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The X-ray background
Xavier Barcons
Instituto de Fsica de Cantabria (CSIC-UC), Spain
The cosmic X-ray background (XRB) above photon energies of
 2 keV is known to be of extragalactic origin. Deep X-ray sur-
veys carried out with ROSAT (Lehmann et al 2000, A&A, 354,
35) and more recently Chandra (Mushotzky et al 2000, Nat, 404,
459) indicate that virtually all the XRB originates at redshifts
z > 1 in galaxies containing some type of Active Galactic Nucleus.
Large-scale structure in the Universe at these redshifts produces
excess spatial uctuations in the XRB, which could be measured
by future missions. The link between the present-day power spec-
trum of density uctuations and the XRB uctuations induced by
large-scale structure at z  1 2 depends basically on the bias
parameter of the cosmic X-ray sources and on the cosmological
parameters
m
and
 . Simulations of the high-galactic latitude
X-ray sky show that on scales of  1 deg 2 the XRB uctuations
due to large-scale structure are of the order of  1%, with a strong
dependence on the above parameters (Barcons, Fabian & Carrera,
1998, MNRAS, 293, 60). The sensitivity for the measurement of
such a quantity is well within reach of proposed or planned all-sky
survey X-ray missions. The XRB can therefore probe the cosmo-
logical parameters at an epoch where most of the stars formed
and where most of the energy output provided by black holes was
released (Franceschini et al, 1999, MNRAS, 310, L5).
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