Abstracts Earth and Environmental Sciences

Earth Occultation Imaging of the Low Energy Gamma-Ray Sky with the Fermi/Gamma-ray Burst Monitor

by James Craig Rodi

The hard X-ray/soft gamma-ray sky is highly variable as source intensities can vary on timescales from fractions of a second to years. The type of sources emitting at these energies often include compact objects such as white dwarfs, neutron stars, and black holes interacting with the surrounding environment, which enables the study of the properties and characteristics of these exotic objects. Because the interactions with the environment is often through accretion, most of the emission is in X-rays and gamma-rays Frank et al. 1992 making this energy range critical to understanding the relevant physical processes and mechanisms. Hard X-ray/soft gamma-ray (10 keV - 1 MeV) photons are difficult to focus requiring alternative observing techniques to monitor sources in this energy range. One method is the Earth Occultation Technique (EOT) that uses the rapid change in count rate (~sim 10 sec) due to a source passing behind (or out from behind) the Earth to measure source intensity. The EOT has been applied to the Gamma-ray Burst Monitor (GBM) on board the NASA Fermi satellite since its launch in 2008 and has been performing all-sky monitoring of a predetermined catalog of ~200 hard X-ray/soft gamma-ray sources. Because the EOT requires prior knowledge of a source's position for monitoring, I have developed a tomographic imaging method Imaging with a Differential filter using the Earth Occultation Method (IDEOM) to generate all-sky images in search of sources absent from the input catalog to construct a catalog as complete as possible. A complete catalog is important for reducing a source of systematic error as the flux from an unaccounted-for source can be attributed to a known source thus biasing its flux measurement. Approximately 4 years of GBM data have been analyzed to produce all-sky images covering the 12-50 keV, 50-100 keV, and 100-300 keV energy bands with 16 sources being added to the GBM catalog. All-sky images are shown for each energy range along with a table listing the sources detected by IDEOM. Also, the list of sources detected by IDEOM has been compared to those detected by EOT in each energy band. In addition, spectral analysis was performed in the four persistent sources detected by GBM and the Fermi/Large Area Telescope (LAT) (NGC 1275, 3C 273, Cen A, and the Crab) thus providing energy coverage from ~10 keV to ~100 GeV to study the high energy emission processes of these sources. Spectral results from GBM and LAT are presented and shown to be in agreement with previously proposed models in the literature. Finally, I have also developed a transient search algorithm to search for flares and outbursts in the light curves of GBM sources. This algorithm was applied to the ~200 sources in the GBM catalog to search for transient events during the first four years of the mission in four energy bands covering 12-500 keV. The search resulted in 168 transient events found from 65 sources with 7 events detected above 50 keV and 1 event detected above 100 keV (XTE J1752-223). A table of start and stop times are…