2004-2008 Onderzoeker in Opleiding (Graduate student) at SRON Netherlands Institute for Space Research (Division of High Energy Astronomy) and Utrecht University (Astronomical Institute Utrecht)

2009 - present Research Associate at the University of Minnesota, School of Physics and Astronomy, as part of the Joint Institute for Nuclear Astrophysics.

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 category 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.

Selected Publications

Keek, L., Galloway, D. K., in't Zand, J. J. M., & Heger, A. 2010, The Astrophysical Journal Multi-instrument X-ray Observations of Thermonuclear Bursts with Short Recurrence Times

Keek, L., Langer, N., & in't Zand, J. J. M. 2009, Astronomy and Astrophysics The effect of rotation on the stability of nuclear burning in accreting neutron stars

in't Zand, J. J. M., Keek, L., Cumming, A., Heger, A., Homan, J., & Méndez, M. 2009, Astronomy and Astrophysics Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?

Keek, L. 2008, Ph.D. Thesis Probing thermonuclear burning on accreting neutron stars

Jonker, P. G. & Keek, L. 2008, The Astronomer's Telegram AX J1754.2-2754: back in outburst

In't Zand, J. J. M., Bassa, C. G., Jonker, P. G., Keek, L., Verbunt, F., Méndez, M., & Markwardt, C. B. 2008, Astronomy and Astrophysics An X-ray and optical study of the ultracompact X-ray binary A 1246-58

Keek, L., in't Zand, J. J. M., Kuulkers, E., Cumming, A., Brown, E. F., & Suzuki, M. 2008, Astronomy and Astrophysics First superburst from a classical low-mass X-ray binary transient

Boirin, L., Keek, L., Méndez, M., Cumming, A., in't Zand, J. J. M., Cottam, J., Paerels, F., & Lewin, W. H. G. 2007, Astronomy and Astrophysics Discovery of X-ray burst triplets in EXO 0748-676

Keek, L., in't Zand, J. J. M., & Cumming, A. 2006, Astronomy and Astrophysics The superburst recurrence time in luminous persistent LMXBs