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High Energy Group Research

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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. Our astrophysical research targets include supernova remnants and neutron stars; accretion- and rotation-powered pulsars; cosmic rays; and both stellar-mass and supermassive  black  holes.  We perform numerical simulations using multi-dimensional codes on supercomputer facilities, and make use of international high-energy observatories including the XMM-Newton and INTEGRAL satellites (ESA), and NuSTAR and Chandra (NASA).

We are also active members of the LIGO Scientific Collaboration and the OzGRav ARC Centre of Excellence for Gravitational Wave Discovery, and carry out research on searches for gravitational waves and their electromagnetic counterparts (see the Gravitational Waves group page for more details). Members of the group are active in the Gravitational-wave Optical Transient Observer (GOTO) telescope project, which is developing robotic hardware to identify optical counterparts of binary mergers.

The group consists of staff members, postdoctoral  researchers  and  students,  from the School of Physics and Astronomy and the School of 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.

Personnel

  • Alexander Heger (Professor, School of Physics & Astronomy)
  • Alina Donea (Senior Lecturer, School of Mathematical Sciences)
  • Duncan Galloway (Associate Professor, School of Physics & Astronomy)
  • Jasmina Lazendic-Galloway (Lecturer, School of Physics & Astronomy)
  • Eric Thrane (Senior Lecturer, School of Physics & Astronomy)
  • Paul Lasky (ARC Future Fellow and Senior Lecturer, School of Physics & Astronomy)
  • Bernhard Müller (ARC Future Fellow and Senior Lecturer, School of Physics & Astronomy)
  • Yuri Levin (Adjunct professor)
  • Tyrone Woods (Postdoctoral associate, School of Physics & Astronomy)
  • Letizia Sammut (Postdoctoral associate, School of Physics & Astronomy)
  • Evert Rol (Postdoctoral associate, School of Physics & Astronomy)
  • Kendall Ackley (Postdoctoral associate, School of Physics & Astronomy)
  • Daniel Reardon (PhD student)
  • Zac Johnston (PhD student)
  • Conrad Chan (PhD student)
  • James Grimmett (PhD student)
  • Ashley Bransgrove (Masters student)
  • Adelle Goodwin (Honours student)

[ High Energy Group Publications ]

Future events

  • High-energy group meetings, Thursday at 11am, room 110, building 19 (contact Lucy McNeill if you would like to attend or be added to the mailing list)

Past events


Thermonuclear burning on neutron stars

Researchers: Heger, Galloway, Johnston, Goodwin

Thermonuclear (type I) X-ray bursts occur in close binary star systems in which one ofthe components is a neutron star, the remnant of a supernova explosionfrom a massive star, and the other star usually is a low-masscompanion. When a sufficiently large layer of material has accretedon the surface  of  the  neutron  star, a thermonuclear runaway may occurthat results in the observable burst in X-rays. At that time thelayer is about 10 m thick, residing on top of a 10 km radius neutronstar — 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 ofhours 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.


Core-collapse supernovae and the formation of compact objects

Researchers: Mueller, Heger, Chan, Grimmett

At the end of their lives, massive stars build up an iron core that eventually collapses under its own gravity. In most cases, the core of the progenitor forms a neutron star, and the outer shells of the progenitor are expelled in a violent explosion – a core-collapse supernova. In other cases, the star may fail to explode and form a black hole instead. How such a supernova is powered is still an enigma; one promising suggestions is that neutrino from the surface of the young neutron star drive the explosion when they are absorbed in the surrounding layers and heat them. Since the supernova “engine” operates for just a few seconds deep inside the star, we explore it by means of numerical simulations of the fluid flow and radiation transport in the core at supercomputer facilities like the NCI and Pawsey. We are also working on predicting the neutrino and gravitational wave signals from the next galactic supernova, which would shed light on the events deep in the core. Further research topics include the contribution of core-collapse supernovae to the production of heavy elements, the prediction and interpretation of the optical transients associated with these explosions, and the birth properties of neutron stars and black holes.

Shock wave (cyan) and neutrino-heated ejecta (red) in the deep interior of the explosion of an 18 solar mass star. Shock wave (cyan) and neutrino-heated ejecta (red) in the deep interior of the explosion of an 18 solar mass star.

Gravitational Waves

Researchers: Levin, Lasky, Galloway, Thrane, Sammut, Rol, Ackley

Gravitational waves (GWs) are propagating ripples in the fabric of space-time, and 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. Gravitational wave astronomy became a reality in 2016 with the spectacular detection of the merger of a pair of black holes each ~30 times the mass of our sun. 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 detect optical counterparts and improve the sensitivity of future GW searches from neutron stars. See the dedicated Gravitational Waves group page for more details

These plots show the strain in the detector, which indicates that a gravitational wave from colliding black holes was detected by LIGO on January 4th, 2017. These plots show the strain in the Hanford and Livingstone detectors, which indicates that a gravitational wave from colliding black holes was detected by LIGO on January 4th, 2017.
Image credit: LIGO Laboratory

Neutron Star Binaries

Researchers: Galloway, Johnston, Goodwin

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 theirextremely 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 final stage of a massive star evolution, creating an expanding shell called the supernova remnant (SNR), and sometimes leaving behind a neutron star or a black hole. Supernova explosions are among the most energetic events in the Universe, and have been suspected to be the sites where cosmic rays are reated. Cosmic rays are protons, ions and electrons, 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. Thus, their places of origin have to be observed indirectly, via interactions with the ambient medium and production of gamma-ray emission. Supernova remnants often reside in denser regions of the Galaxy, and are sometimes found to interact with the surrounding molecular cloud. Research topics include investigation of particle acceleration sites and studying interactions of SNRs with dense molecular clouds using millimetre, radio, X-ray and gamma-ray observations.

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 synchrotron emission at 20-cm from Australia Telescope Compact Array (white contours), tracing distribution of shock-accelerated particles in the supernova remnant G347.3-0.5. Image credit: Lazendic et al. 2004, ApJ, 602, 271

Nuclear activity in active galaxies

Researchers: Donea, Hanson

In the real universe, black holes often come in pairs, and over time will 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  witha  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 very high-energy 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  H.E.S.S.II (The High Energy Stereoscopic System) experiment, which will allow gamma-ray observations down to 20 GeV in galaxies. 

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 byphi_0 and d_0. The star shows the VHE gamma-ray source located at a distance z_0 from themain black hole. The gamma-rays emitted along the jet can interact with soft photons from bothaccretion disks. A simplified BBH model assumes that the primary accretion disk is much more extendedthan the second disk (the disruption effects in the BBH system may cause the external disk of BBH2 to beunstable).