Large-scale variations of the cosmic X-ray background and the X-ray emissivity of the local Universe

¹ Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany e-mail: mikej@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
³ Centre d'Étude Spatiale des Rayonnements, 31028 Toulouse, France

Received: 17 September 2007
Accepted: 19 March 2008

Abstract

We study the cosmic X-ray background (CXB) intensity variations on large angular scales using the slew data from the RXTE observatory. We detect intensity variations up to ~2% on angular scales of 20-40°. These variations are partly correlated with the local large-scale structure, which allowed us to estimate the emissivity of the local Universe in the energy band 2-10 keV at (8±4)10³⁸ h₇₅ erg s^-1 Mpc^-3. The spectral energy distribution of the large-angular-scale variations is hard and is compatible with that of the CXB, which implies that normal galaxies and clusters of galaxies, whose spectra are typically much softer, do not contribute more than 15% to the total X-ray emissivity of the local Universe. Most of the observed CXB anisotropy (after exclusion of point sources with fluxes 10^-11 erg s^-1 cm^-2) can be attributed to low-luminosity AGNs.