X-Ray Line-emitting Objects in XMM-Newton Observations: The Tip of the Iceberg
10 December 2004
Every time a new window on the cosmos, or more simply a new region of the parameter space has been explored, new and often unexpected results have been obtained. During most of 2004, a group of scientists at the Brera Astronomical Observatory, led by Tommaso Maccacaro has developed a new algorithm to detect astronomical sources in the images obtained by XMM-Newton, the most powerful X-ray telescope ever built. This technique has already allowed the discovery of new interesting objects.The idea behind the new algorithm is simple but powerful and it is based on the extension to the energy axis of the usual source detection techniques that are typically restricted to the spatial coordinates. In this way it is possible to discover sources characterized by a peculiar X-ray emission localized in a very narrow energy range; in other words a line emission. These sources have been named XLEO (X-ray Line Emitting Objects).
Among the sources that are expected to show up as a result of this search there is a particular kind of rather elusive active galaxies. Active galaxies (AGNs and Quasars) are the subject of years of research by the Brera Observatory's team (composed, besides Tommaso Maccacaro, by Valentina Braito, Alessandro Caccianiga, Roberto Della Ceca and Paola Severgnini). It is widely accepted by the scientific community that active galaxies and quasars are the site of rather extreme physical phenomena, due to the presence of supermassive black holes in their nucleus. The census of these objects however is still incomplete since quite often the emitted radiation is blocked by the presence of large amounts of absorbing material surrounding the black hole. It is thus extremely difficult, in these cases, to discover the presence of the active nucleus. On the other hand these elusive active galaxies are quite important since they are thought to be responsible for most of the diffuse x-ray background radiation in the "hard" x-rays.
A key to recognize such active galaxies is the presence of an emission line in their x-ray spectrum (the Fe Kα line at 6.4 keV). The new technique developed at the Brera Observatory allows to find specifically these "obscured" AGNs that some believe may well be a "silent majority" among the various kinds of active galaxies.
Indeed the first results obtained by Maccacaro and collaborators (Ap.J. Letters V617, 2004) are extremely interesting and encouraging. About a dozen XMM-Newton observations have been analysed and a number of XLEO candidates have been found. Among these, three are very likely extremely obscured active galaxies at cosmological distances, with a revealing prominent emission line in their spectrum. The emission line (that is emitted at 6.4 keV) is seen redshifted to a lower energy and this allows, directly from the X-ray data, a determination of the distance of the galaxy. This, by itself, is an exceptional result that derives from the detection of the X-ray emission line. Usually, to determine the distance of newly discovered X-ray sources it is necessary to obtain an optical spectrum and often only the largest of the world's telescopes are powerful enough to make a conclusive observation. Now, thanks to the power of XMM-Newton and to the capability to discover Line-Emitting X-ray sources the cosmological distance of these objects can be determined directly from the X-ray data.