Earth Occultation Techniques

Earth occultation is a familiar phenomenon to all residents of the Earth, although most know it as sunrise and sunset. Satellites observe Earth occultations of an astrophysical source when it sets behind or rises above the Earth's horizon (also called the Earth's limb). Two occultations, a rise and a set, occur every satellite orbit. The most extensive use of Earth occultation to measure sources was with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) for 9 years. The entire sky was subject to Earth occultation for some portion of CGRO's precession period. In BATSE data, atmospheric attenuation produced step-like features when astrophysical sources underwent occultation. The observed change in count rate in several energy bands provided a measurement of the source intensity and spectrum without requiring sophisticated background models. I will describe (1) a brief history of occultation techniques in X-ray and gamma ray astronomy; (2) how occultation features were used to locate and monitor astrophysical sources with BATSE and (3) how the Earth occultation technique can be applied to future instruments.