Postdoctoral Research Associate

Laurens Keek

University of Minnesota

School of Physics & Astronomy

116 Church Street S.E. Minneapolis, MN, 55455

342B Tate, 625-2432, email laurens @ physics.umn.edu


Research Description

I study type-I X-ray bursts from accreting neutron stars. When a neutron star accretes matter from a binary companion, a layer of hydrogen and/or helium piles up on its surface. Due to the high gravity at the neutron star surface, this layer is highly compressed. When the layer is just a few meters thick, the temperature and density at the bottom are already high enough for thermonuclear fusion to start. This fusion "burns" hydrogen into helium and helium into carbon. Under certain conditions, this burning can proceed in an unstable manner, causing all the fuel to burn within one second. During such a flash the temperature is high enough for a series of proton captures, that turn carbon into much heavier isotopes, with mass numbers up to 100.
These flashes are observed as X-ray bursts. Since the 1970's several thousands bursts have been observed from approximately 90 sources in our Galaxy. Many of the observed features cannot be explained well with our current models. For example, the observed conditions for which the burning is stable or unstable do not match the theory. An exciting new discovery in recent years are rare long bursts, that last from half an hour up to an entire day. This is much longer than the 10 to 100 seconds a normal burst is observable. The longest and most energetic catagory are referred to as "superbursts". They are thought to be caused by the unstable burning of a carbon-rich layer.
In our Cosmic Explosions Group, we work on both observations and models of type-I X-ray bursts. On the observational side, we collaborate with groups around the world to create large catalogs of thousands of observed bursts. This will give us a better understanding of the different types of burning behavior neutron stars exhibit in a wide range of conditions. This helps us to improve our models. We develop computer codes that model the evolution of the neutron star envelope in time. An important part of these codes are the reaction networks that model the nuclear burning reactions. We use the best available reaction rates, through our collaborations within JINA, the Joint Institute of Nuclear Astrophysics.

Publications

Selected publications: See ADS for the full list of publications.