X-ray astronomy

Tuesday 19th June, 2018

Our speaker for the evening was Prof Martin Hardcastle from the University of Hertfordshire and his talk was entitled "Some like it hot: X-ray astronomy and the lives of galactic clusters". He said that the main interest of his research team was in active galactic nuclei or AGN. He explained the term AGN by saying that many galaxies have a very bright centre that can outshine the rest of the galaxy. This luminous central area is a very bright source of X-rays as the material around it is heated by friction as it swirls towards a central black hole. Some material escapes the black hole as powerful magnetic fields sweep it up into jets that blast out of the galaxy along its axis of rotation.

He then continued by saying that Earth's atmosphere absorbs X-rays, which is fortunate for any life on the planet as this form of radiation can damage living cells and DNA. However, this means that any instrument that is designed to study X-rays outside a laboratory needs to be above the atmosphere. So, the first research into X-rays was carried out by taking X-ray detectors to high altitudes using rockets launched from the White Sands Missile Range in New Mexico using a V-2 rocket in early 1949. An X-ray detector was placed in the nose cone of the rocket and it was sent on a sub-orbital flight that reached just above our atmosphere.

An alternative to using rockets, due their relatively short flight time, is to employ balloons with detectors slung below them, as they can stay aloft for many hours or even days. The only problem is that they do not rise above the atmosphere and so some X-rays will still be absorbed. Even so, this research has been and still is very productive and back in 1964 a balloon experiment recorded for the first time high energy X-rays from the shock wave generated by debris from an exploded star known as the Crab Nebula.

Prof Hardcastle then continued by saying that we are now able to send satellites up beyond the atmosphere to detect X-rays and many, such as NASA's Chandra spacecraft, are still operational. Unlike the light waves that we can see X-rays are very difficult to focus to make an image of an astronomical object. It is not possible to use a standard telescope layout of mirrors and lenses to try and focus X-rays as these rays just pass straight through the lens or mirror and carry on.

There is a method to focus X-rays, though, that is somewhat like skipping a flat stone across the surface of water. By letting the X-rays hit a series of mirrors aligned nearly parallel to their incoming direction the X-rays will graze the surfaces of the mirrors and over a reasonable distance be brought to a focus. In Chandra and similar spacecraft the mirrors are arranged as a series of nested cylindrical mirrors that are incredibly smooth and have been coated with the metal Iridium.

Prof Hardcastle finished his talk by saying that one of the mysteries that his team has been studying is how galaxy clusters stay hot enough to keep emitting X-rays. He proposes that the energy comes from polar jets of material expelled from a galaxy crashing into gas surrounding it and heating it.