Potential research projects

Investigating the black hole at Galactic Centre (Masters/PhD project)

The link between γ-ray and radio emission in AGN (Masters/PhD project)

The physics of bright transient emission (PhD project)

Imaging Extremely Bright Emission from Relativistic Jets using the Interstellar Telescope (PhD project)

The brightness temperature of synchrotron emission is theoretically limited by inverse Compton scattering to 10¹² K. This limit exists because photons emitted by a source are inverse Compton scattered by the same ultra-relativistic electrons that radiate the photons themselves as they escape the emission region. The electrons impart energy to the photons in the process. If the photon energy density is high enough, the energy losses of the electrons become so high that they impart all their energy to the outgoing photons. At this point the source should radiate away all its energy in a large rapid flash, the inverse Compton catastrophe, and cool back down again.

But there is a problem. The emission from some extragalactic radio sources exceeds this limit by up to two or three orders of magnitude. These sources are known as intra-day variable (IDV) AGN, so-called because the intensity in these sources is observed to vary on timescales of 20 minutes to 12 hours, depending on the source. The variability is not caused by the sources themselves. Rather it is due to twinkling, or scintillation, caused by turbulent electron clouds in our own Galaxy.

To study interstellar scintillation is to study the most compact, bright objects in the universe, since only the most compact radio sources can exhibit interstellar scintillation. IDV sources must have angular diameters at most tens of microarcseconds, about 100 times smaller than can be resolved using the best VLBI imaging! However, by studying the statistics of the scintillations themselves, we can "image" these sources using interstellar scintillation itself! It is as if we are looking through these sources through a piece of frosted glass, but that the glass allows us to discern details of these sources that cannot be obtained even with the largest telescopes on Earth. In effect, the interstellar medium is itself acting like a telescope, with a diameter of 108 m. Scintillation imaging is extremely powerful, because it allows us to probe the anatomy of energetic radio sources with a spatial resolution of 0.1 to 0.01 pc. We believe that we are imaging the very regions where the black holes in these radio galaxies are launching a highly relativistic jet.

The z=0.54 quasar J1819+3845 exhibits the extreme variations of all known intra-day variable sources. The source can double change its flux density in the time it takes you to read this notice! Bi-monthly observations of this quasar with the Westerbork radio telescope revealed that the source is not only variable due to interstellar scintillation, but that the anatomy of the source evolves on a timescale of months. We are using the interstellar telescope to make a "movie" to chart the evolution of the microarcsecond structure of this source and help determine how such sources can possibly radiate at such high brightness temperatures.

At the same time, we are also unravelling the enigmatic nature of the interstellar turbulence that is causing the rapid scintillation in this source. Our observations show that the scintillation is caused by a highly turbulent layer of ionised plasma only 15 pc away from Earth. The turbulence is not obviously associated with any nearby astronomical objects. So how is such turbulence generated, and what is driving it? The answers to these questions stand to reveal a great deal about the nature of ill-understood magnetohydrodynamic turbulence in general.

Resolving the Dynamics and Structure of the Interstellar Medium (PhD)

Over the last two decades, the discoveries of extreme scattering events, intermittency in the variability of intra–day variable quasars, and pulsar "fringing" events have revealed that our understanding of the structure of the interstellar – and its associated dynamics – is woefully incomplete.

The purpose of this project is to develop a number of new techniques to probe the ISM on the scales that are most relevant to its dynamics and bring them to bear on the problem. You will employ the technique of interstellar speckle imaging to make snapshots of the actual turbulence responsible for pulsar scintillations, thus resolving how the turbulence evolves with time. You will help develop and employ holographic techniques that can potentially recover the entire field of phase distortions imposed by the ISM.

You will acquire the specialized radio telescope data required for these observations, interpret them and explore their implications for the understanding of interstellar turbulence.

In addition, you will assimilate pulsar dispersion and scattering measurements obtained using existing instruments and use these to construct a map of the turbulent interstellar medium throughout our Galaxy.

Extracting Emeralds from the Ephemeral Universe in the SKA Era (PhD)

Short–timescale radio transients are associated with the most energetic and brightest single events in the Universe. They open new vistas on the physics of extreme states of matter and strong gravitational fields. With a sensitivity exceeding two orders of magnitude over existing instruments and an extremely wide field of view, the Square Kilometre Array (SKA) will be the ultimate radio transients detection machine, and is expected to discover entirely unknown classes of transients.

Unfortunately, the detection of transients is a difficult business. A brute–force search for elusive transient events at the full sensitivity of the SKA will require sifting through exabytes of data each second, a task well beyond the processing capabilities of even the fastest supercomputer on Earth.

The aim of this project is to revolutionise the way the SKA will operate as a transients machine. You will unleash the mathematics of decision theory to construct optimal detection algorithms that resolve whether a transient signal is present in the data and zero in on its characteristics without having to search the entire data stream. You will also investigate the various ways of combining and processing the signals from the SKA's 5000 antennas to determine the hardware configuration that maximises the SKA's sensitivity to transient events.