Imaging high-energy astrophysical sources using Earth occultation

Abstract

HIGH-ENERGY astrophysical sources can be difficult to image. Photons with energies above ∼5 keV are hard to focus, so experiments usually employ coded masks^1–4 or moving collimators^5–9 to modulate the flux received by the detectors; the resulting signals are then deconvolved 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 occulting disk. Changes in the measured signal during a single occultation correspond to the integrated intensity of sources positioned along the arc described by the Earth's edge (limb). The low-altitude, moderately inclined orbit of the spacecraft ensures that the angle at which the limb traverses a source region varies between occultations, and thus data from a series of occultations can be transformed into an image. This imaging process is conceptually and mathematically similar to those used in fan-beam aperture-synthesis radio-astronomy¹⁰ and medical computer-assisted tomography¹¹, and holds great promise for all-sky imaging with relatively simple (and hence inexpensive) detectors.