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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
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NASA NTRS1980

Spectroscopic observations of the optical counterpart of Centaurus X-4

The optical spectrum of the transient X-ray burst source Centaurus X-4 was observed about 5 weeks after the source reached its maximum. The brightness of the optical counterpart had decreased to V = 18.2, and the star had become appreciably redder (B - V = 0.7) compared to its color at maximum. The spectrum of Centaurus X-4 is similar to that of cataclysmic variables showing strong emission lines of H-1 and weaker lines of He-1 and He-2. The N03 lambda 4640 line is not visible. The continuum energy distribution of Centaurus X-4 shows the presence of a main-sequence star in the system, with spectral type between K3 and K7. This is consistent with the orbital period of 8.2 hr proposed by Kaluzienski et al (1980), if the main-sequence star is close to filling its Roche lobe.

Van Paradijs, J.↗
NASA NTRS1981

The distribution and morphology of X-ray-emitting gas in the core of the Perseus cluster

An unresolved source has been found coincident with the nucleus of NGC 1275 in a high-resolution X-ray image of the core of the Perseus cluster. Absorption in the optical features at high velocity with respect to NGC 1275, which are thought to be associated with a foreground galaxy, does not produce any detectable X-ray absorption. The emission tends to become asymmetric in the presence of the lower-velocity filaments, but no obvious, detailed correlation is found between X-ray enhancements and individual filaments. Deprojection of surface brightness to yield temperature and density profiles of the intracluster gas shows results consistent with a quasi-hydrostatic radiative accretion flow onto NGC 1275, and the pressure-driven mass inflow onto the central galaxy is then 200-400 solar masses/yr. The possibility of a problem in the relation of line-of-sight velocity dispersion to cluster gravitational mass is confirmed.

Fabian, A. C.↗
NASA NTRS1982

A medium sensitivity X-ray survey using the Einstein Observatory - The log N-log S relation for extragalactic X-ray sources

The paper presents results of an X-ray survey of about 50 sq deg of the high galactic latitude sky at sensitivities in the range of 7 x 10 to the -14 to 5 x 10 to the -12 ergs/sq cm/s. The number-flux relation is derived for the extragalactic population to yield a best-fit power-law slope of 1.53 + or - 0.16, and the content of the sample is analyzed in terms of types of sources, appearing to be significantly different from the content of similar samples selected at higher fluxes. The medium sensitivity sample of extragalactic sources is dominated by active galactic nuclei, while samples selected at higher fluxes and higher energies are dominated by clusters of galaxies. Thus, the number-flux relation for extragalactic sources may be interpreted to a first approximation as the sum of the two different distributions with flatter and steeper slopes describing clusters and AGNs, respectively.

Maccacaro, T.↗
NASA NTRS1982

The temperature of thermal X-ray and gamma-ray sources

The temperature of a source can be accurately measured, assuming that it is optically thin and exhibits a bremsstrahlung, on the basis of the shape of its spectral distribution. An explicit relation is derived for the temperature in terms of this slope, and as a sum of correction terms to the limiting form of the bremsstrahlung formula. The corrections can be evaluated easily in terms of simple, accurate asymptotic formulae. For the case where there is a distribution of temperatures in the plasma, an additional correction is derived in terms of the rms variation in the plasma temperature. Corrections due to a modified Bessel function, the Born approximation, plasma electron energy distribution and electron-electron bremsstrahlung are discussed

Gould, R. J.↗
NASA NTRS1982

Coma-type radio halos and cluster X-ray morphology

It is pointed out that there seems to be a characteristic X-ray morphology for clusters with Coma-type radio halos. This correlation may be the first evidence that the conditions conducive to formation of a radio halo influence or are influenced by the thermal gas distribution. It is suggested that heating of the intracluster gas by the cosmic rays that power the radio halo may be the source of the correlation. The anomalous gas temperature of Coma may be the result of this process. Future observations of cluster temperatures and velocity dispersions should provide a test of the cosmic-ray heating hypothesis

Vestrand, W. T.↗
NASA NTRS1982

Microwaves and hard X-rays from solar flares - Multithermal and nonthermal interpretations

It is generally accepted that the hard X rays and centimeter-wavelength (microwave) radio emission from impulsive solar flares come from a common source. The present investigation is concerned with the development of a model for impulsive hard X rays and microwaves under the assumption that they arise from the same distribution of electrons, possibly, but not necessarily, from the same volume of space. An examination is conducted of a model in which the hard X rays and microwaves are produced in a homogeneous volume by a power-law electron distribution (nonthermal model). An investigation is also performed of a multithermal model in which the hard X rays and microwaves come from an inhomogeneous source, a nest of magnetic arches in which the highest temperature electrons are confined to a small core, and where, away from the core, the temperature decreases. It is found that more or less satisfactory models can be constructed using either multithermal or nonthermal, power-law electron distributions.

Dulk, G. A.↗
NASA NTRS1983

HEAO A-2 soft X-ray source catalog

The HEAO A-2 low energy X-ray detectors have surveyed over 95% of the sky in the spectral bands 0.18-0.44 and 0.44-2.8 keV, down to a typical limiting sensitivity of 1 x 10 to the -11th and 3 x 10 to the -11th ergs per sq cm per sec in each band, respectively. Using a significance criterion of 6 sigma for existence, 114 sources are cataloged, of which 54 were previously undiscovered and 32 remain unidentified. The catalog contains a listing of all counterpart identifications and a cross-reference to all HEAO 1 A-2 low energy detector team publications on the catalog sources complete through the end of 1981. Supplementary material allows the estimation of spectral parameters for the spectral models from the observed intensities. Simple graphs allow the extraction of energy fluxes at the earth, both with and without the effects of interstellar absorption. The angular distribution of the cataloged sources in the 0.44-2.8 keV band is concentrated toward the galactic plane and resembles that seen at higher energy. The distribution in the 0.18-0.44 keV band is more uniform and contains many more newly discovered and unidentified sources.

Nugent, J. J.↗
NASA NTRS1983

Microwave and hard X-ray imaging of a solar flare on 1980 November 5

VLA and SMM hard X ray data on the solar flares of November 5, 1980 are analyzed and compared with data from other sources. The VLA provided measurements at 15 GHz at 10 sec intervals, using left and right circular polarizations with a 0.6 arcsec resolution. The hard X ray imaging spectrometer on the SMM obtained data in six bands from 3.5-30 keV, with 8 x 8 arcsec resolution and 1.5 sec separation. The data were examined for a possible nonthermal source for the microwave component of the emissions detected, the origin of 16-30 keV excess fluxes, the relation between the X ray and microwave sources, the magnetic connection between observed loops, and the physical characteristics of the radiating loop. The data were consistent with a model that assumes fast electrons are accelerated to a single power-law energy distribution and freely stream along the magnetic field. The data also agreed with a thick-target model for solar flare X ray emission.

Hoyng, P.↗
NASA NTRS1984

Einstein observations of the Crab nebula pulsar

Einstein Observatory (HEAO 2) high-resolution (approximately 4 sec) images of the Crab nebula which clearly resolve the pulsar from the diffuse X-ray emission are presented. The pulsar contributes 3.9 + or - 0.2 percent of the observed 0.1-4.5 keV flux and shows no evidence (other than the 33 ms pulsations) for variations on time scales up to approximately 2.3 days. The pulsar flux decreases to zero over about one-eighth of the cycle, with the upper limit to point source emission during this interval implying a blackbody surface temperature of less than 2.5 x 10 to the 6th K for the neutron star (10 km assumed radius). The spatial distribution of nebular emission near the pulsar closely resembles that of the optical continuum.

Harnden, F. R., Jr.↗
NASA NTRS1984

Results from the Helios galactic and solar cosmic ray experiment (E7)

Energy spectra, charge composition, and flow patterns of solar and galactic cosmic rays were investigated using three nuclear-particle telescopes on Helios to measure electrons (50 keV to 8 MeV) protons (100 eV to 800 MeV) alpha particles (to 600 MeV per nucleon) heavier elements up to neon, (to 200 MeV per nucleon). The experiment includes a proportional counter to monitor solar X-rays in the range 2 to 8 keV. A collection of solar flare associated particle events are observed whereby coronal effects and spatial distribution of flare particles are determined over almost a whole solar cycle; a typical solar particle events are also observed. Earlier explanations concerning corotating energetic particle streams in the inner and outer solar system are confirmed. Solar modulation of galactic cosmic rays is identified. Gamma ray bursts are detected and the sources in the sky are precisely determined, proving the suspected galactic origin.

Trainor, J. H.↗
NASA NTRS1985

Hexagonal uniformly redundant arrays for coded-aperture imaging

Uniformly redundant arrays are used in coded-aperture imaging, a technique for forming images without mirrors or lenses. This technique is especially important for the high energy X-ray and gamma-ray region above 20 kev. In this technique, a mask consisting of opaque (closed) and trasparent (open) areas is placed between the photon sources to be imaged and a position sensitive detector or a detector array. Each source casts a shadow pattern of the mask or aperture onto the detector. This shadow pattern may be viewed as an encoded signal for that source direction. If each possible source code is unique, the detected composite of overlapping shadow patterns may be decoded to produce an image of the source distribution.

Finger, M. H.↗
NASA NTRS1985

Neutrino astronomy and the atmospheric background

Some aspects of neutrino astronomy are illustrated by calculating the neutrino-induced muon flux from Cygnus X-3 binary X-ray source. The signal depends primarily on the power in cosmic rays at the source and on the distance to the source, and only relatively little on details of the matter distribution in the neighborhood of the source.

Gaisser, T. K.↗
NASA NTRS1986

The flare kernel in the impulsive phase

The impulsive phase of a flare is characterized by impulsive bursts of X-ray and microwave radiation, related to impulsive footpoint heating up to 50 or 60 MK, by upward gas velocities (150 to 400 km/sec) and by a gradual increase of the flare's thermal energy content. These phenomena, as well as non-thermal effects, are all related to the impulsive energy injection into the flare. The available observations are also quantitatively consistent with a model in which energy is injected into the flare by beams of energetic electrons, causing ablation of chromospheric gas, followed by convective rise of gas. Thus, a hole is burned into the chromosphere; at the end of impulsive phase of an average flare the lower part of that hole is situated about 1800 km above the photosphere. H alpha and other optical and UV line emission is radiated by a thin layer (approx. 20 km) at the bottom of the flare kernel. The upward rising and outward streaming gas cools down by conduction in about 45 s. The non-thermal effects in the initial phase are due to curtailing of the energy distribution function by escape of energetic electrons. The single flux tube model of a flare does not fit with these observations; instead we propose the spaghetti-bundle model. Microwave and gamma-ray observations suggest the occurrence of dense flare knots of approx. 800 km diameter, and of high temperature. Future observations should concentrate on locating the microwave/gamma-ray sources, and on determining the kernel's fine structure and the related multi-loop structure of the flaring area.

Dejager, C.↗
NASA NTRS1986

IRAS observations of binaries with compact objects

The infrared emission data, obtained on 260 binary systems by the all-sky IRAS survey in wavelengths between 12 and 100 microns, are reported. Of all the 260 sources, which contained compact objects including white dwarfs, neutron stars, or possibly black holes, only 32 contained detectable IR radiation. The X-ray emitting Be-type stars (gamma-Cas and X Per) were found to have their energy flux proportional to frequency in the range of the log nu values of 12.7-14.7. However, the GS304-1 flux distribution is unique, in that its flux rises by several orders of magnitude as the wavelength changes from 4000 A to 60 microns. A static dust cloud was detected, with a radius of about 1 AU, which has formed around the classical nova RR Pic since its 1925 eruption. The post-eruption far-IR light curve of a classical nova provides strong evidence for IR emissions from both dust grains formed during the eruption and dust grains existing from previous eruptions.

Schaefer, B. E.↗
NASA NTRS1986

Evidence for beamed electrons in a limb X-ray flare observed by Hard X-Ray Imaging Spectrometer (HXIS)

The limb flare of November 18, 1980, 14:51 UT, was investigated on the basis of X-ray images taken by the Hard X-ray Imaging Spectrometer (HXIS) and of X-ray spectra from the Hard X-Ray Burst Spectrometer (HXRBS) aboard the Solar Maximum Mission (SMM). The impulsive burst was also recorded at microwave frequencies between 2 and 20 GHz whereas no optical flare and no radio event at frequencies below 1 GHz were reported. The flare occurred directly at the SW limb of the solar disk. Taking advantage of the spatial resolution of HXIS images, the time evolution of the X-radiation originating from relatively small source regions can be studied. Using Monte Carlo computations of the energy distribution of energetic electrons traversing the solar plasma, the bremsstrahlung spectra produced by the electrons were derived.

Haug, Eberhard↗
NASA NTRS1987

High energy gamma ray astronomy

High energy gamma ray astronomy has evolved with the space age. Nonexistent twenty-five years ago, there is now a general sketch of the gamma ray sky which should develop into a detailed picture with the results expected to be forthcoming over the next decade. The galactic plane is the dominant feature of the gamma ray sky, the longitude and latitude distribution being generally correlated with galactic structural features including the spiral arms. Two molecular clouds were already seen. Two of the three strongest gamma ray sources are pulsars. The highly variable X-ray source Cygnus X-3 was seen at one time, but not another in the 100 MeV region, and it was also observed at very high energies. Beyond the Milky Way Galaxy, there is seen a diffuse radiation, whose origin remains uncertain, as well as at least one quasar, 3C 273. Looking to the future, the satellite opportunities for high energy gamma ray astronomy in the near term are the GAMMA-I planned to be launched in late 1987 and the Gamma Ray Observatory, scheduled for launch in 1990. The Gamma Ray Observatory will carry a total of four instruments covering the entire energy range from 30,000 eV to 3 x 10 to the 10th eV with over an order of magnitude increase in sensitivity relative to previous satellite instruments.

Fichtel, Carl E.↗
NASA NTRS1986

Compton scattering in strong magnetic fields

The relativistic cross section for Compton scattering by electrons in strong magnetic fields is derived. The results confirm and extend earlier work which has treated only transitions to the lowest or first excited Landau levels. For the teragauss field strengths expected in neutron star magnetospheres, the relative rates for excited state transitions are found to be significant, especially for incident photon energies several times the cyclotron frequency. Since these transitions must result in the rapid emission of one or more cyclotron photons as well as the Compton-scattered photon, the scattering process actually becomes a photon 'splitting' mechanism which acts to soften hard photon spectra, and also provides a specific mechanism for populating higher Landau levels in the electron distribution function. The results should be significant for models of gamma-ray bursters and pulsating X-ray sources.

Daugherty, Joseph K.↗
NASA NTRS1988

An all sky study of fast X-ray transients

In the HEAO 1 A-2 survey of fast X-ray transients, a search of 2 to 20 keV scanning data was made for brief increases in flux, greater than 4 millicrabs, on timescales approximately 1 to 10 to the 4th s above a 12-hour average. The search was divided into two regions, and all-sky survey of the Galaxy, and a survey of the Magellanic Clouds. In the latter, 37 events were found and identified with 4 of the 5 brighest sources in the LMC, plus 2 as flares from a foreground star. Np X-ray bursts, no gamma-ray bursts, and no events from the SMC were found. In the all-sky survey, after excluding well-known variable sources, out of the equivalent of approximately 104 days of data, 15 events were found which falling into 2 broad categories: flares from coronal sources, roughly isotropically distributed, with optically thin thermal spectra; and harder fast transients apparently distributed near the Galactic plane. The first were identified as flares from ubiquitous active cool dwarf stars. It was hypothesized that the second may have been from distant Be-neutron star binaries. However, at least two of the harder, more luminous events remained unidentified. Future research should examine the nature of these rare events, and how they may fit into a hierarchy of hard fast transients from gamma-ray bursts to outbursts from pulsar systems.

Connors, Alanna↗