X-ray Surveys and the X-Ray background

Supervisors: Dr G C Stewart, Dr R S Warwick, Dr M C Watson.

Although more than 30 years have passed since the discovery of the cosmic X-ray background (XRB), the origin of this radiation field is still keenly debated. The observed isotropy of the XRB at energies above 3 keV establishes an extragalactic origin for the hard XRB emission. Current interest centres on identifying the types of discrete X-ray source which through their integrated light contribute significantly to the XRB and give rise to its observed 40-keV thermal-bremsstrahlung spectral form.

Recent source surveys conducted by ROSAT have provided an almost complete inventory of the constituents of the soft X-ray background (i.e. below 2 keV). Current population synthesis models imply that the background is the ``echo'' of mass accretion onto supermassive black holes, integrated over cosmic time. These X-ray studies impact directly on questions of cosmological interest such as Do all large galaxies contain supermassive black holes? and Were these supermassive objects formed before or after most of the stars in the Universe?

In the future crucial information will come from deep X-ray surveys carried out in the harder energy bands, where the maximum of the energy density of the X-ray background resides. Although preliminary studies have now conducted using the Japanese ASCA satellite, definitive pencil-beam surveys are planned in the 2-10 keV band by AXAF and XMM.

At Leicester, there has been a consistent interest in deep X-ray surveys and the XRB since the days of the early Ariel V All-Sky Survey. More recently Ginga, ROSAT and ASCA observations have been employed in on-going studies of X-ray source populations at faint levels and in defining the global spectral and spatial properties of the XRB. We expect to carry this interest into the new era and to this end we are closely involved in several major extragalactic survey programmes proposed for XMM.

Page maintained by Keith Sohl (kbs@star.le.ac.uk)