Dissertation: Behavior of Low-mass X-ray Binaries and their Formation in Globular Clusters

(Cassiopeia – Autumn/l’automne 2016)

by Arash Bahramian
Thesis defended on June 22, 2016
Department of Physics, University of Alberta
Thesis advisor: Dr. Craig O. Heinke

Abstract
Low mass X-ray binaries (LMXBs) are systems of compact stellar remnants accreting from a low mass companion star. These systems show various levels of mass transfer on various timescales. Many aspects of accretion in these systems are still not fully understood, specifically the emission processes involved in various states of mass transfer. The population of LMXBs has been found to be orders of magnitude higher (per unit mass) in globular clusters (GCs) compared to the Galactic field. This overabundance has been explained as due to the formation of LMXBs by stellar encounters in GCs. In this thesis, we study GC LMXB populations, and the details of accretion in these systems. First, we focus on the population of LMXBs and the role of stellar encounters in their formation in GCs. We calculate model-independent stellar encounter rates for 124 Galactic GCs, and show that core-collapsed clusters tend to have lower numbers of LMXBs compared to other clusters with similar values of the stellar encounter rate. Then, we focus on studying accretion in LMXBs in various classes of systems (quiescent, transient, ultra-compact and symbiotic). We provide evidence for the presence of low-level accretion in the rise and decay of outbursts in transient LMXBs, and the absence of low-level accretion in many quiescent neutron star LMXBs. Finally we study two peculiar LMXBs, and show that one is an ultra-compact X-ray binary, and another one is a symbiotic X-ray binary.

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