High-Energy Astrophysics Group

What are cosmic rays?

Cosmic rays are protons and other atomic nuclei possessing enormous energies. The rarest have energies almost 100 million times greater than protons accelerated by the Large Hadron Collider (LHC), and represent the most energetic particles known in the Universe.

Astrophysicists have been striving to understand how and where Nature accelerates these particles to such extreme energies, and how they fit with the broader astrophysical picture.

The giant Pierre Auger Observatory , covering 3000 square kilometres in western Argentina, has made great strides towards solving these long-standing and important astrophysical problems.

How do we detect cosmic rays?

Extremely energetic cosmic rays initiate enormous cascades of secondary particles when they strike the Earth's atmosphere. Such an air shower can cover tens of square kilometres at ground level, and during its passage through the atmosphere the shower induces a flash of bluish fluorescence light.

The Pierre Auger Observatory observes the showers in two complementary ways to extract information on the nature of the incoming cosmic rays, including their energy, arrival direction and mass.

An array of 1660 particle detectors spread over 3000 square kilometres measures the shower particles as they strike Earth, while a series of 27 large optical telescopes measures the flashes of light from atmospheric nitrogen excited by the shower.

Since full operations began in 2008, the Pierre Auger Observatory has exploited the sensitivity and size of the Observatory to publish many studies on the characteristics of the high-energy cosmic ray flux, including a high-statistics measurement of the suppression of the flux at the highest energies, strong limits on the photon and neutrino content, intriguing indications of large- and small-scale clustering of arrival directions, and an interpretation of air shower measurements indicating an increase in the average mass of the cosmic ray particles at the highest energies.

At the same time, high-energy particle interactions have been studied at energies inaccessible to man-made particle accelerators. A major upgrade of the Observatory commenced in 2016 to enhance the sensitivity of the surface array detectors to the mass of the cosmic ray particles.

What we do at HEAG

The University of Adelaide hosts one of the largest groups in the Auger collaboration. Adelaide was a founding member of the collaboration, and hosted its first design workshop in 1993. Members of the Adelaide group now hold key positions in the collaboration, including leadership roles in the mass composition and analysis foundations tasks, and in the Upgrade Task Force.

Staff and students undertake tasks including development of hardware and software to assist with data analysis and infra-red monitoring of clouds, as well as taking part in observing runs and interpretation of results.

Historically, the group operated a cosmic-ray detection array at the department's Buckland Park field station, for several decades from the 1970's onwards.

Discover how the detector works

Key contact

• Professor Bruce Dawson

What are gamma rays?

Gamma-ray astronomy uses various techniques to observe, either directly or indirectly, photons that have energies above approximately 1.2 x 10⁵ eV (about 2 x 10^-14 Joule). This corresponds to a wavelength of around 10^-11m and a frequency around 3 x 10¹⁹ Hz.

Compare this with green visible light, which has a wavelength around 500 x 10^-9m, a frequency of 6 x 10¹⁴ Hz, and energy around 2.5 eV or 4 x 10^-19 J. That is, a gamma-ray photon carries at least 50,000 more energy than the light you see with your eyes...

Thus, gamma-rays are much more energetic than visible-light photons, and the physical processes that are required to produce such high-energy photons are themselves correspondingly more energetic. This provides constraints on the type of processes that are capable of producing gamma-rays, and so detecting and studying gamma-rays from sources gives us another window onto the nature of these sources.

How do we detect them?

Gamma-rays can be observed via ground-based and space-based detectors.

Satellite-borne detectors can detect gamma-rays directly; for example, the BATSE satellite incorporated counters which measured directly the interaction between gamma-rays and the material in the detectors. Thus, the gamma-ray itself was measured by those detectors.

Ground-based detectors use indirect methods, such as the atmospheric Cerenkov air shower method. Here, when the gamma-ray enters Earth's atmosphere, it is travelling faster than the speed of light in air, and emits Cerenkov radiation.

In essence, this consists of visible or near-visible photons, and we look for these Cerenkov photons. Thus, we do not observe the gamma-ray directly, but the results of its interaction with the Earth's atmosphere.

What we do at HEAG

Staff and students in HEAG utilise both space-based and ground-based techniques. We have a major collaboration with the HESS gamma-ray telescope system in Namibia.

We are also involved with planning and development of the Cerenkov Telescope Array.

Our research is directed at determining the observational characteristics of the sources of the gamma-rays, and elucidating the physical process that produce such high-energy photons.

In particular for galactic gamma-ray sources within our own Milky Way galaxy, those gamma-rays can arise from the interaction between cosmic ray particles and the various components of our galaxy's interstellar medium (such as gas clouds), so a major effort is the examination of that interaction.

Thus, we also undertake research at other wavelengths, particularly in the radio and sub-millimetre regimes, and also examining the nature of the galactic magnetic field.

Key contact

• Professor Gavin Rowell

What are neutrinos?

A neutrino is a subatomic particle with no electrical charge and almost (but not quite) no mass. Its properties make it difficult to detect, but it is a very important particle, at least in part because there are so many of them.

For example, neutrinos can carry away vast amounts of energy from a supernova explosion. Thus, a full understanding of the mechanics of supernovae can come about only with a good knowledge of the processes that take place during these events, and experimental conformation of expected results - such as a neutrino burst preceding detection in optical or other wavelengths, as was seen with supernova SN1987A.

Neutrinos are produced in a range of astrophysical environments, so an experimental understanding of them, in spite of the great difficulty in their detection, provides us with another important insight into astrophysical and nuclear processes.

How do we detect neutrinos?

With difficulty.

The properties of neutrinos greatly limit their interaction with ordinary matter, so we must look at detection strategies which enable us to observe their very rare interactions with matter.

Whilst the initial detection of neutrinos was undertaken with laboratory-scale equipment, experiments to detect astrophysical neutrinos are much larger. Some involve large volumes of water, and they usually are located underground in order to shield the system from other naturally-occurring particles; the neutrino flux essentially is not affected by passing through substantial amounts of rock, but the background from other particles is greatly reduced.

Also, we do not look to detect the neutrino itself; rather, we are looking for some signature of the interaction of a neutrino with matter, and use the detection of such interaction products as an indication that a neutrino was present.

In the IceCube experiment, light-sensitive photo-multiplier tubes (PMTs) are buried deep within Antarctic ice. When a neutrino interacts with a neutron or a proton within H or O atoms in the ice, that interaction can release secondary particles which are travelling faster than the speed of light in ice (similar in principle to the air Cerenkov technique used for gamma-ray telescopes); as a result, Cerenkov light is emitted. The PMTs detect these flashes of Cerenkov light.

Over 5,000 PMT-based detectors are buried in the ice at depths between 1,450-to-2,450 metres below the surface. Analysis of event data from the PMT array can lead to information about the direction of arrival of the neutrino, and also help to determine whether the neutrino was from an astrophysical source or was produced in the earth's atmosphere by other processes.

What we do at HEAG

HEAG members are involved in analysis and interpretation of data from IceCube, particularly with regard to the question of whether or not the distribution of neutrino sources truly is isotropic (i.e., uniform across the sky), and if we are able to correlate neutrino sources with other objects, e.g. gamma-ray sources.

Also, staff have contributed to the installation of the detector array, and are involved in the planning and administration of the collaboration.

How does the ICECUBE work?

Key contact

• Associate Professor Gary Hill

What is radio astronomy?

Radio waves cover the broad range of frequencies from around 3 kilohertz to about 300 gigahertz. Radio astronomy uses various portions of this frequency range. The types of objects that can be explored in this regime range from planets and their immediate environments (for example, Jupiter is a source of radio emission), ordinary stars such as our Sun, peculiar stars such as pulsars, extreme locations such as the areas around black holes, interstellar gas in galaxies, and through to intergalactic space.

The radio spectrum was perhaps the second main spectral domain available to astronomers, after optical methods. Astronomical sources of radio waves were first found in the 1930's, and in the following years, many types of radio emission were discovered- solar, planetary, stellar, galactic.

With the development of instrumentation and techniques in the radio regime, the era of multi-wavelength astronomy began. It was realised that a much clearer understanding of the nature of astronomical sources could be found when our data cover a range of different wavelengths, since we then explore different aspects of these sources.

How do we observe radio sources?

Radio telescopes can produce maps of radio emission on the sky, and if these maps are produced with sufficient resolution, we essentially obtain images of sources at radio wavelengths. Further, if data from widely separated telescopes are combined (known as interferometry), the resolution obtained is equivalent to that of a telescope with a diameter equal to the separation of those telescopes- and this could be thousands of kilometres. This idea can be extended to combining telescopes on Earth with some in space, or space-borne arrays for very large separations.

This method allows us to produce the highest resolution obtainable in images at any wavelength. Also, radio observations can be done with very high time resolution- for example, observations of pulsars with rotational periods measured in milliseconds.

Since people were using radio for communications since the 1890's, the basic technology for detecting (and generating) radio waves had been known for some decades prior to the discovery of astronomical sources of radio waves.

Radio telescopes vary in form from simple antennae (possibly even just fixed wires), through to larger fixed arrays -such as the early Mills Cross in the early days of Australian radiophysics - to the classical steerable radio dish, exemplified by the Parkes radio telescope.

Recent developments include the Square Kilometre Array (SKA) and its predecessor instruments, which utilise arrays of fixed antennae, with sophisticated data-processing and analysis routines to allow simultaneous multi-directional and multi-frequency observations.

What we do in HEAG

Staff and students utilise radio data at millimetre and sub-millimetre wavelengths to complement and support observations in gamma-rays.

Within our galaxy, gamma-ray photons can arise from the interaction of cosmic ray particles with the interstellar medium (ISM) in our galaxy, and thus these radio observations help us to determine more clearly the production of these gamma-rays by mapping and

probing the structure of the ISM.

Also, these radio observations are undertaken in part to look for spatial co-incidence between the radio and the gamma-ray sources, since a considerable fraction of our galactic gamma-ray sources are of an unidentified nature. Multi-wavelength observations can help to elucidate the nature of these unidentified sources.

Most observations are undertaken with the Mopra and NANTEN2 facilities.

Key contact

• Professor Gavin Rowell

Key collaborations

• Australia Telescope National Facility - Mopra telescope
• NANTEN2 - NANTEN Submillimeter Observatory, Chile

Gravitational-wave astronomy is a revolutionary new area of astronomy and astrophysics. These waves provide information about the evolution of the universe which was previously not available through electromagnetic radiation or particle-based observations. Gravitational waves provide a new way of sensing the universe and observing its early history.

Our group was an active contributor to the first successful detection of gravitational waves in 2015. This detection observed stellar-mass black-hole binaries for the first time and provided the most extreme tests of Einstein’s Theory of General Relativity. In 2017 the collision of two neutron stars was observed for the first time, and the resulting waveforms gave insight into the structure of the most extreme nuclear matter in the universe.

The group is currently developing new ‘adaptive optics’ systems with advanced optical diagnostics. These systems will enable the laser beams used in the LIGO and Virgo detectors to be constantly monitored and adjusted during use, which will significantly increase detection rates and fidelity.

The next step will be a range of next-generation detectors. The group is also exploring technology that will use silicon mirrors cooled to about minus 150ºC. This may allow detectors to routinely observe gravitational waves from coalescing black holes and neutron stars, and search the universe for previously undetectable new sources.

The Adelaide Group is a node of the OzGrav ARC Centre of Excellence.

Key contacts

• Professor Peter Veitch
• Professor David Ottaway

What is optical astronomy?

This branch of astronomy utilises the optical wavelengths, which range from about 300 nanometres (nm, 10^-9 metres) to 1000nm. It is the oldest observing regime in astronomy, since the original detector used for observations was the human eye.

Generally, the eye is sensitive from about 400nm to 700nm or so, but with the addition of detector hardware such as photographic emulsions, photo-multiplier tubes and solid-state detectors such as CCD cameras, the range has been extended to include some of the near ultra-violet and the near infra-red.

This range of wavelengths is one to which the Earth's atmosphere is transparent, so ground-based telescopes can cover the full range of these wavelengths.

How do we make observations?

As the technology available to optical astronomers has advanced over the centuries, we now have three main data-gathering methods:

• Imaging, where we want to see what an object or part of the sky looks like;
• Photometry, where we want to measure the brightness of an object; and
• Spectroscopy, where we study the elemental composition of objects; and other parameters such as their motion relative to the Earth.

An optical telescope is used both to form a detailed image of an astronomical object and also, very importantly, to collect more light (i.e. more photons) from that source. This second point means that we can make more accurate measurements of that light, and thus determine the properties of that source more accurately and precisely.

A detector (most commonly an electronic detector such as a CCD camera) is placed near the telescope focus and may be preceded by a spectrograph (for spectroscopic observations) or a filter (in order to restrict the observations to a particular smaller range of wavelengths).

Whilst the majority of observations are done from the Earth's surface, space-based facilities such as Hubble and Kepler take advantage of being above the Earth's atmosphere, obtaining data that can not be obtained from the ground.

What we do in HEAG

Our optical astronomy research is in the areas of cataclysmic variable star photometry, transiting exoplanet photometry, and, most recently, spectroscopy of eta Carinae, southern Be stars, and occasional targets-of-opportunity in collaboration with other facilities such as the Hubble Space Telescope. Observations are undertaken in the University of Adelaide Optical Observatory, which is on the North Terrace campus.

As this is a small facility operated solely by HEAG, there are no restrictions on availability, and we can dedicate the system to whatever observing programs we wish. These observing programs can be part of international campaigns for observing or monitoring particular sources, such as exoplanet host stars, emission-line stars, or gamma-ray counterparts.

Key contact

• Dr. Padric McGee

Academics 

Research staff

Visiting research fellows 

Current MPhil and PhD students

MPhil (coursework/research) students

2024 Honours students

Past members

• Abreu, P., Aglietta, M., Albury, J. M., Allekotte, I., Almeida Cheminant, K., Almela, A., . . . D'Olivo, J. C. (2023). Cosmological implications of photon-flux upper limits at ultrahigh energies in scenarios of Planckian-interacting massive particles for dark matter. Physical Review D, 107(4), 16 pages. DOI 
• Abdul Halim, A., Abreu, P., Aglietta, M., Allekotte, I., Almeida Cheminant, K., Almela, A., . . . Dos Anjos, R. C. (2023). Constraining the sources of ultra-high-energy cosmic rays across and above the ankle with the spectrum and composition data measured at the Pierre Auger Observatory. Journal of Cosmology and Astroparticle Physics, 2023(5), 024-0-024-49. DOI 
• Abdul Halim, A., Abreu, P., Aglietta, M., Allekotte, I., Allison, P., Almeida Cheminant, K., . . . Zavrtanik, M. (2023). A Catalog of the Highest-energy Cosmic Rays Recorded during Phase I of Operation of the Pierre Auger Observatory. The Astrophysical Journal Supplement Series, 264(2), 50-1-50-24. DOI  
• Abreu, P., Aglietta, M., Albury, J. M., Allekotte, I., Almeida Cheminant, K., Almela, A., . . . Pierre Auger Collaboration. (2023). Limits to Gauge Coupling in the Dark Sector Set by the Non-observation of Instanton-Induced Decay of Super-Heavy Dark Matter in the Pierre Auger Observatory Data. Physical Review Letters, 130(6), 061001-1-061001-9. DOI 
• Abreu, P., Aglietta, M., Allekotte, I., Almeida Cheminant, K., Almela, A., Alvarez-Muñiz, J., . . . Engel, R. (2023). Search for photons above 10¹⁹eV with the surface detector of the Pierre Auger Observatory. Journal of Cosmology and Astroparticle Physics, 2023(5), 021-0-021-26. DOI
• Abdul Halim, A., Abreu, P., Aglietta, M., Allekotte, I., Almeida Cheminant, K., Almela, A., . . . Zavrtanik, M. (2023). Search for Ultra-high-energy Photons from Gravitational Wave Sources with the Pierre Auger Observatory. The Astrophysical Journal, 952(1), 91-1-91-11. DOI 
• Damineli, A., Hillier, D. J., Navarete, F., Moffat, A. F. J., Weigelt, G., Corcoran, M. F., . . . Di Scala, G. (2023). The Long-term Spectral Changes of Eta Carinae: Are they Caused by a Dissipating Occulter as Indicated by cmfgen Models?. Astrophysical Journal, 954(1), 65. DOI

• Nazé, Y., Rauw, G., Bohlsen, T., Heathcote, B., McGee, P., Cacella, P., & Motch, C. (2022). X-ray response to disc evolution in two γ Cas stars. Monthly Notices of the Royal Astronomical Society, 512(2), 1648-1657. DOI 
• Lee, S., Einecke, S., Rowell, G., Balazs, C., Bellido, J. A., Dai, S., . . . White, M. (2022). Performance of a small array of Imaging Air Cherenkov Telescopes sited in Australia. Publications of the Astronomical Society of Australia, 39, 8 pages. DOI