PhD Defense Thijs van Putten
A magnetar is a type of neutron star with an extremely strong magnetic field, a hundred million times stronger than the strongest fields made in laboratories. In turn, a neutron star is one of the end products of stellar evolution, and is formed when a massive star has used up all its fuel for nuclear fusion. A neutron star has a mass around twice that of the sun, but a diameter of only twenty km, making it extremely dense. Studying neutron stars, and magnetars in particular, is the only way to study what happens to matter at these extreme densities and magnetic field strengths.
Many magnetars are seen through their bursts, which are short bright flashes of radiation on the edge between X-ray and gamma-ray radiation. When radiation interacts with matter, it exerts a small force on that matter. In magnetar bursts, the intensity of the emitted radiation can let this force become as strong as gravity. In Chapters 2 and 3 of this thesis I have studied what happens when this force does overcome gravity, by making numerical models of magnetar atmospheres during a burst. I conclude that during these bursts magnetars will develop highly irregular outflows of matter.
In Chapter 4 I have developed a numerical model to test how the atmosphere of a magnetar alters the properties of its emission, in particular to model the observed beaming of magnetar emission, an effect whereby radiation is emitted in a preferred direction. I find that a simple model for the atmosphere of a magnetar can recreate the observed amount of beaming. This makes my model a promising way to use observations of beamed magnetar bursts to learn more about the objects emitting these bursts
Location: Agnietenkapel, Oudezijds Voorburgwal 231, Amsterdam