## Doctoral Dissertations

Spring 1990

Dissertation

Physics

#### Degree Name

Doctor of Philosophy

The objective of this project is to study soft $\gamma$-ray emission in the (0.1-10) MeV energy band for selected AGN and explore how much they contribute to the total diffuse $\gamma$-ray background. For this purpose, a series of imaging observations of extragalactic objects in the low-energy $\gamma$-ray region were carried out by the University of New Hampshire's Coded Aperture Directional Gamma-Ray Telescope (DGT). The DGT was successfully flown at stratospheric balloon altitudes on 1988, May 11 when observations were made of the Crab, NGC 1275, MKN 421, and NGC 4151. The measured Crab spectrum is consistent with a featureless power-law of the form; dN(E)/dE = 5.93E$\sp{-2.07\pm\sbsp{.23}{.17}}$ $\gamma$'s cm$\sp{-2}$ s$\sp{-1}$ keV$\sp{-1}$, in agreement with the previous observations of this source. Significant emission was detected up to 500 keV from the Seyfert galaxy, NGC 4151.
To increase the total sky exposure we have analyzed the extragalactic field images, including the 3C 273 region, obtained by the DGT during its previous flight in 1984. We have specifically searched for point source signatures in the MeV energy range in 1.5 sr of the DGT image fields. No positive emission was detected above the flux value of 3.2 $\times$ 10$\sp{-12}$ ergs cm$\sp{-2}$ s$\sp{-1}$ keV$\sp{-1}$ at 1 MeV from any of the four active galaxies. No serendipitous source during a total observation period of 40 hours in the energy range (1-8) MeV was observed. Based on the log N - log S method and the constraint imposed by the CGB flux, we calculate that if the origin of the cosmic $\gamma$-ray background is attributed solely to an active galaxy component, then the ratio of active galaxy $\gamma$-ray luminosity at 1 MeV to X-ray luminosity at 2 keV, L$\sb{\gamma}$/L$\sb{x}$, should be $\leq$ 4.5 $\times$ 10$\sp{-2}$. Using this ratio we also predict the total number of sources detectable with the DGT in the energy range (1-2) MeV to be $\sim$2 for the entire sky; the null result obtained by the DGT is not in contradiction with this prediction. We also calculate the number of $\gamma$-ray sources expected to be detected by the future $\gamma$-ray experiments with better detection sensitivity.