Gamma-ray all-sky imaging with the Burst and Transient Source Experiment

¹ School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK
² Centre d'Étude Spatiale des Rayonnements, 9 avenue Colonel-Roche, BP 4346, 31028 Toulouse Cedex 4, France

Corresponding author: S. E. Shaw, simon.shaw@obs.unige.ch

Received: 30 June 2003
Accepted: 21 December 2003

Abstract

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 < angular resolution and ~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°, 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.