What is the period of low X-ray emission called ?
Deep surveys with X-ray telescopes , such as the Chandra X-ray Observatory , have demonstrated that around 80 % of the cosmic X-ray background is due to resolved extra-galactic X-ray sources , the bulk of which are unobscured ( " type-1 " ) and obscured ( " type-2 " ) active galactic nuclei ( AGN ) .