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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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NASA NTRS1975

X-ray bursts from solar flares behind the limb

Data from the OSO-7 X-ray experiment were used in a search for behind-the-limb solar X-ray bursts. Seventeen active centers were found as likely seats of such bursts. The spectra of 37 over-the-limb X-ray bursts were analyzed. The distributions of maximum temperature, maximum emission measure, and characteristic cooling time do not differ in a statistically significant way from the distributions for center events. The observations support the view that the dominant cooling mechanism is conduction, for the cooling times derived from the temperature histories are too small to be explained by radiative cooling. The fraction of over-the-limb bursts which have an nonthermal component is 2/3, exactly the same as found for the 122 X-ray bursts studied in Datlowe et al. (1974b). Thick-target models of X-ray emission would suggest that hard X-ray sources would be rapidly occulted with increasing longitude of the burst location, yet just the contrary was observed.

Roy, J.-R.↗
NASA NTRS1976

On the distance to the candidate star coincident with A0620-00

High-dispersion echelle spectra of the candidate star for the transient X-ray source A0620-00 are discussed. No absorption or emission lines attributed to the star were found under visual examination of the spectra, but interstellar sodium D lines were clearly visible. Results are presented for measurements of the profiles and central velocities of the sodium absorption lines, and other observations of interstellar features in the general direction of the star are summarized. While the exact distance to the star cannot be determined due to the patchy distribution of the interstellar gas, it is suggested that the star is located between 1.5 and 2.5 kpc; 2 kpc is adopted as the most reasonable value. It is noted that the possibility cannot be ruled out that the star lies beyond 3 kpc and is seen through a hole in the distant neutral hydrogen gas.

Gull, T. R.↗
NASA NTRS1976

Support of selected X-ray studies to be performed using data from the Uhuru (SAS-A) satellite

A new measurement of the diffuse X-ray emission sets more stringent upper limits on the fluctuations of the background and on the number counts of X-ray sources with absolute value of b 20 deg than previous measurements. A random sample of background data from the Uhuru satellite gives a relative fluctuation in excess of statistics of 2.0% between 2.4 and 6.9 keV. The hypothesis that the relative fluctuation exceeds 2.9% can be rejected at the 90% confidence level. No discernable energy dependence is evident in the fluctuations in the pulse height data, when separated into three energy channels of nearly equal width from 1.8 to 10.0 keV. The probability distribution of fluctuations was convolved with the photon noise and cosmic ray background deviation (obtained from the earth-viewing data) to yield the differential source count distribution for high latitude sources. Results imply that a maximum of 160 sources could be between 1.7 and 5.1 x 10 to the -11 power ergs/sq cm/sec (1-3 Uhuru counts).

Garmire, G. P.↗
NASA NTRS1978

The HEAO-B Monitor Proportional Counter Instrument

The Monitor Proportional Counter Instrument for the HEAO-B X-ray Telescope satellite is designed to observe celestial X-ray sources in the range of 1 to 20 keV with intensities exceeding .0002 Crab. It provides both spectral and temporal data, to complement the data from the other HEAO-B instruments, to compare with observations from previous flights, as well as to allow detailed study of time variant X-ray phenomena. The basic detector is a large area proportional counter system, consisting of two counters, high voltage power supplies and preamplifiers, mounted behind a collimator with a 2/3 degree square field of view. The signal processing, power distribution and control electronics are contained in a separate unit, which includes circuitry to perform pulse height analysis, background rejection, time interval measurement, redundancy switching and control, housekeeping and telemetry interfacing.

Gaillardetz, R.↗
NASA NTRS1973

Some aspects of cosmic synchrotron sources

Synchrotron emission is considered from individual particles which have small pitch angles and the general properties of synchrotron sources which mainly contain such particles, as well as the emissivities and degrees of circular polarization for specific source distributions. The limitation of synchrotron source models for optical pulsars and compact extragalactic objects are discussed, and it is shown that several existing models for the pulsar NP 0532 are inconsistent with the measured time variations and polarizations of the optical emission. Discussion is made also of whether the low frequency falloffs in the extragalactic objects PKS 2134 + 004, OQ 208, and NGC 1068 is due to emission from particles with small pitch angles or absorption by a thermal plasma or synchrotron self-absorption. It is concluded that the absorption interpretations cannot account for the turnover in the spectrum of PKS 2134 + 004. Measurements of polarization, angular structure, and X-ray flux are also described.

Epstein, R. I.↗
NASA NTRS1975

The light curve of a transient X-ray source

The Ariel-5 satellite has monitored the X-ray light curve of A1524-62 almost continuously from 40 days prior to maximum light until its disappearance below the effective experimental sensitivity. The source exhibited maximum light on Dec. 4, 1974, at a level of 0.9 the apparent magnitude of the Crab Nebula in the energy band 3-6 keV. Although similar to previously reported transient sources with a decay time constant of about 2 months, the source exhibited an extended, variable preflare on-state of about 1 month at a level of greater than 0.1 maximum light. The four bright (greater than 0.2 of the Crab Nebula) transient sources observed during the first half-year of Ariel-5 operation are indicative of a galactic disk distribution, a luminosity at maximum in excess of 10 to the 37-th power ergs/sec, a frequency of occurrence which may be as high as 100/yr, and a median decay time which is less than 1 month.

Kaluzienski, L. J.↗
NASA NTRS1979

Mapping of electrical potential distribution with charged particle beams

Potentials were measured using a beam of soft X-rays in air at 2 x 10 to the -5 power Torr. Ions were detected by a continuous-dynode electron multiplier after they passed through a retarding field. Ultimate resolution depends upon the diameter of the X-ray beam which was 3 mm. When the fields in the region of interest were such to disperse the ions, only a small fraction were detected and the method of measurement was not very reliable. Yet reasonable data could be collected if the ions traveled in parallel paths toward the detector. Development should concentrate on increasing the aperture of the detector from the pinhole which was used to something measured in centimeters. Also increasing the strength of the source would provide a stronger signal and more reliable data. Measurements were made at an estimated ion current to 10 to the -15 power A from a 10 cm length of the X-ray beam, this current being several orders of magnitude below what would have a perturbing effect on the region to be measured. Consequently, the source strength can be increased and prospects for this method of measurement are good.

Robinson, J. W.↗
NASA NTRS1975

The light curve of a transient X-ray source

The Ariel-V satellite monitored the X-ray light curve of A1524-62 almost continuously from 40 days prior to maximum light until its disappearance below the effective experimental sensitivity. The source exhibited maximum light on approximately 4 December 1974, at a level of 0.9 the apparent magnitude of the Crab Nebula in the energy band 3-6 keV. Although similar to previously reported transient sources with a decay time constant of approximately 2 months, the source exhibited an extended, variable pre-flare on-state of about 1 month at a level of greater than approximately 0.1 maximum light. The four bright (greater than 0.2 of the Crab Nebula) transient sources observed during the first half-year of Ariel-V operation are indicative of a galactic disk distribution, and a luminosity at maximum in excess of 10 to the 37th power ergs/sec.

Kaluzienski, L. J.↗
NASA NTRS1976

Accretion onto magnetized neutron stars - Structure and interchange instability of a model magnetosphere

A self-consistent model is analyzed for the spherical infall of weakly magnetized plasma into the magnetosphere of a slowly rotating, strongly magnetized neutron star. It is shown that spherical infall is probably a good approximation for X-ray sources which accrete from a stellar wind. The location of the standoff shock which halts the hypersonic infall is estimated along with the emission from the shocked layer. The location of the equilibrium magnetopause and the structure of the magnetic field within it are calculated; it is found that the magnetic poles are true cusps and that the entry of gas due to equilibrium flow across a cusp is almost certainly dominated by the interchange instability near the magnetic equator. The energy principle is applied to derive necessary conditions for the occurrence of this instability. The results indicate that the strong magnetic-pressure gradient stabilizes the gas unless moderately strong radiative cooling takes place and that the cooled plasma enters the magnetosphere as long filaments capable of moving between field lines. The rate at which the equilibrium magnetopause can 'absorb' mass and momentum is derived, the validity of the approximations employed is discussed, and the likely evolution of the sinking filaments is outlined to show that the spatial distribution of the plasma is determined mainly by the dynamics and thermodynamics of the filaments rather than the magnetic-field structure.

Arons, J.↗
NASA NTRS1971

Plasma model for thermal X-ray sources

Thermal X ray sources associated with rotating collapsed stars with surrounding plasma shells, discussing plasma density profile and electron distribution in stellar magnetosphere

Coppi, B.↗
NASA NTRS1971

Interaction of fast particles with intergalactic matter.

A discussion is given of the relaxation of power-law cosmic-ray spectra in the intergalactic medium. The theoretical time-dependent spectra obtained are used to calculate the nonthermal radiation produced by bremsstrahlung of subcosmic-ray electrons colliding with ambient protons of the intergalactic gas. A comparison is made between the theory and the observations of the diffuse X-ray and gamma-ray background. The calculated cosmic-ray proton spectra are applied to a computation of the heating of the intergalactic medium, and the resulting thermal bremsstrahlung radiation is compared with the suprathermal proton bremsstrahlung flux. We conclude that nonthermal bremsstrahlung is unlikely to be an important contributor to the isotropic X- and gamma-ray background. However, cosmic-ray heating can provide a plausible heat source for maintaining a hot intergalactic gas, especially when evolutionary effects are included in the distribution of cosmic-ray sources.

Arons, J.↗
NASA NTRS1977

Backscatter of hard X-rays in the solar atmosphere

The solar photosphere backscatters a substantial fraction of the hard X rays from solar flares incident upon it. This reflection was studied using a Monte Carlo simulation which takes into account Compton scattering and photo-electric absorption. Both isotropic and anisotropic X ray sources are considered. The bremsstrahlung from an anisotropic distribution of electrons are evaluated. By taking the reflection into account, the inconsistency is removed between recent observational data regarding the center-to-limb variation of solar X ray emission and the predictions of models in which accelerated electrons are moving down toward the photosphere.

Bai, T.↗
NASA NTRS1978

Hard X-ray imaging from the solar probe

The solar probe offers a platform with particular advantages for studying solar nonthermal plasma processes via the observations of hard X-radiation from energetic electrons in the chromosphere and corona, these include (1) high sensitivity, (2) a second line of sign (in addition to the earth's) that can aid in three dimensional reconstruction of the source distribution, and, (3) the possibility of correlation with direct measurements of the nonthermal particles from the probe itself.

Hudson, H. S.↗
NASA NTRS1977

Recent high energy gamma-ray results from SAS-2

Recent developments in gamma-ray astronomy due to the results from SAS-2 have focused on two areas. First, the emission from the plane of the Galaxy is the dominant feature in the gamma-ray sky. The galactic latitude and longitude distributions are consistent with the concept that the high-energy radiation originates from cosmic rays interacting with interstellar matter, and the measurements support a galactic origin for cosmic rays. Second, searches of the SAS-2 data for emission from localized sources have shown three strong discrete gamma-ray sources: the Crab nebula and PSR 0531 + 21, the Vela supernova remnant and PSR 0833-45, and a source near galactic coordinates 193 deg longitude, +3 deg latitude, which does not appear to be associated with other known celestial objects. Evidence has also been found for pulsed gamma-ray emission from two other radio pulsars, PSR 1818-04 and PSR 1747-46. A localized source near longitudes 76-80 deg may be associated with the X-ray source Cyg X-3.

Thompson, D. J.↗
NASA NTRS1978

Imaging of X-ray aurorae from Spacelab

The Atmospheric X-ray Emission Telescope (AXET) is designed to image and measure the spatial, temporal, and spectral distributions of X-ray aurorae produced in the upper atmosphere by precipitating energetic electrons. The bremsstrahlung-produced X radiation penetrates to stratospheric depths where it can modify ozone, conductivity, and other critical atmospheric parameters. Remote sensing of this radiation from topside will provide vital data on solar-terrestrial relationships and mechanisms that could help trigger tropospheric response to solar activity. The X rays also provide direct information concerning the dynamics of radiation-belt processes within the magnetosphere. AXET is designed to detect sources up to 50 keV and to image them below 25 keV by means of directionally sensitive proportional counters with passive collimators. It will be mounted in the Space Shuttle on a single fixed platform designed to optimize viewing conditions for the anticipated orbits and aspects of early Spacelab missions.

Goldberg, R. A.↗
NASA NTRS1979

Ultraviolet spectroscopic measurements of globular clusters

Ultraviolet spectra of six globular clusters (M15, M92, NGC 1851, NGC 6624, 47 Tuc, and NGC 6752) have been taken with the International Ultraviolet Explorer satellite in order to investigate the surface brightness distributions in the central regions. The ultraviolet emission indicates composite stellar spectra. Short-wavelength emission (1200-1900 A) arises from blue horizontal-branch stars; long-wavelength emission (2100-3000 A) is characteristic of late-type horizontal-branch and giant stars. The surface brightness distribution is more compact at short wavelengths than at long wavelengths, suggesting a segregation of horizontal-branch stars. A dense core appears to be present in X-ray-emitting clusters and absent in those clusters without X-ray sources. The X-ray source in NGC 6624 may have been detected at short wavelengths (1300-1900 A).

Dupree, A. K.↗
NASA NTRS1979

Einstein observations of the Orion Nebula

A single 3.5 hour observation on 1979 March 6 with the imaging proportional counter aboard the Einstein Observatory has revealed the presence of numerous low-luminosity (10 to the 31st ergs/s) X-ray sources in a region centered near the Trapezium in the Orion Nebula. In addition to the Trapezium, 22 discrete sources have been detected in a 1 x 1 deg field at or above the 4 sigma level. These sources are clearly associated with star formation, as indicated by the similarity of their spatial distribution with that of the nebular variables, a class of pre-main-sequence stars. Because the field is quite crowded, however, individual identifications of these sources are difficult. In addition to the discrete sources, there is evidence for diffuse or unresolved emission from two ridges extending 16 deg to the north and 15 deg to the west of the Trapezium. The nature of the discrete sources and of the diffuse emission is discussed.

Ku, W. H.-M.↗