Joris' Research Page
This page is - like the research itself - in a continuing state of evolution and may therefore neither be complete, not up-to-date. Please do feel welcome to contact me with any questions or when I need a nudge to update anything.
I am currently a Marie-Curie post-doctoral researcher at the Max-Planck-Institute for Radio Astronomy in Bonn, Germany, where I use the Effelsberg 100-m radio telescope to search and time millisecond pulsars (MSPs) within the wider framework of the European Pulsar Timing Array .
Before moving back to Europe, I was a post-doc for a year at West Virginia University in the U.S.A. and before that still, I did a PhD as part of the Parkes Pulsar Timing Array (PPTA) project, in Swinburne University of Technology and the Australia Telescope National Facility (ATNF) in Australia. My thesis "Long-Term Timing of Millisecond Pulsars and Gravitational Wave Detection", can be downloaded from astro-ph.
A list of my publications can be found here.
Recent and Ongoing Research Topics
As always, a couple of research projects are going on in parallell. In order of prioritisation, these are the things I am working on as of January 2010.
(Note I provide this information voluntarily and in good faith, assuming stealing projects & ideas as well as agressive scooping do not occur amongst civilised people. Please don't disappoint me!)
- Timing the Relativistic Binary J1141—6545:
This heavy white–dwarf—neutron star binary system is of unique importance for tests of relativistic gravity, as most clearly described here (Figure 5). However, timing efforts have been complicated by geodetic precession, which slowly but surely changes the shape of the pulsar's pulses. (See this reference for more information on that.) Right after the most recent timing solution was published (see this reference), the pulsar glitched, further complicating the timing efforts. Extensive work (mostly by Manchester, Kramer & Stairs) on modelling the changes of the pulse shape has been presented at the Marcel Grossmann meeting and will be published shortly, or so I am told. I am currently updating the timing solution of Bhat et al. by adding the last few years of data, including a model for the glitch. Starting in March 2010, I am hoping to investigate the value pulse-shape modelling (as presented by Manchester, Kramer et al.) can have on the timing effort.
Collaborators: M. Bailes, R. Bhat & M. Kramer.
- Predicting the Value of Future Timing Epochs:
Pulsar timing is built on two inputs: the times-of-arrival (TOAs) of radio pulses and a mathematical model predicting these TOAs. The difference between the predictions and the actual TOAs are called the (timing) residuals. When something is clearly wrong with the timing model, the residuals will show a clear trend as shown in this figure (taken from page 12 of my thesis). As the figure clearly shows, not all TOAs are equally useful, depending on which parameters one is most interested in. I am currently using Monte-Carlo simulations to attempt to quantify the usefullness of potential future timing observing epochs. Initial results suggest that there is indeed quite a difference and that a handful of well-planned observations could prove more valuable than large amounts of randomly planned observing time.
Collaborator: P.C.C. Freire.
- Pulsar Spectral Indices:
We are using pulsar population software to simulate hypothetical galaxies filled with pulsars. By running historic pulsar surveys on these galaxies and comparing their discovery statistics with those obtained in reality, we can investigate the validity of assumptions that went into the originally simulated pulsar population. While this technique is not new, we are currently applying it to find out more about the inherent spectral index distributions of both normal and millisecond pulsars.
Collaborators: D.R. Lorimer & M.A. McLaughlin.
Then there are a few projects that are on the back burner for now, but which I am hoping to get finished in the not-too-distant future. Note that I am not necessarily the lead investigator for all the following projects.
- Timing Model Uncertainties:
It has long been known that the uncertainties on pulsar timing model parameters are badly underestimated. In my first paper, we attempted a spectral Monte-Carlo approach to estimate these uncertainties more accurately. This technique has now been further developed (mostly by Hobbs & Coles) and should be published relatively soon. This implies that it is now also possible to apply this technique to any other MSP timing data set. I intend to apply it to the data sets of my second paper , which should provide the first large sample of MSPs for which the errors will be accurately determined.
Collaborators: G. Hobbs, W. Coles, M. Bailes & many of the PPTA astronomers.
- Limits on the GWB:
Most methods to place limits on the strength of the gravitational wave background using pulsar data, either depend on the data being statistically white (which it sometimes isn't) or on cross-correlations between data sets, therefore requiring more long data sets than one. With the collaborators listed below, I have developed a technique to use a single, long and highly precise pulsar timing data set to place a bound on the strength of the GWB. While an earlier incarnation of this method has been described in some detail in Chapter 5 of my thesis, it has not been published in a journal so far. Since my thesis was written, further details have been ironed out and the technique is now fully ready to be written up in paper form and published properly. This should happen relatively soon.
Collaborators: G. Hobbs, W. Coles, A. Lommen.
Past Research in Astronomy
- Lutz-Kelker Bias:
Recently, I quantified the Lutz-Kelker bias for pulsar parallax measurements. An online tool is now available here . The paper recently got accepted by MNRAS and can be accessed here
Collaborators: D.R. Lorimer & M.A. McLaughlin.
- Timing Stability of Millisecond Pulsars:
Following on the long-term timing of PSR J0437–4715, I repeated this exercise for all 20 of the pulsars in the Parkes Pulsar Timing Array (PPTA). While most of these are not quite as interesting as PSR J0437–4715, this investigation did provide a unique insight into the long-term stability of millisecond pulsars - a vital property for pulsar timing array efforts and gravitational wave detection. As described in our paper, we found the overwhelming majority of our pulsars to be extremely stable and therefore good sources to be used in a timing array. However, we also found that practically all of our timing is white-noise dominated, which means we need more telescope time, bigger telescopes and more, brighter pulsars. Since most MSPs (and, more generally, most pulsars) have only been detected after the year 2000, our data set - with an average data span of 12 years - is by far the largest sample of long-term timing data on MSPs currently available. As part of this research, I wrote some C-code to estimate the sensitivity of pulsar timing arrays to a gravitational wave background. This code is now freely available here.
Main collaborators: M. Bailes, W. Coles, W. van Straten, G. Hobbs and the PPTA team.
- Precision Timing of PSR J0437–4715:
To start off my Ph.D., I combined the 10 years of timing data on the brightest millisecond pulsar in the sky: PSR J0437–4715. This unique data set resulted in a few great finds such as one of the most precise astronomical distances to date and one of the heaviest known pulsar masses, as well as spin-off science including limits on trans-Neptunian objects (TNOs) and a powerful limit on the variation of Newton's Gravitational constant (Gdot). The publication can be found here. Shortly after publication, the distance to this pulsar was independently verified and published (along with an improved limit on Gdot and TNO masses) by Adam Deller (see this page). Our work was also used in a more thorough investigation on possible variability of G by Kosmas Lazaridis (see this paper).
Main collaborators: M. Bailes, W. van Straten, G. Hobbs & R. Manchester
Brief Professional History
1998-2000: I was a member of the Urania Public Observatory, Antwerp, Belgium and took a few introductory evening courses in astronomy there.
2003-2004: The "Master of Science in Advanced Techniques in Radio Astronomy and Space Science" was my real start in (radio) astronomy. The follow-up programme currently being organised at Chalmers University in Göteborg, Sweden, provides the same strong basis that got me started. Another useful link in this context is the Alumni Organisation that I founded together with some of my classmates. My Master's Thesis Satellite Interferometry Simulations can be found here
2005-2009: I worked on my Ph.D. thesis "Long-Term Timing of Millisecond Pulsars and Gravitational Wave Detection" at Swinburne University of Technology in Melbourne, Australia, under supervision of Prof. Matthew Bailes. I was also co-supervised at the Australia Telescope National Facility (ATNF) in Sydney, Australia, by Dr. Richard Manchester and with much help and (both moral and scientific) support from Dr. George Hobbs. I was lucky enough to spend many days, nights and - everything added together - quite a few kilohours observing at the Parkes Radio Telescope.
2009: I worked as a postdoctoral research associate at West Virginia University in Morgantown, U.S.A. Because my stay was only short, I focussed on a few simulation-based statistical analyses of the pulsar population. One of these deals with parallax biases and has been published (see above), the other one relates to the pulsar spectral index distribution and is still ongoing.