204 research outputs found
A search for very high energy gamma rays from PSR1706-44 using the Atmospheric Cerenkov Imaging Technique / by Gavin Peter Rowell.
No full textIncludes bibliographical references.ix, 159 p. : ill. ; 30 cm.Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 199
Aesthetics of urban media façades
No full textThis paper sets out to develop and extend current literature on\ud
design practices for ambient media façades. It does this by\ud
bringing together theories of ambient media, computational\ud
aesthetics, and urban aesthetics. This unique theoretical\ud
combination has informed the design of several exemplars\ud
produced by the author, which are discussed as case studies
Studies of the interstellar medium towards dark TeV gamma-ray sources
No full textRecent advances involving ground based observations of the very-high-energy sky
have led to the discovery of a growing number of Galactic tera-electron volt (TeV)
γ-ray sources. Many of these sources have been associated with other high energy
phenomena, such as supernova remnants (SNRs) and pulsar wind nebulae. However,
a number of TeV γ-ray sources are `dark', with no strong counterparts seen at other
wavelengths. Astrophysical TeV γ-rays are produced via two channels; leptonic and hadronic. The
leptonic process involves the upscattering of background photons by high energy electrons
through the inverse-Compton effect. The hadronic process involves the decay of
neutral pions produced by interactions between highly accelerated cosmic-rays (CRs)
and the ambient interstellar medium (ISM). Dark TeV γ-ray sources may result from
CR and ISM interactions, and studying these sources will shed light on the nature of
the parent particle accelerators. Some of the sources selected for study in this thesis
are considered candidate PeVatrons, an emerging class of extreme accelerators which
produce CRs with energies in the peta-electron volt (PeV) range.
A detailed understanding of the ISM towards dark TeV γ-ray sources is vital in
order to investigate possible origin scenarios. In addition to providing contextual
clues, morphological similarities between TeV γ-rays and the ISM can provide strong
evidence of hadronic CR interactions. This may lead to positive associations between
TeV sources and other sources observed at different wavelengths.
The work in this thesis primarily involves the use of radio telescopes to investigate
the distribution of the ISM towards dark TeV γ-ray sources. The data has been used
to constrain the mechanisms behind these objects, and underpins the investigations
into their mysterious origins. Five TeV γ-ray sources were studied as part of the work
in this thesis. HESS J1640 - 465 and the PeVatron candidate HESS J1641 - 463 are a pair of γ-ray
sources each coincident with a SNR bridged by bright Hii regions. The study of the
ISM suggests that the TeV emission from both sources may be hadronic in origin,
and provides evidence to support the PeVatron nature of HESS J1641 - 463.
HESS J1614 - 518 has no obvious counterparts seen in other wavelengths. The investigation
of the ISM reveals an overlapping ring of gas potentially associated with an
undiscovered SNR or the stellar cluster Pismis 22, which may power the TeV emission.
The nearby TeV source HESS J1616 - 508 has several potential counterparts,
including two SNRs and three pulsars, though there was no evidence in the study of
the ISM to support any association. Spatially matching gas, however, is found coincident
with the TeV source, suggesting a possible link with an undiscovered accelerator.
HESS J1702 - 420 is both a dark TeV γ-ray source and a PeVatron candidate. Though
a SNR and pulsar are seen near the outskirts of this source, no evidence in the study of
the ISM is found to support any association. Morphologically favourable gas overlapping
the TeV source, however, suggests possible scenarios involving nearby acclerators,
such as stellar winds from massive stars or an undetected SNR.Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Physical Sciences, 201
A study of the interstellar medium towards the VHE gamma-ray sources HESS J1614-518 and HESS J1616-508
No full textOne of the most intriguing problems in galactic astronomy is the observation of the highest energy photons. Very high energy (VHE) gamma-ray telescopes such as HESS have located sources of TeV (10¹² eV) gamma-rays which are not associated with any known objects. Whilst these could be a new type of particle accelerator, it is more likely that they are related to supernova remnants (SNRs), pulsar wind nebula (PWN) or massive stellar regions. They may result from high energy cosmic-ray (CR) interactions with interstellar gas (ISM). This project used new radio data which provided information on molecular clouds to model the production of gamma-rays from CR interactions. The densities of protons in these clouds were used in models to determine if potential particle accelerators surrounding the two HESS sources, HESS J1616-508 and HESS J1614-518 were capable of producing the emission. The potential accelerators surrounding HESS J1616-508 were all found to have insufficient gas within their diffusion radius. Thus, it was not possible for those sources to produce gamma-rays through hadronic interactions despite only requiring modest CR energy budgets compared to that provided from a SNR. The same result was also found for WR 73-1 and PSR J1613-5211 near HESS J1614-518. However, Pismis 22 and WR 74 contained CO RoI CO1, CO2 and CO25 within their diffusion radii. The energy in CRs required for each region to generate the overlapping gamma-ray emission was compared to the available energy if a SNR was assumed to be the accelerator. The required energy was found to be less than the energy available. Thus, WR 74 and Pismis 22 could still generate the hadronic gamma-ray emission from HESS J1614-518.Thesis (M.Phil.) -- University of Adelaide, School of Physical Sciences, 2016
Climate change and equity
No full textThe Gavin Mooney Memorial Essay Competition honours the work and memory of the late Professor Gavin Mooney, a health economist who was a tireless advocate for social justice in local, national and international arenas.
Launched in 2013, the competition seeks to draw public attention to social justice and health equity concerns, and to recognise the public-interest value of writing and writers.
The inaugural competition called for essays on the theme of climate change and equity, in recognition of the work of Professor Mooney’s late partner Dr Delys Weston. Each year the competition will call for entries related to a theme around equity and social justice.
Sydney GP Dr Tim Senior took out the inaugural 2013 prize with his essay “Climate Change and Equity: Whose Language Is It Anyway?”. The judges said the winning entry challenges the language of climate change activism, and also incorporates the voices of those who are most likely to be affected by climate change.
The four runner-up entries are by Steve Campbell and Lucie Rychetnik, Oscar McLaren, Peter Boyer and Dora Marinova, and Fergus Green
High-Energy Cosmic-Ray Propagation in the Milky Way and the Associated Diffuse Gamma-Ray Emission
No full textCosmic rays (CRs) are electrons, protons, and other nuclei that have been accelerated to energies ranging from 1MeV to 100 EeV (106 to 1020 eV). As CRs travel through the interstellar medium (ISM) they are deflected by the magnetic fields that permeate interstellar space, propagating for up to millions of years and across distances as large as thousands of parsecs. While propagating, CRs collide with gas particles and interact with the infrared radiation from stars and the cosmic microwave background (CMB). These interactions create gamma-ray photons that can be observed at Earth. Observations of gamma rays show a large-scale background component of emission along the plane of the Milky Way. Even after a century since CRs were discovered there is no widespread consensus on the locations where Galactic CRs are accelerated, their composition, or which processes create most observed gamma rays. Simulations of Galactic CR transport were performed with the software Galprop, with the resulting gamma-ray flux calculated up to the PeV regime. The impact of altering parameters such as the number and distribution of CR sources, the distribution of infrared radiation between stars, and the distribution and strength of the Galactic magnetic field (GMF), were investigated. For the first time the modelling variation in the TeV predictions due to uncertainties in the Galactic distributions was quantified. Additionally, the modelling variation from considering a stochastic placement of the CR sources was quantified up to 1PeV. The simulation results were compared to the most detailed Galactic TeV gamma-ray survey: the H.E.S.S. Galactic plane survey (HGPS). The Galprop predictions were broadly compatible with the large-scale emission from the HGPS after accounting for both the catalogued sources and estimates of the unresolved source fraction. At 1TeV the gamma-ray emission from CR electrons was found to contribute 50% to the large-scale emission, with this leptonic component increasing further as energy increased. The GMF was found to be an important modelling consideration above 1TeV as it impacted the large-scale emission by approximately a factor of two. Additionally, the CR electron flux at Earth above 1TeV was found to vary by over a factor of ten over a period of a few million years due to the short cooling times and diffusion distances of the TeV CR electrons. The Galprop models were found to agree with observations of the diffuse gamma rays in the TeV regime by H.E.S.S., and the PeV regime by LHAASO, extending the demonstrated accuracy of Galprop into the TeV–PeV regime. The results in this thesis will aid in discovering the origin of the diffuse gamma-ray emission and allows constraints to be placed on how the CR sources are distributed in the MW. The results will also inform the next generation of experiments, such as the Cherenkov telescope array (CTA), on possible observation strategies and background considerations. It was also found that the proposed CTA Galactic plane survey will be sensitive enough to observe the large-scale diffuse gamma-ray emission in the TeV regime.Thesis (Ph.D.) -- University of Adelaide, School of Physics, Chemistry and Earth Sciences, 202
Quantifying Australian atmospheric properties for a gamma-ray observatory to operate at energies greater than 10 TeV.
No full textNewly proposed ≥ 10 TeV (1 TeV = 10¹² eV) imaging atmospheric Cherenkov technique (IACT) γ-ray experiments will need optimal sites for their construction and operation at energies above 10 TeV. Quantifying the optical properties of Australian atmospheres will be part of a site survey to determine the viability of possible IACT Australian sites. Atmospheric optical properties need to be quantified as a TeV IACT γ-ray array detects Cherenkov radiation that has propagated through the atmosphere within the visible spectrum. For the optimum collection of Cherenkov radiation, sites with the highest optical atmospheric transmission are ideal. Another optical property of concern is the night sky background (NSB) level. NSB photons are a form of background noise that can cause accidental triggering, therefore sites with the lowest levels of NSB are desired. Further information on IACT and the NSB is presented in Chapter 2. The optical properties of the Australian atmosphere which I quantified for the site surveys were the optical transmission and the NSB. The optical transmission was measured through the star extinction coefficient. The star extinction coefficient and the NSB were found through the method of photometry using a Pentax K10D digital single lens reflex (DSLR) camera. Because a Pentax K10D DSLR camera has not previously been used for this purpose, I initially conducted viability tests on its ability to measure the NSB and star extinction coefficients. I will present these results in Chapter 5. Data collected from multiple trips to Fowlers Gap in New South Wales, Australia, were used for this purpose. I will present the results of star extinction coefficient and NSB level measurements in Chapter 6. To further help understand the effects of atmospheric conditions, I have investigated the effects of differing levels of aerosols and the effects of cirrus clouds on the performance of a simulated ≥ 10 TeV IACT γ-ray array. I give more information about simulated IACT γ-ray array's in Chapter 3 and present the results in Chapter 4. The effects of cirrus clouds were investigated as these clouds occur at a similar height to the maximum emission of Cherenkov photons for γ-ray shower energies around 10 TeV.Thesis (M.Sc.) -- University of Adelaide, School of Chemistry and Physics, 201
Cloudy ammonia: a multi-wavelength molecular line survey of the molecular clouds surrounding the W28 supernova remnant.
No full textThis thesis is a multi-wavelength study of high energy objects interacting with the interstellar medium (ISM). One such object is the W28 supernova remnant (SNR) with several sites of TeV (10¹ ² eV) γ-ray emission detected from its boundary by the H.E.S.S. telescopes. The γ-ray emission is spatially well matched to giant molecular clouds which surround the remnant. Inorder to understand the distribution of dense (n > 10³ cm⁻³) molecular gas in the region, multi-wavelength (λ ∼12mm and 7mm) molecular line surveys are presented. Utilising the NH₃, CS and SiO molecules, the broad-scale distribution of the dense and disrupted gas in the W28 region is revealed. Additional multi-wavelength data towards W28, including radio (λ ∼ cm), molecular line (λ ∼ mm), infra-red (λ ∼ 10⁻⁶m), X-ray (λ ∼ 10⁻¹⁰m) and γ-ray (λ ∼ 10⁻¹ ²m) data are compared and discussed in context with the results of this work. Other high energy ISM interacting objects, microquasars, are also studied. TeV γ-ray observations towards three candidate microquasars with jets and/or jet/ISM interactions are presented. In all three cases, no significant point-like or extended TeV γ-ray emission is seen and upper-limits are set.Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 201
The cold, the dense and the energetic : cosmic ray bombardment of molecular cores near supernova remnants.
No full textOne of the oldest unsolved mysteries in astrophysics is the origin of cosmic rays, particles that travel at speeds close to the speed of light. A plausible theory to explain the acceleration of these particles is shock-acceleration in the expanding shells of supernova remnants (SNRs) within our galaxy. In this thesis, the interstellar medium towards supernova remnants that display indicators of particle acceleration, ie., gamma-ray emission, are investigated. More specifically, results from mm-wavelength molecular gas surveys towards two gamma-ray emitting SNRs, RXJ1713.7−3946 and CTB37A are presented. Chapter 1 summarises astrophysics at high energies, including what cosmic rays are, how they may be accelerated, their connection to gamma-ray emission and how gamma-ray astronomy is performed from the ground. On the opposite (low-energy) side of the energy spectrum, Chapter 2 describes some of the theory of single dish radio astronomy, which allows us to probe molecular environments. By tuning the receiver to home-in on particular molecular species, different interstellar environoments can be targeted. Some specific molecular species are outlined in Chapter 3, before utilising these species in following chapters. The bulk of chapters 4 and 5 are composed of published articles presenting interstellar gas observations and investigation. Chapter 4 is an in-depth analysis of the molecular environment towards the supernova remnant RXJ1713.7−3946 (in 3 articles) using several independent molecular gas tracers, including transitions of the CS, NH₃ and N₂H⁺ molecules. In addition to various specific mm-phenomena, the presence of dense gas was confirmed via our observations. The issue of cosmic ray transport into dense star-forming cores was then addressed. Due to enhanced magnetic turbulence, cosmic ray propagation may be slower than the galactic average, so predictions for several slow-diffusion scenarios are made. Through modeling, scenarios where low energy cosmic rays are excluded from the centres of molecular cores were identified. Such cases may result in a lower proportion of low energy gamma-rays coming from core centres relative to higher energy gamma-rays (ie. a hardening of the gamma-ray spectrum). Chapter 5 is an overview of the molecular gas towards the entire gamma-ray emission region of the supernova remnant CTB37A (in 1 article), allowing the estimation of the mass of cosmicray target material, found to be ∼10⁴M⊙. In a hadronic scenario for gamma-ray emission, this corresponds to a cosmic ray density of ∼80-1100 times that seen Earth. This may have implications for the supernova remnant energetics, distance and age, which are discussed. Finally, in Chapter 6, an investigation of the subtleties of cosmic ray diffusion near supernova remnants is carried out, and techniques to simulate effects that may result from diffusion into molecular gas are outlined. Hard conclusions concerning the spectrum of gamma-rays resulting from molecular cores are left for future work.Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 201
Design studies for a multi-TeV γ-ray telescope array: PeX (PeV eXplorer).
No full textThis thesis presents work towards the design of a new array of Image Atmospheric Cherenkov Telescopes (IACTs) to detect multi-TeV (E > 10¹ ² eV) γ-ray sources. The array consists of 5 telescopes in a square layout with one central telescope, known as the Pevatron eXplorer or PeX. PeX is a PeV (10¹ ⁵ eV) cosmic ray explorer that aims to study and discover γ-ray sources in the 1 to 500 TeV range. The initial PeX design has been influenced by the HEGRA CT-System and H.E.S.S. configurations. One important feature of multi-TeV air showers is their ability to trigger telescopes at large core distance (> 400 m). PeX will utilise large core distance events to improve the performance and illustrate the viability of a sparse array for multi-TeV γ-ray astronomy. In Chapter 1, I will discuss the astrophysical motivation behind multi-TeV observations. A number of γ-ray sources have shown emission that extends above 10 TeV, for example unidentified source HESS J1908-063. A new multi-TeV detector can provide a new look at the Galactic plane and work towards uncovering the origin of Galactic cosmic ray acceleration. In Chapter 2, I will look at the physics of air showers, which involves the interaction of protons and γ-rays with the atmosphere to form a cascade of particles. I will discuss the lateral distribution for γ-rays and show the importance of large core distance shower for multi-TeV events. Gamma-ray showers with an image size > 60pe can be detected up to 700 m away from PeX for 500 TeV showers. In Chapter 3, I introduce PeX in detail along with the simulation programs used to model it. I discuss the standard shower reconstruction algorithm (Algorithm 1) and an advanced shower reconstruction algorithm (Algorithm 3). I also introduce the image parameters that I will investigate while optimising PeX, which include; site altitude, image triggering conditions, image cleaning conditions, telescope separation and image size cut. In Chapter 4, I have optimised the PeX cell for a low altitude (0.22 km) observational site using Algorithm 1. Parameters such as telescope separation, triggering combination, cleaning combination and image size cut have been varied over a range of values to provide the optimum results for PeX. In Chapter 5, I have optimised the PeX cell for a higher altitude (1.8 km) observational site using Algorithm 1. The same parameter variations considered in Chapter 4 have been used in Chapter 5. It appears that scaling the H.E.S.S. values to appropriate values for PeX provides the near optimum results. A comparison between the site altitudes suggests that a 0.22 km altitude provides the slightly better performance for energy > 10 TeV. In Chapter 6, a new time cleaning cut has been investigated. The arrival time between photons in two adjacent pixels in the camera is used to apply an extra cut which helps mitigate night sky background. To illustrate the robustness of the time cleaning cut, various level of night sky background have been considered. These levels include: off-Galactic plane, on-Galactic plane and towards the Galactic centre. The most important result is that PeX performance with a time cleaning cut improves results when a high level of night sky background is present. For a Galactic centre level of night sky background there is a factor of 1.5 improvement in angular resolution, effective area and quality factor when a time cleaning cut is applied compared to using no time cleaning cut. In Chapter 7, Algorithm 3 has been considered. A smaller sample of parameter variations has been simulated to confirm that the same trends found in Chapters 4 and 5 appear for Algorithm 3. The site altitude and time cleaning cut have also been considered. Algorithm 3 provides a direction reconstruction improvement over Algorithm 1 especially for large core distance events which are important for PeX. In Chapter 8, I consider some possible enhancements to PeX. These enhancements include: varying pixel size and pixel arrangement in the camera, further cuts to rejection proton events and possible separation between proton and γ-ray pulses. Chapter 8 also provides the flux sensitivity results for multiple PeX configurations. The final configuration and flux sensitivity for PeX is presented in this Chapter. This work shows the value of a sparse array of Cherenkov telescopes to open up the > 10 TeV energy regime.Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 201
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