Neutron Star Astrophysics

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Neutron stars are remarkable objects born in supernovae when a massive star collapses at the end of its life. The core of the massive star shrinks under its own gravity until it is only 10km in radius, but still contains as much mass as the Sun. The neutrons and protons inside the neutron star are closer together than in the nucleus of an atom, and so neutron stars are important laboratories for studying the strong nuclear force. Neutron stars are also important because of the extreme conditions in their vicinity: strong gravity, strong magnetic fields, and rapid spin. The research proposed here is an investigation of physical processes in and around neutron stars. The goal is to find out what the dense matter inside neutron stars is like, understand why many neutron stars are magnetically active (like an extreme version of our Sun), and to use neutron stars as laboratories to investigate extreme conditions. We do this by making theoretical models of processes that we think are happening inside neutron stars and then comparing to astronomical observations, often with orbiting X-ray telescopes. Of particular interest are neutron stars in binary systems. If the neutron star orbits close enough to its companion star, the huge tides raised on the surface of the companion cause gas to flow onto the neutron star. The hydrogen and helium builds up on the surface of the neutron star for a few hours and then undergoes a thermonuclear runaway, rapidly burning up and causing a bright flash of X-rays lasting a few seconds. For those few seconds, we can see the neutron star directly and study it. One of the projects in this proposal is to understand the plasma wind that is driven from the surface of the star as it undergoes one of these flashes. By observing in X-rays, we can try to measure the radius of the neutron star and from there work out the density of its core. Some neutron stars have strong magnetic fields and show flares and eruptions similar to our Sun. This is quite surprising because a neutron star should have a relatively calm surface, unlike the Sun whose outer layers are convective so that plasma is continuously bubbling up to the surface and disturbing the magnetic field. Somehow neutron stars are also magnetically active. We hope to find out why by studying how quickly the neutron star cools down after a heating event.