Lennart van Haaften, Ph.D.

Binary stars

I specialize in binary stars, simply called binaries, which are pairs of stars orbiting each other because of the gravitational pull between them.

I work mostly on X-ray binaries. These are binaries in which one of the two stars is a neutron star or a black hole (these are extremely compact remnants of very massive stars) that pulls gas off of its companion star, which is a more normal star, like the Sun for example. This gas falls towards the compact remnant, forms an disk around it, and heats up due to friction. Because of the very high temperature reached by the gas in the disk, it emits large amounts of X-ray radiation, hence the name X-ray binaries.

We observe this radiation using X-ray telescopes in space, above Earth's atmosphere. X-rays are absorbed by Earth's atmosphere, so we cannot observe the X-rays of X-ray binaries from the ground. But they also emit light and radio waves for example, and these we can see from the ground. By observing X-ray binaries in multiple bands of radiation, we can better understand their accretion disks and other regions and processes that produce the observed radiation.

Current research

At Texas Tech I'm investigating populations of X-ray binaries in galaxies in the Virgo Cluster using observations by the Chandra X-ray Observatory. The Virgo Cluster is a very large cluster of galaxies about 50 million light-years away, within reach of our X-ray telescopes, and therefore one of the best places to look for large populations X-ray binaries.

In one galaxy, called NGC 4472, I'm looking for bright X-ray binaries. In particular I want to know where they are located: closer to the center of the galaxy, or further towards the outskirts. Because the chemical composition of gas and stars is different in different locations within this galaxy, their preferred locations could tell us about where X-ray binaries most easily form, and how there formation and evolution depends on chemical composition.

In other galaxies, also in the Virgo Cluster, I'm searching for X-ray binaries that become much brighter or fainter over a timespan of several years, which could mean that they host a black hole, and not a neutron star (the latter are more common overall). This is because neutron stars have a very hot surface that warms up the disk of gas around it, making its structure more stable and its luminosity less variable in time. Black holes do not have a solid surface, which could explain why their luminosities are observed to be more variable.

Publications

List on NASA ADS

List on Google Scholar

List on ArXiv