Study of the Origin of the Cosmic X-ray Background from ASCA Deep Sky Observations

Akiko Yamashita

We have studied the origin of the cosmic X-ray background (CXB) through an arc-minute scale fluctuation of the CXB observed with the solid-state imaging spectrometer (SIS) onboard the ASCA satellite. For this study, we performed the deepest observation ever done (exposure of 400 ks with ASCA) in the energy range above 2 keV (up to 6 keV) in the Selected Area 57 (SA57) field. The surface brightness distribution of the CXB in the SIS field of view was obtained after eliminating the instrumental non-uniformity. The mean surface brightness of the observed field is (5.44±0.10±0.27)×10-8 erg s-1 cm-2 sr-1, and the 2-10 keV flux of the brightest source in the field is 5×10-14 erg s-1 cm-2. The surface brightness distribution shows significant fluctuation. This can be attributed to a Poisson fluctuation of a number of faint sources in the effective field of view determined by the point spread function of the X-ray telescopes onboard ASCA. The surface brightness of the dimmest region of the field is only 40% of the average CXB flux at most, and it can be concluded that the contribution of a number of discrete sources to the CXB flux in 2-6 keV should be more than 60%. This is the first observational confirmation that more than a half of the CXB emission comes from discrete sources in the energy band above 2 keV. We set a new constraint on the logN-logS relation down to the 2-10 keV flux of (1-2)×10-14 erg s-1 cm-2. The source count at the flux level of 1×10-14 erg s-1 cm-2 ranges between 200 and 750 deg-2. The average energy spectrum of the fluctuating component in 2-6 keV is very hard and its photon index is 1.26±0.12 which is slightly smaller than that of the average CXB. This strongly suggests that the sources composing the fluctuating component are responsible for making the CXB spectrum very hard. We also show some evidence which suggests that the faint sources detected with the ROSAT deep survey are just the counterparts in the 0.5-2 keV band of the sources responsible for the 2-6 keV fluctuation observed with ASCA. According to the optical identification of the ROSAT sources, most of the faint ROSAT sources are AGNs exhibiting broad emission lines in the optical band, having X-ray luminosity of 1043-1045 erg s-1 and located at z~ 0.5-3. Thus, a natural consequence of this study is that AGNs with broad optical lines, X-ray luminosity of 1043-1045 erg s-1, and z~0.5-3 would be the main constituents of the CXB. However, currently known nearby AGNs with broad optical lines generally have significantly softer X-ray spectra than that of the CXB, and it remains unsolved why the distant AGNs with broad optical lines have such a hard spectrum similar to the CXB.