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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Balloon observations of hard X-rays from NGC 4151 and an X-ray transient source

An X-ray telescope consisting of 400 sq cm phoswich detectors (NaI(Tl)/CsI(Na)) was flown from Hyderabad (India) on 31 October 1980; the field of view was 11 deg x 1 deg FWHM. During a five hour observation at 5 millibars two X-ray sources were seen: (1) the variable Seyfert Galaxy NGC 4151; and (2) a transident of approximately 8 minutes duration in the vicinity of the variable source 4U1444 43 which may be associated with the galaxy NGC 5608. X-ray fluxes and spectra in 18-120 keV X-rays are presented.

Kunte, P. K.↗
NASA NTRS1985

Radial distribution of cosmic ray intensity in the galaxy from gamma-ray data

The radial distributions of Galactic gamma-ray emissivity and cosmic ray intensity are derived by unfolding the SAS2 line flux data. The results show a marked gradient, with the cosmic ray intensity in the region 3,R,7 kpc being 3 to 4 times as much as locally. There is also an indication for a higher gradient for cosmic rays of lower energy.

Goned, A.↗
NASA NTRS1984

HEAO 1 high-energy X-ray observations of three bright transient X-ray sources H1705-250 (Nova Ophiuchi), H1743-322, and H1833-077 (Scutum X-1)

The bright X-ray transients H1705-250 (Nova Ophiuchi) and H1743-322 in the energy range 12-180 keV were observed, and the presence of high-energy spectral components was revealed. On the basis of X-ray spectra in the 1-10 keV range, transients are classified as 'soft' (kT = 4 keV) or 'hard' (kT = 17 keV); and both H1705-250 and H1743-322 should be in the 'soft' category. Data have been reexamined for the archetypal 'soft' transient A0600-00 taken by the SAS 3 satellite, but no evidence is found there for a high-energy spectral component. Thus consideration of a wider X-ray energy range makes the 'hard'/'soft' distinction between X-ray transients much less clear.

Cooke, B. A.↗
NASA NTRS1983

Detection of cosmic ray electrons above 10 to 14th eV using gamma ray observatories

A quantitative evaluation of high energy gamma ray observatories for the study of cosmic ray electrons is made. This is based on the principle that the synchrotron photons emitted by the electrons in the earth's magnetic field is collinear in the detector. It is shown that the size and the gamma ray detection efficiency of the SAS II instrument is so small, that no useful information can be derived from it. On the other hand, one may be able to set useful upper limits to the flux of electrons by making use of the high energy gamma ray detector in the GRO.

Stephens, S. A.↗
NASA NTRS1986

Preliminary designs for X-ray source modifications for the Marshall Space Flight Center's X-ray calibration facility

The objective of this investigation is to develop preliminary designs for modifications to the X-ray source of the MSFC X-Ray Calibration Facility. Recommendations are made regarding: (1) the production of an unpolarized X-ray beam, (2) modification of the source to provide characteristic X-rays with energies up to 40 keV, and (3) addition of the capability to calibrate instruments in the extreme ultraviolet wavelength region.

Croft, W. L.↗
NASA NTRS1986

Rapid X-ray and optical variability in the X-ray selected BL Lacertae object IE 1402.3 + 0416

Results from X-ray and optical observations of the X-ray-discovered BL Lac object 1E 1402.3 + 0416 are presented, where the X-ray measurements were carried out with the Channel Multiplier Array (CMA) and Medium Energy experiment (ME) detectors on board Exosat. These measurements revealed an intensity decrease by a factor of two on a time scale of a few hours. At maximum flux, the source was significantly greater than at the time of the Einstein Image Proportional Counter (IPC) discovery observation. The 2-6 keV X-ray spectrum was determined by the ME experiment, and IPC, HRI, and CMA data were subsequently compared. The source varied over the years by a factor of five; its brightest state was within the last three years. It is noted that a redshift greater than 0.2 would require that anisotropic emission mechanisms be invoked.

Giommi, P.↗
NASA NTRS1987

EX56a study of extended X-ray emission around isolated galaxies EX56b identification and spectra of bright X-ray sources at high galactic latitude

The EXOSAT observations confirmed the identification and extended nature of PKS 2345-35. It gave a good 2 to 10 keV X-ray spectrum and a detailed spatial profile indicating asymmetry of the structure. In the high galactic latitidue investigation, the BL Lac object identified with the HEAO-1 source 1430+423 was detected, and the first X-ray spectrum was obtained. Several simulataneous observations of H0323+022 were obtained over a broad range of electromagnetic spectrum. Studies of luminous active galactic nuclei have given significant information on the spectrum of the quasar PKS 0558-504. In a study of Southern sky cataclysmic variables, the EXOSAT was used to determine the X-ray spectrum and search for periodicities in two objects. Studies of complete identifications have revealed that X-ray sources in two high galactic latitude fields are stars, and therefore are to be excluded from the Piccinotti extragalactic sample. Only one Piccinotti source remains to be identified.

Schwartz, Daniel A.↗
NASA NTRS1986

Modifications of the X-ray source and monitor at the X-ray Calibration Facility

In order to test the instruments aboard the Advanced X-ray Astrophysics Facility (AXAF) some modifications will need to be made in the X-ray Calibration Facility at Marshall. Several of these modifications involve the X-ray source and the monitor. The source was redesigned to increase the spectral purity of the beam and decrease its polarization by minimizing the number of bremsstrahlung photons in the beam. This was accomplished by utilizing an annular electron gun which allowed the beam to take off antiparallel to the direction at which electrons are incident on the anode. Two other features of the source are the conical anode which decreases the effective spot size and a rotatable anode and filter wheel which allow the operator to change targets without breaking vacuum. The monitor is an important part of the facility because it is used to determine the X-ray flux at the target. A commercially available solid-state detector, Si(Li), should be used along with appropriate proportional counters for monitoring. This detector will be particularly useful when energy or wavelength dispersive instruments are tested because of its good resolution.

Newbolt, W. Barlow↗
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 NTRS1986

A Fourier transform telescope for sub-arcsecond imaging of X-rays and gamma rays

This paper describes a Fourier transform telescope designed to image solar flare X-rays and gamma rays at energies up to 1 MeV with arcsec resolution. The imaging technique makes use of a bigrid collimator divided into a number of smaller areas called subcollimators. The grids in each subcollimator consist of a set of linear apertures so configured that each subcollimator provides a measurement of a single Fourier component of the angular distribution of the source. The imaging concept is therefore a mathematical analog to aperture synthesis in radio astronomy. For X-ray and gamma-ray astronomy, this approach has significant advantages in terms of relaxed requirements for astronomy, this approach has significant advantages in terms of relaxed requirements for position sensitivity in the detector and for control of grid alignment in the large scale telescope structure. The concept of the Fourier transform telescope will be illustrated with numerical parameters of a version now under study for the Pinhole/Occulter Facility.

Crannell, C. J.↗
NASA NTRS1987

X-ray studies of quasars with the Einstein Observatory. IV - X-ray dependence on radio emission

The X-ray properties of a sample of 114 radio-loud quasars observed with the Einstein Observatory are examined, and the results are compared with those obtained from a large sample of radio-quiet quasars. The results of statistical analysis of the dependence of X-ray luminosity on combined functions of optical and radio luminosity show that the dependence on both luminosities is important. However, statistically significant differences are found between subsamples of flat radio spectra quasars and steep radio spectra quasars with regard to dependence of X-ray luminosity on only radio luminosity. The data are consistent with radio-loud quasars having a physical component, not directly related to the optical luminosity, which produces the core radio luminosity plus 'extra' X-ray emission.

Worrall, D. M.↗
NASA NTRS1987

SMM hard X-ray observations of the soft gamma-ray repeater 1806-20

Six bursts from the soft gamma-ray repeater (SGR) 1806-20 have been recorded with the SMM Hard X-ray Burst Spectrometer during a highly active phase in 1983. Rise and decay times of less than 5 ns have been detected. Time profiles of these events indicate low-level emission prior to and after the main peaks. The results suggest that SGRs are distinguished from classical gamma-ray bursts by repetition, softer nonvarying spectra, short durations, simple temporal profiles, and a tendency for source locations to correlate with Population I objects. SGR characteristics differ from those of type I X-ray bursts, but they appear to have similarities with the type II bursts from the Rapid Burster.

Kouveliotou, C.↗
NASA NTRS1988

On the high-energy gamma-ray signature of cosmic-ray sources

Monte Carlo simulations of the gamma-ray emission from hypothetical cosmic-ray sources are performed. Sources which might correspond to acceleration by supernova shocks in 'average' interstellar conditions and deep within giant molecular clouds are considered. The consequences of dropping the common assumption that the cosmic-ray spectrum at the sources is the same as that observed at earth are examined. Spectral effects which can be related to the depth of the material shroud and the population of accelerated particles are explored using these simulations and are described. The results are compared with the COS B catalog of gamma-ray sources, and the implications for the underlying particle populations and source mechanisms are discussed.

Ormes, J. F.↗
NASA NTRS1988

Gamma-ray and hard X-ray imaging of solar flares

The scientific and technical aspects of high-resolution gamma-ray and X-ray imaging of solar flares are discussed. The scientific necessity for imaging observations of solar flares and the implications of future observations for the study of solar flare electrons and ions are considered. Performance parameters for a future hard X-ray and gamma-ray imager are summarized. Techniques for high-energy photon imaging including direct collimation imaging, coded apertures, and modulation collimators are surveyed. The technique of Fourier-transform imaging is examined. The options for detectors and grid fabrication are reviewed. Several planned future high-energy imagers are described.

Prince, T. A.↗
NASA NTRS1989

The relation between X-ray emission and rotation in late-type stars from the perspective of X-ray selection

An X-ray-selected sample of 128 late-type (F-M) stars analyzed. These stars were identified as optical counterparts to serendipitous X-ray detections made by the Einstein Observatory Extended Medium Sensitivity Survey. Once identified as X-ray sources, the sample was followed up by an extensive program of optical observations, including high-resolution and low-resolution spectroscopy and photometry. Spectral types, luminosity classes, distances, X-ray luminosities, projected rotation rates (v sin i), radial velocities, and binary status have been determined for the sample.

Fleming, Thomas A.↗
NASA NTRS1989

Study of the polarization dependence of the photoelectric effect in the soft X-ray band - A focal plane photoelectric stellar X-ray polarimeter for the Spectrum-X-Gamma mission

An experimental study of the polarization dependence of the photoelectric effect in cesium iodide in the soft X-ray band was started (Heckler et al., 1989). At a grazing angle of 10 degrees and a photon energy of 2.6 keV, it is found that the photoelectric yield from a thin layer of evaporated cesium iodide varies by 12.4 percent as the polarization vector of the incident X-ray beam is rotated about the line-of-sight. The rotation angle corresponding to the maximum photoyield is displaced by 16 degrees from the normal to the photocathode. This modulation and phase shift are in good agreement with the results recently reported by Fraser, et al. (1989) It is shown that a focal plane stellar X-ray polarimeter based on this photoelectric effect will be substantially more efficient than convential X-ray polarimeters such as those based on either Bragg reflection or scattering from low atomic number targets.

Heckler, A.↗
NASA NTRS1991

X-ray echoes from gamma-ray bursts

The identification of an echo of reflected radiation in time histories of gamma-ray burst spectra can provide important information about the existence of binary companions or accretion disks in gamma-ray burst systems. Because of the nature of Compton scattering, the spectrum of the echo will be attenuated at gamma-ray energies compared with the spectrum of the primary burst emission. The expected temporal and spectral signatures of the echo and a search for such echoes are described, and implications for gamma-ray burst models are discussed.

Dermer, Charles D.↗
NASA NTRS1991

Cosmic-ray transport and gamma-ray emission in supernova shells

The mixing and transport of cosmic rays accelerated by a pulsar inside an expanding supernova remnant are examined, and the resulting high-energy gamma-ray emission from nuclear interactions of these accelerated particles in the shell is investigated. Rayleigh-Taylor instability at the interface between a pulsar wind cavity and the inner supernova envelope is assumed to be the mixing mechanism. The analysis is applied to the model of Gaisser, Harding, and Stanev (1987), where protons are accelerated at the reverse shock in the pulsar wind. The instability time-scale is estimated from the dynamics of the pulsar wind cavity, and model the injection, diffusion, and interaction of protons in the shell. The resulting gamma-ray flux is lower than previous estimates due to proton adiabatic losses in the expanding pulsar wind. The protons mix and diffuse only into the innermost regions of the envelope before interacting. Energy-dependent diffusion causes the higher energy gamma-ray light curves to decay faster than those at lower energy.

Harding, A. K.↗