2007A&A...468..571G

Almost the totality of the bright foreground sources in the WMAP Cosmic Microwave Background (CMB) maps are blazars, a class of sources that show usually also X-ray emission. However, 23 objects in a flux-limited sample of 140 blazars of the WMAP catalog (first year) were never reported before as X-ray sources. We present here the results of 41 Swift observations which led to the detection of all these 23 blazars in the 0.3-10keV band. We conclude that all micro-wave selected blazars are X-ray emitters and that the distribution of the micro-wave to X-ray spectral slope (α_µx) of LBL blazars is very narrow, confirming that the X-ray flux of most blazars is a very good estimator of their micro-wave emission. The X-ray spectral shape of all the objects that were observed long enough to allow spectral analysis is flat and consistent with inverse Compton emission within the commonly accepted view where the radiation from blazars is emitted in a Sychrotron-Inverse-Compton scenario. We predict that all blazars and most radio galaxies above the sensitivity limit of the WMAP and of the Planck CMB missions are X-ray sources detectable by the present generation of X-ray satellites. An hypothetical all-sky soft X-ray survey with sensitivity of approximately 10^–15erg/cm2/s would be crucial to locate and remove over 100000 blazars from CMB temperature and polarization maps and therefore accurately clean the primordial CMB signal from the largest population of extragalactic foreground contaminants.