High Energy Astrophysics

The high-energy astrophysics group at Monash University carries out observational and theoretical research into the most violent and energetic processes and objects in our universe. These include supernova remnants and neutron stars; pulsars and their associated wind nebulae; cosmic rays and supermassive black holes. We make use of international high-energy observatories including NASA's Chandra, ESA's XMM-Newton and INTEGRAL satellites, and also the H.E.S.S. II TeV telescope located in Namibia. We are also active members of the LIGO Scientific Collaboration, and carry out research on quantum noise sources and searches for gravitational waves from accreting neutron stars. The group consists of staff members, postdoctoral researchers and students, from the Schools of Physics and Astronomy and Mathematical Sciences.

We welcome intelligent and highly motivated students to study astrophysics for their PhD or as a component of their honours year. Details of project opportunities can be obtained from individuals listed below. More details of the course structure and the many benefits of an honours or PhD degree can be found on the School of Physics and Astronomy home pages.

The high-energy group meets at 2pm on Thursdays, in room 139, building 19 (School of Physics & Astronomy)

Personnel

  • Alexander Heger (Professor, School of Physics & Astronomy)
  • Alina Donea (Senior Lecturer, School of Mathematical Sciences)
  • Duncan Galloway (Senior Lecturer, School of Physics & Astronomy)
  • Jasmina Lazendic-Galloway (Lecturer, School of Physics & Astronomy)
  • Yuri Levin (ARC Future Fellow and Senior Lecturer, School of Physics & Astronomy)
  • Bernhard Müller (Lecturer, School of Physics & Astronomy)
  • Eric Thrane (Lecturer, School of Physics & Astronomy)
  • Paul Lasky (Postdoctoral fellow, School of Physics & Astronomy)
  • Evert Rol (Postdoctoral fellow, School of Physics & Astronomy)
  • Daniel Reardon (PhD student)
  • Chen Hou (PhD student, U. Minnesota)
  • Lawrence Hanson

[ High Energy Group Publications ]

Future events

Past events


Thermonuclear burning on neutron stars

Researchers: Heger, Galloway

Thermonuclear (type I) X-ray bursts occur in close binary star systems in which one of the components is a neutron star, the remnant of a supernova explosion from a massive star, and the other star usually is a low-mass companion. When a sufficiently large layer of material has accreted on the surface of the neutron star, a thermonuclear runaway may occur that results in the observable burst in X-rays. At that time the layer is about 10 m thick, residing on top of a 10 km radius neutron star — the height of a house on top of an object the size of a city! The bursts only last seconds to minutes and recur on time scales of hours to days.

Whereas the timescales above seem very close to those of everyday life, these bursts yet are among the most exotic and lest well understood phenomena in astrophysics. The thermonuclear burning during the burst involves tens of thousands of nuclear reactions on thousands of individual isotopes. Many of the key reaction rates are poorly known as they involve highly unstable nuclei and can be measured in terrestrial laboratories only with extreme difficulty. Research at Monash includes numerical models of these burst to predict burst recurrence times, lightcurves, and energies for comparison with our extensive observational datasets. Research priorities include determining ignition conditions for observed burst sources; understanding so-far unexplained burst phenomena, such as double and triple bursts, and constraining the rates of individual thermonuclear reactions from astrophysical observations.


Gravitational Waves

Researchers: Levin, Lasky, Galloway, Thrane

Gravitational waves (GWs) are propagating ripples in the fabric of space-time, and their existence is one of the major predictions of Einstein's theory of General Relativity. They are generated in highly energetic astrophysical events that involve rapid bulk motions of massive bodies, e.g. black holes. Once generated, the GWs propagate throughout the Universe without absorption, and carry detailed information about the general-relativistic astrophysical processes that produced them. To date, they have not been detected directly. At Monash, researchers are involved in two distinct efforts in this field; the first, using Galactic millisecond pulsars as a "timing array" sensitive to very low frequency signals, such as those which might be produced by the motions of super-massive black holes (see e.g. the Parkes Pulsar Timing Array and the European Pulsar Timing Array projects); and the second, interferometric instruments (such as the Laser Interferometric Gravitational Wave Observatory, or LIGO in the US) sensitive to much more rapid motions originating from merging or rapidly rotating neutron stars. Activities involve development and testing of data algorithms for pulsar timing array data analysis; characterisation of noise sources for interferometric detectors; and optical and X-ray observations to improve the sensitivity of future GW searches from neutron stars.

Aerial view of the Hanford LIGO interferometer Aerial view of the 4km Hanford interferometer, one of two comprising LIGO in the US. From 2010, these instruments are being upgraded for operations from 2014 or so expected to result in the first direct detection of gravitational waves.
Image credit: LIGO Laboratory

Neutron Star Binaries

Researchers: Galloway, Hanson

The stellar remnants of supernova explosions consist of matter under extreme conditions of temperature, density and magnetic field. Neutron stars in binary systems appear as bright X-ray sources to space-based observatories, thanks to gas donated from the stellar companion and heated to tens of millions of degrees in the process. These objects exhibit thermonuclear bursts, in which the accreted fuel is ignited and burns in a bright flash of X-rays once every few hours, and a few exhibit pulsations at hundreds of cycles per second, which allows measurement of their extremely rapid spin rates. Research priorities include the detailed burst properties and underlying thermonuclear reactions; searches for, and characterisation of, new transient pulsing systems; measurements of the mass and radius so as to constrain the interior composition; and optical measurements to improve system parameters and hence boost sensitivity of current and future gravitational wave searches.

artist's impression of a spinning neutron star Artist's impression of a spinning neutron star
Image credit: NASA/Dana Barry

Supernova remnants

Researchers: Jasmina Lazendic-Galloway

A supernova explosion marks the endpoint of a massive star evolution, resulting in an expanding shell, the supernova remnant (SNR), consisting of a blast wave accompanied by slower moving stellar ejecta. Thus, SNRs are often observed as whole or partial shells of optical, X-ray and radio emission. The input of energy and nuclear fusion products into the interstellar medium make SNRs, dynamically and chemically, one of the most important objects in galaxies. The research here at Monash involves:

  • the general physical properties of SNRs, using mainly X-ray and radio observations - as telescopes at different wavelengths continue to improve in sensitivity and spatial resolution, we are able to study SNRs in more detail, rather than their spatially averaged properties;
  • a special class of "mixed-morphology" remnants - an intriguing class of SNRs that show different morphology in X-ray and radio band, and, more importantly, different X-ray properties than expected from standard SNR theory;
  • using high-resolution X-ray imaging and spectroscopy for SNR studies - instruments in X-ray band are approaching optical, IR, and UV bands in their ability to measure precise kinematics and add a third dimension to the data;
  • studying particle acceleration in SNRs using non-thermal X-ray emission and TeV Gamma-ray emission in attempt to solve the question about the origin of low-energy cosmic rays;
  • studying the interaction of SNRs with dense molecular clouds using X-ray, millimetre and infrared observations - the shocks driven by SNRs into dense molecular clouds compress, accelerate and heat the gas, exciting higher molecular transitions and activating chemical reactions forbidden in cold molecular clouds;
Radio and X-ray images of supernova remnant MSH 61-11A.
ASCA satellite image

Overlay of radio data (contours) from the MOST telescope and X-ray data (colour image) from the ASCA satellite.

Chandra satellite image

Overlay of the same radio data (contours) and X-ray data (colour image) from the Chandra satellite.


Cosmic Rays

Researchers: Jasmina Lazendic-Galloway

Cosmic rays are the most energetuc particles known, and have been blamed for many things from the extinction of the dinosaurs to climate change, but astronomers are still not sure where they come from.

Cosmic rays are high-energy protons, ions and electrons; ordinary matter which has undergone some extraordinary process to gain huge energies. Since cosmic rays are electrically charged, their trajectories are deflected as they travel through the large-scale Galactic magnetic field. Unlike photons, the origin of cosmic rays cannot be deduced from their arrival direction.

The most energetic cosmic rays (E~1020eV) are believed to come from the active centres of distant galaxies. The more-prevalent lower-energy (E~1015eV) cosmic rays are believed to be particles accelerated in the expanding shockwaves of supernova remnants (SNRs). Testing this hypothesis has turned out to be more challenging than first thought when cosmic rays were discovered about a century ago. The major evidence comes from observation of synchrotron emission in both radio and X-ray bands, as well as gamma-ray emission - but this is only detected in a handful of SNRs. In addition, other sources (such as pulsar wind nebulae, binary stars and star-forming regions) have been shown to emit gamma rays, as have 'dark accelerators' which display no corresponding X-ray or radio emission.

It appears that the field of particle acceleration and cosmic-ray studies has just begun to scratch the surface of possibilities. Much further work is needed in both theory and observations in order to solve the mystery of cosmic rays.

A number of projects in this field are available to Honours and PhD students, involving observations in radio, millimetre, X-ray and TeV bands, as well as some theoretical work in topics such as broadband emission modelling.

Synchrotron X-ray emission from Chandra matches well with radio emission from ATCA

Synchrotron X-ray emission from Chandra (colour image) matches well with radio emission from ATCA (white contours). Image credit: Lazendic et al. 2004, ApJ, 602, 271

SNR G347.3-0.5 from XMM and Chandra

X-ray images of TeV-emitting SNR G347.3-0.5 from XMM (large image) and Chandra (smaller inset). Image credits: NASA/CXC/SAO/P. Sloan et al; ESA/RIKEN/J. Hiraga et al.


Nuclear activity in active galaxies

Researchers: Donea, Hanson

In the real universe, black holes may not necessarily be isolated - they often come in pairs. In such a pair, the two black holes orbit around each other and gradually spiral inward until they eventually merge to form a single black hole. Physicists and astrophysicists have struggled to understand this merging phenomenon since the 1960's, but understanding the dynamical spacetime of two merging black holes and their environment has proved much more difficult than understanding the stationary spacetime of a single isolated black hole. The unification scheme for active galactic nuclei populates their core with a supermassive black hole, an accretion disk feeding the monster, two bipolar relativistic jets, a dusty torus obscuring the black hole and many broad line emitting clouds. We then ask: What is the structure of the inner part of an active galaxy when TWO black holes devour its content? What can we learn from Tera electronvolt gamma ray observations about feeding of the monsters, what are the emitting signatures from these, how do we observe it, and what shall we predict for future telescopes with unprecedented resolution? We collaborate with the German team from Landessternwarte Heidelberg, Germany on the experiment H.E.S.S.II (The High Energy Stereoscopic System), which will allow gamma-ray observations down to 20 GeV in galaxies. Heidelberg is one of the main astrophysics centres in Germany with five institutes involved in most fields of astrophysics and particle-astrophysics.

Simple geometry of a binary black hole model. The direction of the radio jet and observer defines the z-axis. The primary black hole (BH1) is located at z=0. The position of the secondary black hole (BH2) is given by phi_0 and d_0. The star shows the VHE gamma-ray source located at a distance z_0 from the main black hole. The gamma-rays emitted along the jet can interact with soft photons from both accretion disks. A simplified BBH model assumes that the primary accretion disk is much more extended than the second disk (the disruption effects in the BBH system may cause the external disk of BBH2 to be unstable).