Student Projects

Possible research projects with the optical/x-ray astronomy group for honours and Ph.D. candidates include:

Modelling microlensing light curves to measure exoplanets

The recent discovery of three exoplanets by the microlensing technique (Bond et al, 2004, ApJ 606 L155, Udalski et al, 2005, ApJ 628 L109, Beaulieu et al, 2006 Nature Jan 26th) is likely to lead to many further discoveries and increased research into the method. Already the PLANETmicrolensing collaboration has strong evidence for the existence of a third system harbouring one or more planets and several other promising microlensing events which require further detailed analysis. An important advantage of microlensing is that it is particularly sensitive to solar system like planets whereas the principle alternative techniques (the radial velocity and planetary transit methods) are strongly biassed towards massive, short period planets – the so called “hot Jupiters”. Furthermore the detection of a planet in the OGLE-BLG-2005-071 event has shown that Earth-mass planets can be detected by ground based observatories using 1m class telescopes. Microlensing is the only currently available method which can do this. It is likely that in the next few years microlensing measurements will provide details of the mass and radius distributions of exoplanets from 1 to 10 AU from their parent stars – information essential for our understanding of planetary system formation and evolution.

The technique involves multi-parameter modelling of photometric light curves. The shape of the curves is affected by many factors including unresolved (blended) light from the lens and background stars, the relative size of the source star and the planetary caustic (finite source effect), parallax due to Earth's motion and rotation of the planet or planets about the primary lens star (see eg Gaudi & Gould, astro-ph/9610123, Rattenbury et al, MNRAS 335 (2002), 159). The modelling is very demanding of computer time especially where several planets orbit the lens star. So far most modelling has been done with a bank of dedicated PC's but in future supercomputers may be required.

There are several degeneracies which can seriously affect the analysis. The most significant are the degeneracies in the mass ratio q of the planet to its parent star and the lens-source star proper motion µ due to finite source effects (Gaudi & Gould, 1997). These can lead to an uncertainty of at least a factor of 10 in the mass ratio (q) if the quality and frequency of sampling of the light curve is inadequate. There has been a continuing improvement in modelling since the search for exoplanets microlensing began about 10 years ago but much remains to be done both to better understand and minimise the effects of the degeneracies and to optimise computing methods. The PLANET collaboration is short of persons with modelling skills and has a large database of anomalous events inadequately analysed. Several of these have almost degenerate solutions with binary lens models almost as probable as those with planetary system lenses. These need more detailed analysis allowing for finite source effects in order to break the degeneracy in the planet-to-lens mass ratio q.

You will be required to investigate the various degeneracies in microlensing modelling and develop strategies to minimise their impact on determination of planetary system parameters. You will be able to make use of the University supercomputer operated by the Tasmanian Partnership for Advanced Computing. You will use these methods to analyse known and future ambiguous events.

This project is suitable for a new Ph.D. candidate. We have a scholarship avaliable. The link leads to infomation and an application form. Other UTas scholarship infomation can be found here

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Optimising photometry in the search for exoplanets

The microlensing method is particularly suited to the detection of solar system like planets. The recent detection of a Jupiter-mass planet in the OGLE-2005-BLG-071 event (Udalski et al, astro-ph/0505451) has demonstrated that even Earth mass planets can be detected by microlensing. The success of the method is critically dependent on the quality or the photometry. We have data on at least 3 events which probably harbour planets but which currently are ambiguous because an alternative binary lens model also gives a satisfactory although slightly less probable fit to the light curve. This arises in part because of the inadequacies of the photometry in the crowded galactic bulge fields where exoplanets are being discovered. The PLANET international microlensing collaboration uses an automatic photometry pipeline based on the Dophot 2 profile fitting method. Dophot 2 is particularly suited to use in pipelines but it is clearly not the most precise photometric method available.

In this project you will investigate the relative performance of several other photometric methods (Dophot 3, Daophot and image subtraction) comparing them with Dophot 2. PLANET data for several of the ambiguous events will be used in the tests. If the quality of photometry is significantly improved you will be able to use modelling software developed by PLANET to test for planetary and binary lens solutions. There will be an opportunity to adapt the best method for use in the PLANET pipeline.

This project is suitable for honours research, but could lead to a Ph.D project.
We have two optical astronomy honours scholarships avaliable

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Modelling accreting milli-second X-ray pulsars

Milli-second X-ray pulsars are binary systems containing a neutron star which is being spun up by accretion from its companion. They are believed to be progenitors of milli-second radio pulsars. Transient outbursts of radio, IR and optical emission occur at irregular intervals usually several years apart. We have detailed 4 colour (BVRI) photometry of two of these systems - SAX J1808.4 –3658 and RXTE J0929-314 during outbursts. In both cases there are slow changes in the broadband (BVRI) spectral shape and evidence of short duration flux increases (flares) which are restricted to the I band. Two possible mechanisms have been proposed for the I band flares. Synchrotron emission cut off at I band wavelengths may be making a transient contribution to the optical flux. This may arise from matter spiralling out of the system along bipolar jets. Alternatively, it could be due to thermal emission from cool matter dumped into the outer regions of the accretion disc. The detection of synchrotron emission in these systems would have important consequences for our understanding of X-ray binary systems and accretion processes in general.

In this project you will use disc modelling methods developed by Wang et al (ApJ 563:L61, 2001) to constrain system parameters and determine the evolution of the discs during transient outbursts. The method involves use of standard X-ray heated accretion disc models (see eg Chakrabarty, ApJ 492:342, 1998), the observed BVRI spectra and constraints from X-ray, IR and radio observations. Using these models you should be able to determine the I band excess during the flares and test its compatibility with the hypothetical emission mechanisms. In the event of a future outburst from these or other similar systems you will have an opportunity to make further observations using the Mt Canopus 1m telescope or spectroscopy and IR band photometry using other telescopes.

Recently we have obtained evidence that synchrotron emission in these systems may be triggered by type I thermonuclear X-ray burts from the neutron star. We are planning a multi-wavelength campaign to further investigate this phenomenon.

This project is suitable for honours research, but could lead to a Ph.D project.
We have two optical astronomy honours scholarships avaliable

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Outbursts of the Black Hole Candidate 4U1630-47

This black hole candidate system is unique in that it has semi-regular outbursts. There is now over ten years of observations by the Rossi X-ray Timing Explorer satellite of this source. Each outburst appears different. However, all black hole candidates show a series of states defined by the unique combinations of spectral and timing properties. The spectral properties are determined either by fitting x-ray spectra, or using x-ray colours. The timing properties are in terms of the presence and strength of various components within the power spectra. Quasi-periodic oscillations (QPO) are of particular interest because their frequency seems closely tied to the state of the source. These oscillations are thought to come from the inner regions of the accretion disk and hence probe conditions close to the black hole.

In this project we will use data from each outburst to map the spectral and timing properties of 4U1630-47. We will examine and compare between outbursts answering; how this source progresses from one state to another, explore how much of the possible state space is covered by the source, and if states are truly unique.

A starter project is to look at one particular set of suspected state transitions. During part of it’s 1998 outburst, 4U1630-47 showed semi-regular dips. Power spectra showed that during these dips the QPO changed dramatically in frequency from high to low frequencies. However, the colours/spectra of these dips has not been measured and compared to the overall pattern of the outburst. If these dips represent state transitions, the colours should match up to when 4U1630-47 was showing QPO with the same low frequency.

The project will involve using a large data analysis package. Unix shell script programming, and high level programming language skills will be used/developed. We expect this work to result in a peer reviewed publication.

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Orbital period changes in OY Car

Over the last few years the binary star OY Car has been monitored using the Mt Canopus 1m telescope. Roughly every 1 ˝ hours the star undergoes a deep eclipse. By timing these eclipses we have found that the period is changing. There are 2 aspects of this project:

  1. Continue the long term monitoring of the orbit. This would involve the occasional observing of OY Car, reduction of the images, and period change analysis.
  2. The short-term period variations. This would involve monitoring OY Car every clear night for a month or two, reduction of the images, and measuring the period changes. There could be an extension to the analysis of the timing measurements.

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17 Nov 2005