INTEGRAL observations of the cosmic X-ray background in the 5–100 keV range via occultation by the Earth

¹ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany
³ ESA, ESTEC (SCI-SA), Keplerlaan 1, 2201, AZ Noordwijk, The Netherlands
⁴ IASF/INAF, via Fosso Del Cavaliere 100, 00133 Rome, Italy
⁵ Service d'Astrophysique, DAPNIA/DSM/CEA, 91191 Gif-sur-Yvette, France
⁶ APC-UMR 7164, 11 place M. Berthelot, 75231 Paris, France
⁷ Centre d'Étude Spatiale des Rayonnements, UPS, CNRS, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
⁸ Max-Planck-Institut fuer extraterrestrische Physik, Giessenbachstr., 85748 Garching, Germany
⁹ Danish National Space Center, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹⁰ INTEGRAL Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland
¹¹ Centro de Astrobiología (CSIC-INTA), POB 50727, 28080 Madrid, Spain
¹² Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), POB 50727, 28080 Madrid
¹³ Goddard Space Flight Center, Bldg. 2, Room 245, Code 661.0, Greenbelt, MD 20771, USA
¹⁴ ISOC, ESAC/ESA, Apartado 50727, 28080 Madrid, Spain
¹⁵ ESA-ESOC, Mission Operations Department, Robert-Bosch Str. 5, 64293 Darmstadt, Germany

Received: 11 August 2006
Accepted: 8 February 2007

Abstract

Aims. We study the spectrum of the cosmic X-ray background (CXB) in energy range ~5-100 keV.

Methods. Early in 2006 the INTEGRAL observatory performed a series of four 30 ks 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.

Results.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 normalization is ~10% higher. This difference in normalization can (at least partly) be traced to the different assumptions on the absolute flux from the Crab Nebulae. 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 CXB origin.