Very High Energy Gamma-Rays from Binary Systems

Abstract

This thesis presents a study of the very high energy (VHE) gamma-ray emission from X-ray binary systems using the H.E.S.S. imaging atmospheric Cherenkov array.

The historical background and basic principles of ground-based gamma-ray astronomy are briefly reviewed and an overview of the design and capabilities of the H.E.S.S. telescope system is presented. The broadband observational properties of X-ray binary systems and their relevance in a broader astrophysical context is also discussed.

A review of the radiative emission mechanisms which relate to VHE gamma-ray emission in X-ray binaries is presented, with emphasis given to the leptonic emission processes of synchrotron radiation and inverse-Compton scattering. Intrinsic absorption processes which act to attenuate the emitted flux of VHE gamma-rays are also discussed. Three computer models are introduced which simulate aspects of the gamma-ray emission and absorption in X-ray binary systems.

A detailed analysis of the VHE gamma-ray emission from the X-ray binary LS 5039 is presented and the relevant procedures for data selection, gamma-hadron separation and background estimation are discussed in some detail. Methods for the determination of detection significance and the calculation of gamma-ray fluxes are also reviewed and results are derived which apply specifically to LS 5039. A detailed temporal analysis of the gamma-ray signal from LS 5039 is presented, applying tests for secular, excess and periodic variability. Strong evidence is found for modulation of the observed gamma-ray flux on the orbital period of ~3.9 days. Following a brief discussion of the procedures required for spectral analysis of VHE gamma-ray data, results are presented for LS 5039 which reveal evidence for spectral variability which is correlated with the observed gamma-ray flux and therefore, the orbital phase of the binary system. The spectral and temporal characteristics of LS 5039 are then compared with the predictions of theoretical models in an attempt to explain the observed behaviour.

Contemporaneous X-ray and VHE gamma-ray observations of three galactic microquasars using the Rossi X-ray Timing Explorer and H.E.S.S. are presented. Although no gamma-ray detections are reported, the observations permit the derivation of upper limits to the VHE gamma-ray flux which correspond to episodes of known X-ray behaviour. The X-ray characteristics of each target are compared with pre-existing observational data to infer the presence or otherwise of relativistic outflows at the H.E.S.S. observation epochs. The implications of the gamma-ray non-detections are then discussed in the context of these inferred system properties.

The results of a survey of the VHE gamma-ray emission associated with the positions of 125 known X-ray binaries are presented. Although no conclusive detections were obtained, tentative indications were found for a population of faint, spectrally hard gamma-ray sources associated with high-mass X-ray binary systems. The inferred characteristics of the indicated population show broad agreement with the measured properties of known gamma-ray-emitting X-ray binary systems like LS 5039.