Solving the X-ray background mystery
2 November 2000The long standing uncertainty over the origins of the X-ray background (XRB) may perhaps be a thing of the past. XMM-Newton observations are backing up the view that this faint glow of X-rays pervading the cosmos comes essentially from many individual but so-far undetected celestial objects and not just from the hot environment within galaxies.
To date, it has been difficult to account for all this X-ray background. The question has gradually been elucidated by different X-ray observatories, considering several ranges of XRB emission. In the "soft" X-ray domain (between 0.5-2keV) ROSAT had resolved, or accounted for three-quarters of the XRB flux. NASA's Chandra had resolved a further 10% in this band, leaving but some 10% that could still originate from the hot gas within our Galaxy or clusters of galaxies.
Progress in the higher-energy component has been more difficult. Between 2-10 keV, a quarter of this background emission, has been resolved in ASCA and BeppoSAX surveys, and more than 60% of the XRB is accounted for in this range when recent Chandra observations are included. In the very hard range (5- 10 keV) BeppoSAX has resolved 30% of the XRB. XMM-Newton's contribution, with its unprecedented sensitivity in the hard X-ray band, was eagerly awaited.
In five separate orbits during April-May, XMM-Newton peered in the direction of the Lockman Hole for over 52 hours. This region of the sky, in the northern hemisphere constellation Ursa Major, is well studied at all wavelengths. Observations are made easier because absorbing material such as dust and galactic hydrogen is at its most tenuous.
With the help of spectral diagnostics and sets of previously identified ROSAT objects in the same field of view, XMM-Newton could immediately single out a significant number of new sources. For instance, with 61 sources detected in the 5-10 keV bands, the XMM observations are the deepest X-ray survey ever, around 20 times more sensitive than the previous BeppoSAX observations.
Combining the images from all three EPIC cameras provides a "real-colour" representation of the sources, coded according to their hardness. The red, green and blue colours refer respectively to the 0.5-2, 2-4.5 and 4.5-10keV energy bands. Several diffuse sources with red colours are X-ray clusters of galaxies already identified by ROSAT data. But XMM-Newton clearly reveals a number of green and blue objects and these correspond to obscured faint sources.
These observations fit previously elaborated models which explain the hard spectrum component of the X-ray background by the presence of Active Galactic Nuclei (AGN) with X-ray emitting accretion disks which are seen through obscuring gas and dust clouds. According to these models most of the AGN spectra and about 80% of the light from the disks is absorbed by gas and dust clouds that may be feeding the AGN.
Analysis of this data, which resolves about 60% of the 5-10 keV X-ray background, tends to indicate that many of these sources are probably obscured Seyfert galaxies and quasars. Seyfert galaxies, which are galaxies with small bright nuclei and broad emission lines, have AGNs that produce strong radiation stemming from accretion disks around massive black holes.
Guenther Hasinger, of the Astrophysikalisches Institut Potsdam led the team of astronomers who analysed the data from the observation.
"We have here evidence that in the heart of almost every larger galaxy resides a supermassive black hole. It has long been believed that the X-ray background is the radiation emitted from the vicinity of these black holes in their growing phase. However, most of this energy is obscured by intervening dust clouds so that so far we could not see the hidden objects. The new XMM observations now finally show us the most extremely obscured black holes."
According to Hasinger, the XRB mystery will remain a slowly converging process which will still take several years to solve. The optical observations will be a bottleneck. Intergalactic matter will have to be detected at some point, he believes, and is expected to contribute to about 10% of the total X-ray background.
Meanwhile XMM-Newton is making its contribution. The Lockman Hole XRB investigation is the deepest X-ray survey ever. In the very hard 5-10 keV band, pioneered by BeppoSAX, the European X-ray observatory has entered new territory, going more than an order of magnitude deeper that its predecessor.
Acknowledgements to the first author of the paper:
(Astrophysikalisches Institut Potsdam - AIP)
"XMM-Newton Observation of the Lockman Hole" to be published in Astronomy and Astrophysics.