Dissertation: Exotic Binaries in Galactic Globular Clusters: Identification, Classification, and their Formation

(Cassiopeia – Autumn / l’automne 2021)

by / par Dr. Yue Zhao (Cory)
Thesis defended on July 6, 2021
Department of Physics, University of Alberta
Thesis advisor: Prof. Craig Heinke

Abstract

Globular clusters (GCs) are dense and massive stellar populations, which provide a unique environment where the high stellar density facilitates frequent dynamical encounters, creating many exotic binaries. These exotic binaries generally have short orbits and often harbour compact objects, namely neutron stars (NSs), black holes (BHs), and white dwarfs (WDs). With the unprecedented sensitivity and angular resolution of the Chandra X-ray Observatory, GCs are found to host an overabundance of X-ray binaries.

This thesis identifies and classifies exotic binaries in multiple GCs and presents their relation to cluster dynamics, incorporating X-ray, UV, optical, and radio observations. In the GC M3 (NGC 5272), we discovered 16 X-ray sources within the half-light radius (rh), where the second brightest source (M3-CX2) is a newly discovered low-mass X-ray binary candidate. In a study of NGC 6397, we incorporate deep radio imaging observations from the MAVERIC radio survey and find a strong “hidden” millisecond pulsar candidate. A deep observation of M30 reveals 10 new X-ray sources within rh and suggests a difference between the radial distributions of bright and faint X-ray sources. Finally, a census of radio sources in multiple GCs indicates that they are likely a mixture of millisecond pulsars (the numbers of which, per cluster, scale with the rate of stellar encounters in each cluster) and quiescent black hole binaries (which do not show a simple scaling with the number of stellar encounters per cluster).

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