INTEGRAL hard X-ray spectra of the cosmic X-ray background and Galactic ridge emission

摘要

Aims. We derive the spectra of the cosmic X-ray background (CXB) and of the Galactic ridge X-ray emission (GRXE) in the 20-200keV range from the data of the IBIS instrument aboard the INTEGRAL satellite obtained during the four dedicated Earth-occultation observations in early 2006. Methods. We analyze the modulation of the IBIS/ISGRI detectorcounts induced by the passage of the Earth through the field of view ofthe instrument. Unlike previous studies, we do not fix the spectralshape of the various contributions, but model instead their spatialdistribution and derive for each of them the expected modulation of thedetector counts. The spectra of the diffuse emission components areobtained by fitting the normalizations of the model lightcurves to theobserved modulation in different energy bins. Because of degeneracy, weguide the fits with a realistic choice of the input parameters and aconstraint for spectral smoothness. Results. The obtained CXB spectrum is consistent with the historic HEAO-1 results and falls slightly below the spectrum derived with Swift/BAT.A 10% higher normalization of the CXB cannot be completely excluded,but it would imply an unrealistically high albedo of the Earth. Thederived spectrum of the GRXE confirms the presence of a minimum around80keV with improved statistics and yields an estimate of 0.6for the average mass of white dwarfs in the Galaxy. The analysis alsoprovides updated normalizations for the spectra of the Earth's albedoand the cosmic-ray induced atmospheric emission. Conclusions. This study demonstrates the potential of INTEGRALEarth-occultation observations to derive the hard X-ray spectra ofthree fundamental components: the CXB, the GRXE and the Earth emission.Further observations would be extremely valuable to confirm our resultswith improved statistics. Key words: earth - Galaxy: disk - galaxies: active - diffuse radiation - X-rays: diffuse background - X-rays: general