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The Burst and Transient Source Experiment Earth Occultation TechniqueAn Earth orbiting detector sensitive to gamma-ray photons will see step-like occultation features in its count rate when a gamma-ray point source crosses the Earth's limb. This is due to the change in atmospheric attenuation of the gamma rays along the line of sight. In an uncollimated detector, these occultation features can be used to locate and monitor astrophysical sources provided their signals can be individually separated from the detector background. We show that the Earth occultation technique applied to the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) is a viable and flexible all-sky monitor in the low-energy gamma-ray and hard X-ray energy range (20 keV-1 MeV). The method is an alternative to more sophisticated photon imaging devices for astronomy and can serve well as a cost-effective science capability for monitoring the high energy sky. Here we describe the Earth occultation technique for locating new sources and for measuring source intensity and spectra without the use of complex background models. Examples of transform imaging, step searches, spectra, and light curves are presented. Systematic uncertainties due to source confusion, detector response, and contamination from rapid background fluctuations are discussed and analyzed for their effect on intensity measurements. A sky location-dependent average systematic error is derived as a function of Galactic coordinates. The sensitivity of the technique is derived as a function of incident photon energy and also as a function of angle between the source and the normal to the detector entrance window. Occultations of the Crab Nebula by the Moon are used to calibrate Earth occultation flux measurements independent of possible atmospheric scattering effects.
Document ID
20020048592
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Harmon, B. A.
(NASA Marshall Space Flight Center Huntsville, AL United States)
Fishman, G. J.
(NASA Marshall Space Flight Center Huntsville, AL United States)
Wilson, C. A.
(NASA Marshall Space Flight Center Huntsville, AL United States)
Paciesas, W. S.
(Alabama Univ. Huntsville, AL United States)
Zhang, S. N.
(Alabama Univ. Huntsville, AL United States)
Finger, M. H.
(Universities Space Research Association Huntsville, AL United States)
Koshut, T. M.
(Universities Space Research Association Huntsville, AL United States)
McCollough, M. L.
(Universities Space Research Association Huntsville, AL United States)
Robinson, C. R.
(Universities Space Research Association Huntsville, AL United States)
Rubin, B. C.
(Universities Space Research Association Huntsville, AL United States)
Date Acquired
August 20, 2013
Publication Date
January 1, 2002
Publication Information
Publication: The Astrophysical Journal Supplement Series
Publisher: American Astronomical Society
Volume: 138
Subject Category
Space Radiation
Distribution Limits
Public
Copyright
Other

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