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Observation of the Crab Nebula in Soft Gamma Rays with the Nuclear Compton Telescope

  • Author(s): Bandstra, Mark ShenYu
  • Advisor(s): Boggs, Steven E
  • et al.
Abstract

Soft gamma rays (100 keV-10 MeV) are indispensable probes of the most violent and extreme processes in the cosmos. Gamma rays are produced by non-thermal processes in such disparate objects as neutron stars, X-ray binaries, and Active Galactic Nuclei (AGN), and they also result from the decays of many radioactive nuclei, such as certain isotopes produced in supernova explosions. The penetrating nature of gamma rays allows the astrophysicist to probe deep within these often obscured systems and make unique and complementary observations of their gravitational fields, magnetic fields, and nuclear reactions.

The challenges to soft gamma-ray measurements are numerous. First, the dominant interaction is Compton scattering, which necessitates careful imaging and simulation. Second, there is high background, predominantly from charged particle reactions and the activation of passive material in the instrument. Special care must be taken for background reduction, such as using active shielding and clever event selections. Third, atmospheric absorption of gamma rays necessitates being in space or at balloon altitudes to observe them.

Over the last four decades, various types of telescopes have been developed to detect and image soft gamma-rays. One promising technology is the Compton telescope, which exploits the Compton effect to perform direct imaging of gamma-ray photons. The current generation of Compton telescopes are compact Compton telescopes, which rely on both fine position and fine energy resolution of gamma-ray interactions within the detector volume in order to perform Compton imaging. The development of soft gamma-ray telescopes, and Compton telescopes in particular, is reviewed in Chapter 1.

The Nuclear Compton Telescope (NCT) is one such compact Compton telescope. NCT is a balloon-borne telescope designed to perform imaging, spectroscopy, and polarization analysis on soft gamma rays from astrophysical sources. NCT detects gamma rays using ten crossed-strip high-purity germanium detectors, each with a 2 mm strip pitch and a 15 mm thickness. This dissertation gives an overview of NCT's detectors, gondola systems, and data analysis pipeline (Chapter 3), as well as detailed descriptions of the detector calibrations -- the depth calibration, energy calibration, cross-talk correction, and charge loss correction (Chapters 4-6).

The NCT instrument has flown twice, both times from the Columbia Scientific Balloon Facility in Fort Sumner, New Mexico. The first flight took place in 2005 with a prototype instrument housing only two germanium detectors. Due to the brief flight (6 hours), the only analysis that could be performed was a characterization of the gamma-ray background at float altitudes (40 km). The second flight, on 17-18 May 2009, is detailed in this thesis (Chapter 7). The full ten-detector instrument was flown for a total of 37 hours. The primary goals of the flight were to observe the Crab Nebula and Cygnus X-1, both bright gamma-ray continuum sources (see Chapter 2 for a review of the Crab Nebula). The Crab Nebula was observed for 9.2 hours of the flight and was detected at a significance of 4 sigma (Chapter 8). This is the first detection of an astrophysical source by a compact Compton telescope. This work is an important step in establishing the viability of the compact Compton telescope design for future space-based wide-survey instruments.

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