IMAGING HIGH-ENERGY ASTROPHYSICAL SOURCES USING EARTH OCCULTATION

HIGH-ENERGY astrophysical sources can be difficult to image. Photons w ith energies above approximately 5 keV are hard to focus, so experimen ts usually employ coded masks1-4 or moving collimators5-9 to modulate the flux received by the detectors; the resulting signals are then dec onvolved to form the images. Here we demonstrate a new approach which makes use of the large-area, non-collimated detectors of the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory. As the spacecraft moves in its orbit, the Earth itself acts as a stable o cculting disk. Changes in the measured signal during a single occultat ion correspond to the integrated intensity of sources positioned along the arc described by the Earth's edge (limb). The low-altitude, moder ately inclined orbit of the spacecraft ensures that the angle at which the limb traverses a source region varies between occultations, and t hus data from a series of occultations can be transformed into an imag e. This imaging process is conceptually and mathematically similar to those used in fan-beam aperture-synthesis radio-astronomy10 and medica l computer-assisted tomography11, and holds great promise for all-sky imaging with relatively simple (and hence inexpensive) detectors.