Earth occultation imaging of the low energy gamma-ray sky with GBM

¹ Dept. of Physics & Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
e-mmail: jrodi1@lsu.edu
² Physics Dept., La Sierrra University, Riverside CA 92515, USA
³ Institut de Ciències de l’Espai, (IEEC-CSIC), Campus UAB, Fac. de Ciències, Torre C5, parell, 2a pl., 08193 Barcelona, Spain
⁴ Universities Space Research Association, Huntsville AL 35805, USA
⁵ Dept of Physics, University of Alabama in Huntsville, Huntsville AL 35899, USA
⁶ Marshall Space Flight Center, Huntsville AL 35182, USA

Received: 3 April 2013
Accepted: 19 December 2013

Abstract

Aims. The Earth Occultation Technique (EOT) has been applied to Fermi’s Gamma-ray Burst Monitor (GBM) to perform all-sky monitoring for a predetermined catalog of hard X-ray/soft γ-ray sources. In order to search for sources not in the catalog, thus completing the catalog and reducing a source of systematic error in EOT, an imaging method has been developed – Imaging with a Differential filter using the Earth Occultation Method (IDEOM).

Methods. IDEOM is a tomographic imaging method that takes advantage of the orbital precession of the Fermi satellite. Using IDEOM, all-sky reconstructions have been generated for ~4 years of GBM data in the 12–50 keV, 50–100 keV and 100–300 keV energy bands in search of sources otherwise unmodeled by the GBM occultation analysis.

Results. IDEOM analysis resulted in the detection of 57 sources in the 12–50 keV energy band, 23 sources in the 50–100 keV energy band, and 7 sources in the 100–300 keV energy band. Seventeen sources were not present in the original GBM-EOT catalog and have now been added. We also present the first joined averaged spectra for four persistent sources detected by GBM using EOT and by the Large Area Telescope (LAT) on Fermi: NGC 1275, 3C 273, Cen A, and the Crab.