We carried out the first wide-area unbiased survey with the ASCA satellite in the 0.7–10 keV band around a north Galactic-pole region covering a continuous area of 7 deg ^ { 2 } ( Large Sky Survey ; LSS ) .
To make the best use of ASCA capability , we developed a new source-detection method where the complicated detector responses are fully taken into account .
Applying this method to the entire LSS data independently in the total ( 0.7–7 keV ) , hard ( 2–10 keV ) , and soft ( 0.7–2 keV ) band , we detected 107 sources altogether with sensitivity limits of 6 \times 10 ^ { -14 } ( 0.7–7 keV ) , 1 \times 10 ^ { -13 } ( 2–10 keV ) , and 2 \times 10 ^ { -14 } erg s ^ { -1 } cm ^ { -2 } ( 0.7–2 keV ) , respectively .
A complete list of the detected sources is presented .
Based on detailed studies by Monte Carlo simulations , we evaluated effects of the source confusion and accurately derived Log N - Log S relation in each survey band .
The Log N - Log S relation in the hard band is located on the extrapolation from the Ginga and HEAO1 results with the Euclidean slope of -3 / 2 , while that in the soft band is consistent with the results by ROSAT .
At these flux limits , 30 ( \pm 3 ) % of the CXB in the 0.7–7 keV band and 23 ( \pm 3 ) % in the 2–10 keV band have been resolved into discrete sources .
The average spectrum of faint sources detected in the total band shows a photon index of 1.63 \pm 0.07 in the 0.7–10 keV range , consistent with the comparison of source counts between the hard and the soft energy band .
Those detected in the hard band show a photon index of 1.49 \pm 0.10 in the 2–10 keV range .
These spectral properties suggest that contribution of sources with hard energy spectra become significant at a flux of 10 ^ { -13 } erg s ^ { -1 } cm ^ { -2 } ( 2–10 keV ) .
The most plausible candidates are type-II AGNs , as indicated by on-going optical identifications .