Exploring Gamma-Ray Binaries at Very High Energies: VERITAS Observations and the Detection of PSR J2032+4127/MT91 213

Abstract

Gamma-ray binaries consist of a compact object orbiting with a massive star. The violent interactions within these systems, due either to the collision of the pulsar/stellar winds or the relativistic jets of a black hole, are capable of accelerating charged particles to TeV energies. Due to the orbital motion of the binary system, this acceleration occurs in a constantly-evolving environment. Gamma-ray binaries therefore act as “natural laboratories” for the study of non-thermal acceleration and radiation mechanisms, as well as a means to study the physics of compact objects.

This dissertation is concerned primarily with the discovery of a new gamma-ray binary system: PSR J2032+4127/MT91 213. At the time of its discovery, this was only the second gamma-ray binary system with a firmly identified compact object, and just the seventh gamma-ray binary known. This system’s unique 50-year orbital period sets it apart from other gamma-ray binary systems and underscores the significance of the 100-hour observing campaign to cover the periastron passage in the autumn of 2017 by VERITAS, MAGIC, and Swift-XRT. As a result of these observations, detailed light curves and spectra were constructed which exhibit complex variability. This variability maps the continually-changing environment at the interface of the pulsar and stellar winds, allowing for examination of the underlying physical processes.

The analysis of gamma-ray emission from PSR J2032+4127/MT91 213 was complicated by the fact that the binary system lies in the same area of the sky as another gamma-ray source: TeV J2032+4130. This fact motivated the development of a more sophisticated data analysis method, based on the method of maximum-likelihood estimation, which is capable of simultaneously detecting and characterizing the emission of two or more sources in the same field of view. This analysis was tested on several known sources and shown to be consistent with standard analysis methods. Its ability to distinguish between multiple sources in the same field was demonstrated in a re-analysis of the PSR J2032+4127/MT91 213 + TeV J2032+4130 system. Finally, the method was used to search for “hidden” sources in the vicinity of other gamma-ray binary systems, yielding negative results but providing insight into the nature of the wind interaction in these systems.

The results presented in this dissertation are based on extensive observations carried out by VERITAS: the Very Energetic Radiation Imaging Telescope Array System. VERITAS, which comprises an array of four Imaging Atmospheric Cherenkov Telescopes (IACTs), is uniquely suited for detecting gamma-ray emission in the GeV-TeV range, providing the high sensitivity and precision needed to track the complex variability in PSR J2032+4127/MT91 213’s periastron passage. The physical instrumentation of VERITAS and the standard VERITAS software/data analysis tools, both discussed in detail in this work, were critical in obtaining the results here presented.