Astronomers using ESA's orbiting gamma-ray observatory, Integral, have taken an important step towards estimating how many black holes there are in the Universe.
An international team, lead by Eugene Churazov and Rashid Sunyaev, Space Research Institute, Moscow, and involving scientists from all groups of the Integral consortium used the Earth as a giant shield to watch the number of tell-tale gamma rays from the distant Universe dwindle to zero, as our planet blocked their view.

"Point Integral anywhere in space and it will measure gamma rays" - Pietro Ubertini, INAF, Italy and Principal Investigator on Integral's gamma-ray imager.

Most of those gamma rays do not come from nearby sources but from celestial objects so far away that they cannot yet be distinguished as individual sources. This distant gamma-ray emission creates a perpetual glow that bathes the Universe.

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Title: INTEGRAL observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth
Authors: E. Churazov, R. Sunyaev, M. Revnivtsev, S. Sazonov, S. Molkov, S. Grebenev, C. Winkler, A. Parmar, A. Bazzano, M. Falanga, A. Gros, F. Lebrun, L. Natalucci, P. Ubertini, J.-P. Roques, L. Bouchet, E. Jourdain, J. Knoedlseder, R. Diehl, C. Budtz-Jorgensen, S. Brandt, N. Lund, N. J. Westergaard, A. Neronov, M. Turler, M. Chernyakova, R. Walter, N. Produit, N. Mowlavi, J. M. Mas-Hesse, A. Domingo, N. Gehrels, E. Kuulkers, P. Kretschmar, M. Schmidt

We study the spectrum of the cosmic X-ray background (CXB) in energy range ~5-100 keV. Early in 2006 the INTEGRAL observatory performed a series of four 30ksec observations with the Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background(CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. The spectrum of the cosmic X-ray background in the energy band 5-100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalisation is ~ 10% higher. The CXB spectrum obtained by INTEGRAL agrees well with the measurements of RXTE/PCA, the latest recalculation of HEAO-1 measurements and ASCA and CHANDRA observations. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20-50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CBX origin.

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