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  • ASTROPHYSICS  (19)
  • 1
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    2-165 keV observations of active galaxies and the diffuse background (1983)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: HEAO 1 spectral observations of 12 active galaxies in the 12-165 keV and 2-50 keV ranges are reported. The spectra of these galaxies in the 2-165 keV range are well represented by a single power law model; within experimental uncertainties a narrow dispersion in power law index attributable to the individual galaxies is observed, while the 2-165 keV luminosities of these galaxies ranged from 3 x 10 to the 43rd to 3 x 10 to the 45th ergs/s. An apparent universality of the spectral form is found which can be interpreted as due to a common electron distribution with a temperature of tens of keV in the Compton scattering region or as a common nonthermal power-law distribution generating the observed flux through synchrotron-Compton processes. The data indicate that relativistic particles are likely to be responsible for the X-rays from cores of active galaxies through synchroton-Compton processes. In addition, it is noted that only weak number evolution, if any at all, is present in active galaxies.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 269; June 15
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  • 2
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    Hercules X-1 hard X-ray pulsations observed from HEAO 1 (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The 1.24-sec pulsations of Her X-1 in the energy range 13-75 keV have been analyzed in data obtained from the UCSD/MIT experiment on HEAO 1 during observations of the source on three dates in February 1978 and three in August 1978. Observational results are (1) the main pulse broadens somewhat with increasing energy; (2) the pulsation light curve undergoes pronounced changes at the leading edge of the main pulse from day to day; (3) spectral hardening within the main pulse is confirmed; (4) a 40-60 keV spectral feature in the spectrum is confirmed; (5) this feature is resolved, the pulse width broadening is greater than 20%, and its centroid varies with pulsation phase; and (6) the 13-75 keV spectrum does not noticeably vary from day to day, except for an overall intensity factor. Some implications of these results for the prevailing models of Her X-1 and the HZ Her-Her X-1 system are briefly discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 240
    Format: text
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  • 3
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    Hard X-ray spectrum of Cygnus X-1 (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Long-term measurements of the hard X-ray spectrum from 3 keV to 8 MeV of the black-hole candidate Cygnus X-1 in its low state are reported. Observations were made from October 26 to November 18, 1977 with the A2 (Cosmic X-ray) and A4 (Hard X-ray and Low-Energy Gamma-Ray) experiments on board HEAO 1 in the spacecraft's scanning mode. The measured spectrum below 200 keV is found to agree well with previous spectra which have been fit by a model of the Compton scattering of optical or UV photons in a very hot plasma of electron temperature 32.4 keV and optical depth 3.9 or 1.6 for spherical or disk geometry, respectively. At energies above 300 keV, however, flux excess is observed which may be accounted for by a distribution of electron temperatures from 15 to about 100 keV.
    Keywords: ASTROPHYSICS
    Type: Nature; 293; Sept. 24
    Format: text
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  • 4
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    A high sensitivity determination of the hard X-ray spectrum of Sco X-1 (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Nature; 286; Aug. 21
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  • 5
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    Observations of 12-200 keV X-rays from GX 339-4 (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: X-ray spectra of GX 339-4 measured on three occasions in 1977 and 1978 are presented. These are the first reported measurements above 10 keV. The spectra can be described as the superposition of a soft component, which is dominant below about 20 keV, and a hard component at higher energy. Simultaneous measurements at lower energy show that the soft component vanished during the observation in early 1978. The behavior of these two components is similar to that of the spectrum of Cygnus X-1; this reinforces the previously noted resemblance in rapid X-ray variability.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 262
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  • 6
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    HEAO 1 high energy X-ray observations of the Virgo cluster and A2142 (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Observations are reported of the Virgo cluster and Abell 2142 in the energy range of 15-150 keV, detected by using hard X-ray and low energy gamma-ray instruments on board the HEAO-1 spacecraft, during 1977 and 1978. The Virgo cluster was detected at a mean flux of (1.3 + or - 0.3) x 10 to the 0.001 photons/sq cm-s in the 20-100 keV band, and A2142 was detected marginally at a flux of (2 + or - 0.8) x 10 to the 0.001 photons/sq cm-s in the 15-40 keV band. Inverse Compton emission was not observed in these clusters. Results are used to derive limits to the intracluster magnetic field of B approximately greater than 5 x 10 to the -7th gauss and B approximately 5 x 10 to the -8th gauss in the Virgo cluster and A1242, respectively. There is some evidence for variability in these sources, which implies that the hard X-ray emission originates in a relatively compact object or region rather than from the cluster as a whole.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 246
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  • 7
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    Rapid variability of 10-140 keV X-rays from Cygnus X-1 (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: On five occasions in 1977 and 1978, Cygnus X-1 was observed using the low-energy detectors of the UCSD/MIT Hard X-ray and Low-Energy Gamma Ray experiment on the HEAO 1 satellite. Rapid (times between 0.08 and 1000 sec) variability was found in the 10-140 keV band. The power spectrum was white for frequencies between 0.001 and 0.05 Hz and was proportional to the inverse of the frequency for frequencies between 0.05 and 3 Hz, indicating correlations on all time scales less than approximately 20 s. The shape of the energy spectrum was correlated with intensity; it was harder at higher intensity. If the emission is produced by Comptonization of a soft photon flux in a hot cloud, the heating of the cloud cannot be constant; it must vary on time scales up to approximately 20 s. A variable accretion rate could cause the observed effects.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 246
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  • 8
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    The X-ray spectrum of AM Herculis from 0.1 to 150 keV (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: No significant flux at 100 keV was detected in the observations by the HEAO 1 satellite (March and April of 1978) and in several OSO 7 observations. The spectrum above 2 keV can be fitted by a composite thermal bremsstrahlung model that includes an approximation to the albedo expected from the white dwarf. The bremsstrahlung kT sub e from this model (30.9 + or - 4.5 keV) implies a white dwarf mass in excess of 0.6 solar mass. An emission feature at 6.5 + or - 0.15 keV and equivalent width of 0.8 + or - 0.1 keV is confirmed; it is thought that this might be due to fluorescence from the white dwarf by the bremsstrahlung from a small thin shocked region. It is noted that the continuum could also have been steepened at high energy in scattering in the accretion column, but the line photons cannot have gone through the same optical depths.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 250
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  • 9
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    HEAO-1 observations of HER X-1 at energies above 13 keV in February 1978 (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Keywords: ASTROPHYSICS
    Type: X-ray astronomy; May 29, 1978 - Jun 10, 1978; Innsbruck; Austria
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  • 10
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    HEAO 1 observations of high-energy X-rays from 3C273 (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The first detection of high energy (13 to 120 keV) X rays from the quasar 3C273, made by the HEAO 1 satellite, is reported. Observations were made with the 13 to 180 keV slat collimated detectors of the high energy X-ray and low energy gamma-ray (A4) experiment during December 1977-January 1978 and June-July 1978. Results are consistent with the previously observed X-ray flux variability on a scale of months. Photon count rates are presented for each of five energy bands and count rate and photon spectra for the June through July 1978 observations are derived. A comparison of the data obtained with that at lower X-ray energies and higher gamma-ray energies indicates that there is an overall spectral steepening from low to high energies and a possible break near 20 keV, which may be due to the gamma rays originating from a different region than that of the X rays.
    Keywords: ASTROPHYSICS
    Type: Nature; 278; Mar. 15
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