Gamma-Ray All-Sky Imaging with the Burst and Transient Source Experiment

The BATSE mission aboard CGRO monitored the whole sky in the 20 keV - 1 MeV energy band continuously from April 1991 until June 2000. Although BATSE had very poor intrinsic angular resolution, the data can be used to survey the entire soft gamma-ray sky with < 1 degree angular resolution and ~ 1 mCrab sensitivity by using the Earth occultation method. This method determines flux by measuring the step in the count rate profile in each BATSE detector as a source rises above or sets below the Earth's limb. A maximum likelihood imaging technique can then be used to build up all-sky maps from the images of the Earth's limb produced by occulting sources. However, since the Earth seen from BATSE has a radius of ~ 70 degrees, the limb images that intersect at the positions of bright point sources have a significant effect over the area of the all-sky map. A method for performing image cleaning on likelihood data has also been developed and is used to effectively remove artefacts from the all-sky maps. This paper describes the 'LIMBO' imaging technique and presents preliminary all-sky maps of 25 - 160 keV emission, the first to be made since the HEAO1-A4 mission of 1978-79.