XMM-Newton reveals a Be star and an absorbed QSO
19 October 2000ESA XMM-Newton X-ray observatory, launched last December and now in its routine phase of observations, has a field of view large enough that whilst observing known targets it is also registering X-ray emission from many other objects in the neighbouring area of the sky.
A typical XMM-Newton observation is detecting between 50-200 X-ray sources in addition to the target object. Several thousands of these hitherto unknown sources have already been discovered, of which we know only what XMM-Newton tells us, and a few dozens have now been identified. It has been estimated that in the course of a year, 50,000 serendipitous X-ray sources will be revealed.
The XMM-Newton Survey Science Centre (SSC) led by the University of Leicester in the UK has the responsibility of following-up these newly discovered X-ray sources. The work involves cataloguing the new X-ray objects and, for a sample of these, observing the potential counterparts at optical and infra-red wavelengths so as to determine the precise nature of the X-ray emitting objects.
Part of the SSC's efforts rely on AXIS (An XMM-Newton International Survey) coordinated by the Instituto de Fisica de Cantabria in Spain, which is following up the X-ray space observations with ground-based optical telescopes. Using observatory facilities in the Canary Islands, AXIS astronomers have in just a few nights identified 37 serendipitous X-ray sources.
Thirteen have been identified in the plane of our Galaxy: 11 stars with active coronae, a Be star system, and a low latitude cluster of galaxies. A further 24 X-ray emitting sources pin-pointed by XMM-Newton have been identified looking away from the Milky Way: 16 quasi-stellar objects (QSO) and other Active Galaxies, 4 so-called "Narrow Line X-ray Emitting Galaxies" (NELG), one "normal" galaxy and three stars.
The case of the 'Be' star illustrates how the AXIS investigators have proceeded in their work. During its calibration and performance verification phase, XMM-Newton has used a Crab-like supernova remnant (SNR; The extended emission region remaining after the explosion of a star at the end of its 'life cycle') named G21.5-0.9 as a calibration source. An EPIC-MOS image revealed an unresolved object situated about 2 arcmin to the south-west of the heart of the SNR (lower right in the picture).
Although this object appears to be embedded in the remnant's outer halo, the X-ray measurements of the absorption show that it is in fact a foreground object. (X-ray absorption in the plane of the Galaxy is a good indication of an object's distance.)
An optical observation was made using the 4.2m William Herschel Telescope at the Roque de los Muchachos Observatory in the Canary Islands - one of the appropriately named Isaac Newton Group of Telescopes! - and its ISIS high-efficiency spectrometer.
The optical spectrum obtained is that of a "Be" star, a spectral type "B" star which exhibits hydrogen emission lines. This class of star often consists of a rapidly rotating and interacting pair of stars. Further analysis suggests that the newly identified XMM-Newton source is a low-luminosity 'X-ray binary', an accreting binary containing either a neutron star or white dwarf.
Another exciting member of this first batch of XMM-Newton sources identified by AXIS is a quasi-stellar object which has prominent absorption troughs in its optical spectrum. This absorption is produced by material that the central engine of the QSO (a supermassive black hole to which matter is falling in the form of an accretion disk) has ejected at a speed close to the speed of light.
AXIS research scientist Xavier Barcons comments: "Broad-Absorption-Line (BAL) QSOs such as this one have only very rarely been discovered by their X-ray emission. The reason is that the material ejected at relativistic speed also absorbs most of the X-rays produced by the QSO, especially those with lowest photon energies. Here we see the enormous power of XMM-Newton at work, as its impressive ability to collect higher energy X-ray photons has enabled the discovery of this object".
The deep X-ray sky is populated by sources surrounded by vast amounts of absorbing material. It is believed that normal QSOs - by normal is meant that we have a clean view of their central engine - are a minority among the most general absorbed or obscured sources.
These obscured sources are the key to understanding the, perhaps, most important milestone in X-ray astronomy: the origin of the cosmic X-ray background. This radiation which fills the Universe at X-ray wavelengths, is thought to arise mainly in absorbed QSOs, which have been so far elusive to direct detection with previous X-ray telescopes.
Dr Mike Watson, XMM-Newton Survey Scientist based at Leicester University said: "The follow-up of serendipitous X-ray sources is one of the main tasks of the SSC. Only a few months after the start of normal science operations we are delighted to be able to make such good progress with this aspect of our work. This is due, of course, to our success in getting the AXIS programme approved in the current year. Although this is a long-term programme that will take several years to complete, it's very encouraging to the team that some real surprises have already emerged."
XMM-Newton is already building up what promises to be a very comprehensive survey of the X-ray Universe. And if presently known classes of celestial objects will probably dominate what is seen, further surprises are in store and the possibility exists of discovering entirely new types of X-ray sources.