Leicester University X-ray and Observational Astronomy Group

Julian Osborne : Research

In the last few years I have been concentrating on Swift observations of gamma-ray bursts and of novae. I have also had an interest in the variability monitoring capabilities the SuperWASP telescopes. Prior to the the launch of Swift I used XMM-Newton, first to look at binaries in the nearby Andromeda Galaxy (M31), and then to look at gamma-ray burst afterglows. My original research interests were related to the X-ray emission from the accreting white dwarfs in Cataclysmic Variables. Somewhere between research and project work lies another large project currently under way: the creation of the XMM-Newton catalogues within the SSC.

Swift observations of Gamma-ray Burst afterglows

[Swift GRB light curves]
Example bursts showing the various behaviour patterns seen by Swift: a steep-to-shallow transition (GRB 050315); a large flare (GRB 050502B), and a gradually declining afterglow (GRB 050826, devided by 100 for clarity). BAT data are shown as crosses, XRT data as circles. (From O'Brien et al, Willingale, Osborne and Goad; New Journal of Physics, 8, 121, 2006).

The US/UK/Italian Swift satellite has revolutionised the study of gamma-ray bursts by detecting the bursts and autonomously repointing the spacecraft so that within a minute or so the sensitive X-ray and optical/ultra-violet telescopes are pointing at the burst position. Prior to the launch of Swift in November 2004 such observations typically took place many hours to days later, by which time the intensity of the flux would have fallen by many orders of magnitude. The University of Leicester provided the CCD camera and other elements of the X-ray telescope on Swift, and now provides the UK Swift Science Data Centre, as well as XRT calibration and data analysis.

Gamma-ray bursts are short bursts of high energy radiation, lasting a few milliseconds to a few minutes, that were extremely mysterious until 1997 when the first counterpart to a GRB was found at another wavelength. This first 'afterglow' discovery saw a rapidly fading X-ray source. Shortly afterwards the galaxy in which the burst occurred was found, and finally the distance could be measured. This showed that gamma-ray bursts were coming from very distant galaxies, and so their energy had to be truely enormous - near the rest-mass energy of a star like the Sun. Even though we now think the energies are generally somewhat smaller, since then gamma-ray bursts have represented a frontier in high energy astrophysics, being the most energetic explosions since the big bang.

Swift has been observing around 100 bursts a year. It has shown that the early X-ray afterglows are much more complex than was anticipated, sometimes even showing massive flares. These may be clues to a much longer-lived accretion disk around the newly formed black hole. Swift has provided the first locations of the so-called short bursts, apparently confirming a different origin to the long bursts as they are seen near non-star forming regions and galaxies (and so must come from old progenitors, like colliding neutron star - neutron star binaries, rather than the young massive star supernova in current star-forming regions which are the progenitors of the long GRBs). It is also finding very high redshift bursts (Swift currently holds the record for the most distant stellar object every seen: GRB 050904), allowing galaxies only 5% of the age of the universe (ie 13 billion year old) to be probed. Since 2005 there have been over 1000 publications on Swift results.

Swift observations of Novae

[impression of the nova RS Oph 2006]
A depiction of a nova outburst similar to the RS Ophiuchi system, where a white dwarf, which has steadily been accreting material from a red giant companion, undergoes a thermonuclear explosion on its surface. The ejecta gain speeds around 3000 km/s and pass the red giant in less than 2 days. (Credit: David Hardy/PPARC)

Swift is a soft gamma-ray, X-ray and UV-optical observatory with a unique rapid response capability. This makes it near ideal for observing unpredictable events such as the thermonuclear burning of the gas accreted from a binary companion star onto a white dwarf in a nova explosion. Prompted initally by Prof Mike Bode of Liverpool John Moores University, Swift has made a huge number of observations of the recurrent nova RS Ophiuchi 2006, from 3 days after the outburst to more than 250 days later. The UK Swift Science Data Centre has a nice movie of the explosion.

Only a few novae have been seen to explode more than once, these are the recurrent novae. RS Oph had its last explosion in 1985. It is unusual in that instead of being in a close orbit with a relatively normal star, RS Oph is in a 455 day orbit with a red giant. The 2006 explosion was followed by a large array of astronomical facilities, including XMM-Newton, Chandra, Spitzer, the Hubble Space Telescope, and radio interferometers. Results of this campaign are just appearing now:

RS Oph is an important object, first because the passage of the shock through the red giant wind has the appearance of a supernova remnant evolving thousands of times faster than any real SNR (which takes thousands of years), and second because the white dwarf is near to the critical Chandrasekhar mass and accretion may push it over the limit so that it explodes as a type 1a supernova. It is these supernovae which are behind the accelerating universe dark energy idea, so understanding them has a high priority.

As well as the intensive study of RS Oph, Swift has been observing other novae at various stages of their outbursts. In this way we hope to gain an understanding of the evolution of the nuclear burning and how it depends on the properties of the white dwarf and its companion star. Soft X-rays from the hot white dwarf may only be visible for a few weeks, we would like to understand why.


[SuperWASP lightcurves of two transiting planets]
Transit profiles for 2 new planets, WASP-1 and WASP-2, discovered by SuperWASP. From Collier-Cameron et al (2006).

It is amazing that a telescope with such a rich potential as SuperWASP can be built from off-the-shelf components. Using Cannon SLR lenses and fairly normal CCD detectors (and a rather heavy-duty robotic mount), after first light in July 2003, the first SuperWASP in La Palma surveyed 61 square degrees of sky with each of the 5 lenses every minute each night. There is now also a SuperWASP south, in South Africa, and both have the full 8 lenses. The two telescopes are monitoring millions of stars down to magnitude 16.5. At Leicester we provide the SuperWASP archive; it is a major challenge to give useful high speed access to the tens of terabytes of reduced data that the telescopes produce.

The SuperWASP telescopes are being used to discover new planets by observing the loss of starlight as the planet passes in front of the star. The telescopes are particularly sensitive to so-called 'hot Jupiters', that is large gas giants close to their parent star. We do not have a hot Jupiter in our Solar System, and we believe that during planetary formation, when there is still a proto-planetary disk around the star, the planets can be moved inwards or away from the star by their interaction with the disk. This is not a well understood process, and so finding more such planets is a good way to learn about it.

There are now hundreds of planets now known around other stars, the SuperWASP telescopes have been highly productive both as discovers of exoplanets and also as monitors of variability in the sky. Pulsating stars, accreting cataclysmic variable stars, Active Galactic Nucleii, and even the brighter gamma-ray bursts can all be seen.

XMM-Newton survey of the Andromeda Galaxy (M31)

[XMM X-ray colour picture of central 30 arminutes of M31]
The first XMM-Newton image centred on the core of M31. Soft X-ray sources are shown red, hard X-ray sources are blue. The relatively cool gas (around 3 million degrees) in the core can be seen as a diffuse orange/red glow (from Shirey et al 2001).
In collaboration with Dr Bill Priedhorsky and his team at Los Alamos, and more recently with Robin Barnard, Lindsey Shaw Greening and Ulrich Kolb at the Open University, I have been working on a large XMM-Newton survey of M31 (the Andromeda galaxy). This survey consists of 4 long looks at the central region of the galaxy, and a series of long exposures along the axis of the galaxy. The observations are each half a day long or more, and are designed to collect as many counts as possible from the bright X-ray sources previously discovered by ROSAT and earlier X-ray satellites. Because XMM-Newton has the largest collecting area of any X-ray observatory, we can get the best quality X-ray spectra and light curves from the sources in M31. In the core of the M31 these X-ray sources are mostly low mass X-ray binaries, while further out we expect to see more high mass binaries. With our survey we hope to classify these more precisely, aiming to learn the black hole to neutron star ratio in the galaxy and to better understand the history of formation of these exotic objects.

Although there is still more to do, 9 refereed papers have resulted from the survey work so far:

XMM observations of gamma-ray burst afterglows

XMM-Newton has more collecting area than any other X-ray telescope with good quality CCD detectors, and so is a prime source of high quality spectral data. When it is able to point towards gamma-ray bursts, it can get the best spectra possible. Of course, the X-ray afterglows of gamma ray bursts fade very quickly, so prompt action by the XMM controllers is required, and this is naturally rather difficult. Even so, a few GRB afterglows have been observed within 11-48 hours.

James Reeves, Darrach Watson and I, together with others, have analysed the XMM-Newton X-ray spectra of 3 GRB afterglows (with one more in progress). We detected emission lines from Mg to perhaps Ca in the first of these: GRB011211. This was an important result, as all previous afterglow emission line detections were claimed to be due to Fe. While Fe emission might be due to fluoresence, emission from light elements was not expected, and suggested a supernova explosion shortly before the GRB. In addition, the line energies suggested that the emitting gas was rapidly expanding away from the GRB. We published this result in Nature.

This work has been criticised, both from a data reduction perspective, and in its statistical analysis. Theoretical difficulties have also been highlighted. The analysis criticisms are rejected in a fuller paper, in 2003 in Astronomy & Astrophysics, which describes our GRB011211 analysis in detail.

The XMM-Newton afterglow spectra of GRBs 001025a and 010220 also show emission lines. A paper describing these results was also published by us in Astronomy & Astrophysics.

A few years on from the intial reports, the line detection issue appears to have been taken as far as it can be. While Butler et al. (2003) claimed a low significance line detection using Chandra in a grating observation of 020813, further re-analysis of the spectra of the original XMM bursts by Butler et al. (2005) has essentially confirmed the original low significance result. Swift, which gets to GRBs much earlier than XMM can, has not seen any line emission. It has however sometimes seen blackbody emission in the spectrum in GRBs, perhaps this affected the line detections (which were based on an assumed underlying power law spectrum). The final statement of non-detection in the Swift data was made by "Hurkett et al. (2008).

[XMM EPIC evolving halo from GRB031203]
XMM-Newton's EPIC X-ray cameras show the expanding rings caused by a flash of X-rays scattered by dust in our Galaxy. The X-rays were produced by a powerful gamma-ray burst that took place on 3 December 2003. The slowly fading afterglow of the GRB is at the centre of the rings. Other, unrelated, X-ray sources can also be seen. The time since the burst is shown. At its largest, the inner ring is about one fifth the size of the full moon. Credit: ESA, S. Vaughan (University of Leicester)
XMM has discovered a new phenomenon, time variable X-ray halos around gamma-ray bursts caused by scattering due to dust in our own Galaxy. Papers by Vaughan et al. (2004) and by Watson et al. (2004, 2006) describe how the longer path length of larger angle scattering causes an apparent expanding ring from a single flash of X-rays. Such observations provide a precise measurement of the dust distance from us, and constrain the proerties of the dust and of the original X-ray emission (which is often not directly observed).

Similar expanding X-ray halos have since also been seen around the GRBs 050713A, 050724 and the soft gamma repeater 1806-20.

Cataclysmic Variables

My original scientific interest was in magnetic Cataclysmic Variables, close binary stars in which a strongly magnetic white dwarf accretes gas from a low mass near-main sequence companion. In some, the AM Her type systems (or polars), the magnetic field is so strong that the stars are rotationally locked and the gas flows onto the white dwarf in a narrow steam.

[Animated polar]
Polar animation by Dr Andy Beardmore, Keele University (check his other CV animations).

At lower magnetic fields (and wider separations) the accreting gas probably forms a disk of some sort, although close in the gas must follow the dipole-like field lines onto the white dwarf. These systems are called intermediate polars. Dr Koji Mukai (GSFC, USA) has a web page giving details on these systems.

[Animated intermediate polar]
Intermediate polar animation by Dr Andy Beardmore, Keele University (check his other CV animations).

At lower fields still, the disk probably extends down to the white dwarf surface, where we expect a hot interaction region to be formed.

Dr Mark Garlick makes excellent 'artists impressions' of all sorts of astronomical obejcts, including magnetic cataclysmic variables. I would recommend exploring his web site (although it has been broken recently). Dr Rob Hynes has made some very nice illustrations and a non-magnetic CV movie.

More directly derived from the physics of CVs are the simulations provided by the UK Astrophysical Fluids Facility, here at Leicester. The movies of the simulations of the intermediate polar EX Hya, and of the accretion disk in non-magnetic CVs are wonderful to see.

One of the most interesting CVs is AE Aqr. It is the only known propeller system, ie it was previously like an intermediate polar with the white dwarf spun up by strong disk accretion, but now has a much lower accretion rate so that the gas lost from the companion star is batted out of the system by the very rapidly spinning magnetic field. I have high spectral resolution XMM-Newton data showing the high velocities expected from this model. This accretion scenario is very nicely illustrated by the UKAFF simulation movie (36 MB) made by Drs Richard West and Graham Wynn. (The colours and the companion star in this movie are artificial, while the gas stream density and flow are properly simulated.)

My most recent foray into CVs was the work of my PhD Student (now Dr) Darren Baskill, who published papers in 2001 and 2005 on the X-ray emission of non-magnetic CVs as observed by the Japanese satellite ASCA.

XMM-Newton X-ray source catalogues

[XMM sources in the catalogue]
Some of the sources detected in the EPIC X-ray images that have gone into the first XMM-Newton catalogue.
Over the latter half of 2002 we re-processed all of the XMM-Newton EPIC data in a pipeline specifically modified for catalogue production. The modifications were made to ensure the most recent versions of the individual SAS tasks are used, and also to incorporate position rectification of X-ray source positions with respect to the USNO-A2 catalogue, and to produce some new products aimed at catalogue verification. The first XMM catalogue was released on April 7, 2003. A first presentation of this work was made at the Santander X-ray surveys conference in September 2002.

In July 2006, a pre-release version of the second generation XMM catalogue (2XMMp) was made available. This contains 153,000 detections of 123,000 distinct X-ray sources, and is the largest X-ray source catalogue available. Based on a XMM observations up to April 2006, this catalogue not only features significantly improved source detection and characterisation, but also spectra and lightcurves for the brightest sources. The full 2XMM catalogue is in production now, it will improve on 2XMMp by having extensive manual quality flagging. Since the launch of Swift, my involvement in this work has diminished quite a bit.

My refereed publication list from 2002-2011 is available here, and the non-refereed list is here. A more useful source of these publications is the ADS, this link returns mostly my publications.

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Julian Osborne: 21-Sep-2011