Explaining the cause and spectra of X-ray bursts from magnetars

Supervisor: Dr. Silvia Zane ([Email Address Removed])


Observationally, one of the most distinctive properties of magnetars is the emission of short, intense bursts of hard X-rays/soft gamma-rays. In the magnetar scenario bursts are believed to originate from a hot, magnetically confined pair plasma (a "fireball") which forms above the star surface because of the huge energy injected in the magnetosphere as a consequence of a crustal displacement (a "starquake").

During 2006 we observed, with the Swift X-ray satellite an intense burst `"forest" from SGR 1900+14 which lasted for ~30 s, during which 7 intermediate flares (IFs) were detected. These events have properties in between those of normal bursts and giant flares, i.e. ~1 s duration and luminosity up to ~ 1E42 erg/s. This unique data set provided us with the most complete information to date on SGR bursts. Data were well represented by a two-blackbody spectrum with peculiar characteristics, that lead theoreticians to suggest we are observing for the very first time the two different photospheres of photons with different polarization properties (i.e. the so called ordinary and extraordinary waves which are expected in a strongly magnetized medium - Israel et al. 2008). This would be an unprecedented observational confirmation of our understanding of wave propagation in strong field!

However, the suggestion must be corroborated by a detailed treatment of radiative transfer in the trapped fireball: the lack of this important analysis has prevented up to now a quantitative comparison between theory and observations (see e.g. Liubarsky et al. 2002), which is mandatory to constrain the properties of the fireball. The main goals of this PhD project are: i) to build a complete, self-consistent model for the transport of radiation in the trapped fireball, and ii) to apply results to fit observed burst spectra in order to validate the model and derive physical parameters. The student will develop specific competences in high magnetic field physics, numerical radiative transfer and the application of theoretical models to observations. As a first step, he/she will derive expressions of the relativistic scattering cross below resonance for a thermal distribution of electrons/positrons in a form suitable to numerical calculations. Secondly, a complete transfer code will be developed, including non-conservative scattering. Finally, a spectral archive (obtained varying the model parameters) will be produced and used to interpret all available/forthcoming bursts observations. This will be the first ever state-of-the-art model for burst emission spectra.

UCL was one of the first universities in the world to become involved in making scientific observations in space. Since MSSL was established in 1966, we have participated in more than 35 satellite missions and over 200 rocket experiments. Our groups of research scientists and development engineers work together to ensure that the instruments we produce are as relevant and competitive as possible. The subsequent scientific interpretation of data benefits from the fundamental understanding of the instruments gained from in depth knowledge of their development and testing.

Further details on our PhD programme and other available projects can be found at:
http://www.ucl.ac.uk/mssl/research-degrees and http://www.ucl.ac.uk/astro/phd