The Burst and Transient Source Experiment Earth Occultation Technique

An Earth orbiting detector sensitive to gamma ray photons will see step-likeoccultation features in its counting rate when a gamma ray point source crossesthe Earth's limb. This is due to the change in atmospheric attenuation of thegamma rays along the line of sight. In an uncollimated detector, theseoccultation features can be used to locate and monitor astrophysical sourcesprovided their signals can be individually separated from the detectorbackground. We show that the Earth occultation technique applied to the Burstand 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 andhard X-ray energy range (20 keV - 1 MeV). The method is an alternative to moresophisticated photon imaging devices for astronomy, and can serve well as acost-effective science capability for monitoring the high energy sky. Here we describe the Earth occultation technique for locating new sources andfor measuring source intensity and spectra without the use of complexbackground models. Examples of transform imaging, step searches, spectra, andlight curves are presented. Systematic uncertainties due to source confusion,detector response, and contamination from rapid background fluctuations arediscussed and analyzed for their effect on intensity measurements. A skylocation-dependent average systematic error is derived as a function ofgalactic coordinates. The sensitivity of the technique is derived as a functionof incident photon energy and also as a function of angle between the sourceand the normal to the detector entrance window. Occultations of the Crab Nebulaby the Moon are used to calibrate Earth occultation flux measurementsindependent of possible atmospheric scattering effects.Comment: 39 pages, 24 figures. Accepted for publication in the Astrophysical Journal Supplement