High Energy Astrophysics

• Alina Donea (Research Staff)
• Duncan Galloway (Future Fellow)
• Zdenka Misanovic (Postdoctoral researcher)
• Jasmina Lazendic-Galloway (Margaret Clayton Fellow)

[ High Energy Group Publications ]

Neutron Star Binaries

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; and measurements of the mass and radius so as to constrain the interior composition.

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

Supernova remnants

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

Nuclear activity in active galaxies

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).