Abstract:
This thesis is divided into two parts reflecting the two different fields of work
presented within. The first half is dedicated to studies relating to indirect dark
matter detection, while the second half details work undertaken in the very-high energy
astronomy domain, specifically on the observations of gamma-ray binaries.
The phenomenon of unseen mass in the Universe is as all-pervading as it is mysterious.
The plethora of evidence in support of this conclusion spans both the baryonic
and non-baryonic sectors, and the fingerprints of this unseen anomaly are detected
through both direct and indirect methods making it a fundamental and ubiquitous
part of the Universe. This "dark matter" remains one of the greatest challenges to
modern cosmological models and to our understanding of the Universe. While many
theories regarding its nature exist, it is as yet undetected and the identification of
dark matter (composing 85% of the visible Universe) remains a very active field
of research. Indirect detection of dark matter is a method that aims to infer the
presence and properties of dark matter through the observation of secondary particles
(mostly photons) resulting from dark matter interactions. In many dark matter
models, secondary particles are produced through dark matter’s decay, annihilation,
or other interactions with standard model particles. In dark matter-dominated
astrophysical objects, these secondary particles would form an excess on top of the
known astrophysical populations. This excess can be probed for using modern X-ray
and gamma-ray telescopes, allowing for their distinction and association with dark
matter. In most cases, the flux of these secondary particles would allow for the
inference of dark matter and for the derivation of its properties. A lack of dark
matter-associated flux detected by an instrument allows one to place limits on a
given dark matter candidate particle’s properties. This work, in part, focuses on the
field of indirect dark matter detection, specifically on the search for dark matter
in astrophysical objects and the capacity of upcoming missions to fulfil this task.
As part of this, the results of an annihilating dark matter search with Fermi-LAT in
nearby galaxy clusters are presented. Moreover, studies into the potential of the
upcoming THESEUS and eXTP missions in the detection of well-motivated decaying
dark matter models are detailed.
Gamma-ray Emitting Binary (GREB) systems are a category of high-mass binaries
with unique and variable non-thermal emission, even between objects within the
class. They are categorised as systems containing a massive young star and a compact
object, whose energy spectra peak at above 1 MeV (but typically at E ≳ 100 MeV). These spectra, however, often extend to extremely high energies (E ≳ 10 TeV).
The physical mechanisms for the production of this emission are not well established,
making GREBs exciting and novel objects for study. The second part of this thesis
focuses on the study of GREBs and, specifically, on the system of PSR B1259-63.
This system is comprised of a pulsar in a highly eccentric 3.4 year orbit around
the O9.5Ve star LS 2883 and is known to emit a range of non-thermal radiation
from radio to Very-high-energy (VHE) gamma rays. The bulk of this non-thermal
emission occurs around the periastron and is thought to be connected to the orientation
of the star’s decretion disc of gas and dust to the orbital plane of the pulsar.
This orientation means the pulsar crosses the disc ∼ 16 days before and after the
periastron. With the exception of the GeV energy band, in which intra-periastron
variability has been observed, the emission from the source across the wavelengths
follows a similar form between periastron passages. However, the 2021 periastron
saw a plethora of previously unseen and interesting behaviours in the system across
different wavelengths. This thesis will detail the work undertaken to analyse and
the results of the VHE data taken on the system, by the H.E.S.S. array during the
2021 periastron.
Dark Matter