Phil Evans' Home
Phil's research
I am currently active in two areas of research, Gamma Ray Bursts (GRBs) and Intermediate Polars (IPs). In both cases my involvement is predominantly with X-ray data, although observatories such as Swift and XMM-Newton provide optical/UV data as well, which I exploit when I can.
Note, I am in the process of producing this page. It is not complete.
GRBs
Image Credit: NASA E/PO, Sonoma State University, Aurore Simonnet
Gamma Ray Bursts are the biggest explosions currently known to mankind. A typical GRB gives off a similar amount of energy to a supernova, but it does it in a few seconds, making GRBs around 10,000,000 times brighter than your average supernova. But because they are so short-lived, observing them is not easy. Fortunately, most GRBs also have an afterglow -- a longer lived glow of radiation caused when the original fireball smashes into the surrounding medium. These afterglows fade rapidly, so to observe a GRB it is important to start observing as soon as possible after a trigger. This is where Swift comes in.
Swift is a NASA-UK-Italian satellite designed to detect GRBs, and respond to them rapidly. To this end it has a Burst Alert Telescope (BAT) which monitors 1/6 of the sky at any given point, in the 15-350 keV band. If BAT detects a GRB, Swift immediately (automatically) slews to the new GRB, so that the narrow field instruments can observe the burst. The BAT position is also reported at once to the astronomical community via the GCN system. BAT positions are usually accurate to 3 arcmin (the size of a pinhead half a metre away).
Once Swift is pointing at the new burst, usually about a minute after detecting it, the X-ray telescope (XRT) and UV/optical telescope (UVOT) begin observing. These instruments have much smaller fields of view than the BAT and are collectively as the narrow field instruments, or NFIs. They provide much more accurate positions than the BAT though. XRT finds positions for nearly every burst Swift observes. When it first observes a GRB it can generally produce positions accurate to 3 arc seconds (a pinhead 35 metres away) within 20 minutes. Once the full dataset has been received on the ground a few hours after the burst this is improved to as little as 1.5 arc second (a pinhead 70 metres away). The UV/optical telescope finds counterparts for about 30% of BAT-detected bursts, and can localise them to accuracies of about half an arc second (a pinhead 200 metres away). All of these positions are made available to the astronomical community as soon as they are available, allowing ground-based followup both with dedicated robotic telescopes, and large international facilities like VLT.
Swift's instruments do much more than work out positions. They monitor the brightness of the burst and its spectrum. Because the three instruments cover a wide energy range, this can provide unique insights into the physics of GRBs. For example, Swift data appear to have shown that the internal shock mechanism for producing the prompt emission is favoured over the external shock. They have also revealed that the afterglow decay shows a shallow `plateau' phase which was not predicted and is still not understood.
My work to date has focussed largely on automating analysis of Swift XRT data, to give more information to the GRB community as rapidly as possible. This includes:
- The Swift-XRT light curve repository which contains XRT light curves of all GRBs Swift has observed. This is updated automatically.
- Improving XRT positions of GRBs, reducing the typical uncertainty from over 3.5 arcseconds to less than 2 arcseconds.
- Producing information from the rapidly available XRT data, to give indications of the position, brightness and spectrum within minutes of a burst.
I also have a few active research projects, details of which will appear when they are published.
Intermediate Polars
Image: (c) Mark Garlick /space-art.co.uk
Intermediate Polars are a type of interacting binary system. That is, they consist of two stars which exchange matter. More specifically, an IP is a Cataclysmic Variable (CV), which means the primary star -- the more massive of the two -- is a white dwarf. The secondary star is usually a red dwarf which is filling it's Roche lobe. This means that matter at the edge of the star is actually being pulled more strongly by the white dwarf than by the red dwarf, so matter flows from the red dwarf to the white. Because the stars are orbiting their centre of mass, the matter can't just fall in a straight line, but spirals in and forms a disc (an accretion disc), through which matter transfers to the white dwarf. In an IP the white dwarf has a moderately strong magnetic field, which disrupts the disc. Matter near the star leaves the disc and flows along the magnetic field lines and onto the white dwarf. The image to the right shows Mark Garlick's representation of such a system. As you can see, the magnetic axis of the white dwarf is at an angle, so for parts of the disc gravity makes it easier to flow to one pole, whereas for other parts the other pole is more accessible. This results in the two funnels of material which are often called accretion curtains.
When the matter from the accretion curtains approaches the white dwarf it is in free-fall and travelling much faster than the speed of sound. As it crashes into the white dwarf suface, just above the magnetic polecaps, a shock forms. As more matter hits this shock it is decelerated and cools via a process known as bremsstrahlung (German for "breaking radiation"), which results in the emission of X-rays.
While all this is going on, the white dwarf is rotating, typically one revolution every few hundred seconds. Depending on the exact geometry of the system, and our viewing angle, this may cause the accretion curtains to periodically pass through our line of sight to the X-ray emitting regions, reducing the flux that we see. Also, one or both of the emitting regions may periodically disappear behind the white dwarf, again, affecting the flux. This causes strong periodic modulations in the X-rays (and at UV and optical wavelengths) which we can detect and study.
A large part of my PhD was spent studying these modulations. An advantage of XMM (and Swift) is that it provides simultanous UV or optical data as well as X-ray data. This allowed us, for example, to show that the UV modulation in FO Aquarii lags behind the X-ray, suggesting that the accretion curtains are twisted. Another issue which interests me is why some intermediate polars appear to show lots of soft X-rays which do not come from the bremsstrahlung emission described above, whereas other IPs do not. In a paper in 2007 I suggested that this may be a simple geometric effect. Hopefully, with more data from Swift and XMM, we will be able to test this theory more rigorously.