X-ray and Observational Astronomy

News October 2003: Untangling the Galactic Centre

A large-scale view of the Galactic centre in X-rays as seen by the XMM-Newton EPIC MOS cameras. Sources of predominantly low energy X-rays are shown red, these are mostly relatively nearby stars. Higher energy X-ray sources are shown blue-white, soft X-rays from these are absorbed hence these sources are near or beyond the Galactic centre. The 1E and SAX sources are low mass black holes or neutron stars accreting from a binary companion. The Galactic centre black hole is within Sgr A.

The centre of our Galaxy is a crowded and complicated place. One of the most spectacular recent results about it is the discovery of a massive black hole (3 million times the mass of the sun). Almost all black holes are discovered by the light that they cause to be emitted (as visible or X-ray radiation) from accreting gas as it is heated by falling through the strong gravitational field of the black hole. However, the massive black hole at the centre of our Galaxy is completely dark, and has been revealed only by the very fast motions of nearby stars orbiting around it. (A nice animation of these orbiting stars, together with other information on the central black hole is available here.)

In such an unusual environment, it is important to try to understand all the features that can been seen nearby. They may provide clues about the history of the central black hole, which in turn could have had a major influence on a much larger region if it were very bright in the past. (After all, most celestial X-ray sources are very luminous, actively accreting massive black holes in the centres of galaxies, the so-called Active Galactic Nucleii.)

Because the solar system lies in the plane of our Galaxy, our view of the centre is heavily obscured by dust (see this beautiful HST image of the Sombrero galaxy showing dust in the plane of a similar galaxy). Thus a visible light image is not very useful, and we have to rely on more penetrating forms of radiation to see into the centre.

Radio images show a bright source with a complex shape, called Sgr A (the Galactic centre is in the constellation Sagittarius). Sgr A west is a strange spiral structure which has at its centre Sgr A*, where the central black hole lies. Sgr A east has a shell-like structure encompassing Sgr A west. (These images show different colours for different intensities. They are taken at different radio frequencies, and so show structures differently. Sgr A* is visible as a red dot in the image of Sgr A east, the spiral of Sgr A west is just visible in pale green in this image; the shell of Sgr A east is just visible in dark blue in the image of Sgr A west. Please see this page for more information and image credits.)

Dr Masaaki Sakano and Prof Bob Warwick from the University of Leicester, together with Dr Decourchelle from CE Saclay and Dr Predehl from MPE Garching, have been making a study of this region in X-rays, which are also able to penetrate the gas and dust around the Galactic centre. They have been using the European Space Agency's X-ray observatory XMM-Newton to survey the Galactic centre region. XMM-Newton is well suited to this task because of its very large mirrors, these collect large numbers of X-rays allowing detailed imaging and spectral work to be carried out. The image at the top of this page shows the richness and complexity of the Galactic centre in X-rays.

The Sgr A east radio ring appears as a diffuse, centrally concentrated source in the X-ray image, with most of the X-rays coming from inside the radio ring. This distribution of radio and X-ray emission is characteristic of a 'mixed-morphology' supernova remnant, likely to be the result of a supernova explosion in a relatively dense environment. The supernova explanation for Sgr A east is strengthened by the X-ray spectrum. The spectrum is dominated by emission lines, and suggests multi-temperature emission (10 - 40 million degrees) from around 3 solar masses of gas. The thermal energy of this gas is 3x10⁴⁹ ergs, also consistent with a supernova origin. The supernova probably occurred 5 - 10,000 years ago, the high abundance of iron in the X-ray spectrum (3 times solar) points to a supernova of type Ia or a supernova of type II in a progenitor of less than 20 solar masses.

Not all X-ray features in the Galactic centre region have such straightforward explanations. The same authors have also discovered that the isolated radio filament called the "Sgr A east wisp" is also an extended hard X-ray source. There are very few such associations, the power law X-ray spectrum suggests sychrotron radiation (emission from ultra-fast electrons in magnetic field), and it seems likely that the X-rays locate the site of electron acceleration, while the radio emission maps out the region in which the electrons interact with the magnetic field. Because the electrons radiate their energy rapidly, there has to be a continuing acceleration process operating here (even if we do not know exactly what this is). Such non-thermal emission can occur in supernova remnants, and this seems probable for this newly discovered "wisp" also.

Papers detailing the results from the XMM Galactic centre survey are available from the arXiv service; The XMM-Newton view of the Galactic Centre, Sgr A east and its surroundings observed in X-rays, and The discovery of a new non-thermal X-ray filament near the Galactic Centre. The paper "XMM-Newton observations of Sagittarius A East" has been submitted to the Monthly Notices of the Royal Astronomical Society, and will appear on the arXiv when it is accepted for publication.

Other news items are available here.

Back to group page.

Last updated: 2003 October 16 by Julian Osborne