal mass ejections is presented to explain many of the characteristics of these gradual flares.

Dennis, B. R.↗
NASA NTRS1985

Solar flare development during the impulsive phase as deduced from correlated radio and hard X-ray observations

A correlative study of metric type III/V bursts and hard X-ray events during the preflash and the impulsive phases of solar flares is presented. A total of 55 groups of type III bursts were observed with the Nancay radiospectrograph, and 32 of these were associated with hard X-ray bursts detected with instruments on ISEE 3, SMM, or both. X-ray bursts associated with type III or U bursts, and type V continuum emission lasting less than 10 s are relatively small, with peak fluxes at 30 keV of not more than 1 photon/sq cm per sec per keV. X-ray bursts associated with type III bursts and continuum emission lasting longer than 30 s are considerably more intense, with peak fluxes at 30 keV exceeding 1 photon/sq cm per sec per keV and spectra extending to not less than 100 keV. For these latter events, type III emission was observed at 169 MHz with the Nancay radioheliograph from several locations during the preflash phase. At the time of the rapid increase in the hard X-ray flux, a new source appeared at 169 MHz and proceeded to fluctuate in phase with one of the original sources that brightened at the same time. This type of behavior, observed in 13 flares, is interpreted as being caused by the rapid interaction of two or more magnetic structures giving rise to the impulsive phase of the flare.

Raoult, A.↗
NASA NTRS1986

The possible importance of synchrotron/inverse Compton losses to explain fast mm-wave and hard X-ray emission of a solar event

The solar burst of 21 May 1984, presented a number of unique features. The time profile consisted of seven major structures (seconds), with a turnover frequency of greater than or approximately 90 GHz, well correlated in time to hard X-ray emission. Each structure consisted of multiple fast pulses (0.1 seconds), which were analyzed in detail. A proportionality between the repetition rate of the pulses and the burst fluxes at 90 GHz and greater than or approximately 100 keV hard X-rays, and an inverse proportionality between repetition rates and hard X-ray power law indices were found. A synchrotron/inverse Compton model was applied to explain the emission of the fast burst structures, which appear to be possible for the first three or four structures.

Correia, E.↗
NASA NTRS1986

Quasi-periodic pulsations in solar hard X-ray and microwave flares

For more than a decade, various studies have pointed out that hard X-ray and microwave time profiles of some solar flares show quasi-periodic fluctuations or pulsations. Nevertheless, it was not until recently that a flare displaying large amplitude quasi-periodic pulsations in X-rays and microwaves was observed with good spectral coverage and with a sufficient time resolution. The event occurred on June 7, 1980, at approximately 0312 UT, and exhibits seven intense pulses with a quasi-periodicity of approximately 8 seconds in microwaves, hard X-rays, and gamma-ray lines. On May 12, 1983, at approximately 0253 UT, another good example of this type of flare was observed both in hard X-rays and in microwaves. Temporal and spectral characteristics of this flare are compared with the event of June 7, 1980. In order to further explore these observational results and theoretical scenarios, a study of nine additional quasi-periodic events were incorporated with the results from the two flares described. Analysis of these events are briefly summarized.

Kosugi, Takeo↗
NASA NTRS1986

The generation of rapid solar flare hard X-ray and microwave fluctuations in current sheets

The generation of rapid fluctuations, or spikes, in hard X-ray and microwave bursts via the disruption of electron heating and acceleration in current sheets is studied. It is found that 20 msec hard X-ray fluctuations can be thermally generated in a current sheet if the resistivity in the sheet is highly anomalous, the plasma density in the emitting region is relatively high, and the volume of the emitting region is greater than that of the current sheet. A specific mechanism for producing the fluctuations, involving heating in the presence of ion acoustic turbulence and a constant driving electric field, and interruption of the heating by a strong two-stream instability, is discussed. Variations upon this mechanism are also discussed. This mechanism also modulates electron acceleration, as required for the microwave spike emission. If the hard X-ray emission at energies less than approx. 1000 keV is nonthermal bremsstrahlung, the coherent modulation of electron acceleration in a large number of current sheets is required.

Holman, Gordon D.↗