Measurement of the
Cosmic Diffuse Gamma-Ray Spectrum
from 800 keV to 30 MeV
S. Cheenu Kappadath
- University of New Hampshire, May 1998
Abstract
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[pdf-file]
[postscript]
The Cosmic Diffuse Gamma-Ray Spectrum
The COMPTEL CDG flux measurements in the 1 to 10 MeV range are about 5-10 times lower than the pre-COMPTEL estimates. The CDG measurements show no evidence of the MeV bump in the 1 to 10 MeV range. The integrated flux from 0.8 to 9 MeV is (4.7 ± 1.7) x 10^-3 photons/cm^2-s-sr and that from 9 to 30 MeV is (1.5 ± 0.3) x 10^-4 photons/cm^2-s-sr. This represents the first significant detection (statistical significance of 7.2 sigma) of the CDG emission in the 9 to 30 MeV range. The preliminary COMPTEL CDG estimates reported earlier (Kappadath et al. 1995; 1996; 1997) are in good agreement with these results.
The COMPTEL CDG spectrum is fit to power-law function, A x (E/E0)^-a, where A is the flux normalization, E is the photon energy, E0 is taken to be 5 MeV (the logarithmic-midpoint energy between 0.8 and 30 MeV) and a is the spectral index. The resulting differential photon spectrum from 800 keV to 30 MeV is well described by a photon spectral index of -2.4 ± 0.2 and a flux normalization of (1.05 ± 0.2) x 10^-4 photons/cm^2-s-sr-MeV at 5 MeV.
Thesis Contents
Title Page
- [pdf-file] [postscript]
Table of Contents
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Abstract
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Chapter I: Introduction
- [pdf-file] [postscript]
Chapter II: The COMPTEL Instrument
- [pdf-file] [postscript]
Chapter III: Overview of the Instrument Background
- [pdf-file] [postscript]
Chapter IV: Data
- [pdf-file] [postscript]
Chapter V: The Instrument Background
- [pdf-file] [postscript]
Chapter VI: The CDG Analysis Method
- [pdf-file] [postscript]
Chapter VII: Results and Discussion
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Chapter VIII: Conclusions
- [pdf-file] [postscript]
Bibliography
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Dissertation Director:
Dr. James M. Ryan
Other Relevent Figures
The observed Virgo and SGP regions represent large areas towards the North and South Galactic Poles. The measured CDG emission from these polar directions can be inspected for any large scale differences. The measured CDG spectrum above 4.2 MeV from the Virgo and SGP directions are plotted in the above together with the power-law fit to the total COMPTEL CDG spectrum. The spectrum from the Virgo and SGP directions are consistent with one another. Over large scales (Virgo and the South Galactic Pole), the 4.2-30 MeV CDG measurements are compatible with an isotropic emission to within 50% of the measured flux.
The COMPTEL CDG photon spectral index of -2.4 in the 0.8 to 30 MeV energy range is of an intermediate value between the steeper slope (-2.9) at lower energies and the harder slope (-2.1) at higher energies. The diffuse spectrum evolves from a soft to hard spectrum in the COMPTEL energy range.
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