4 The survey

The X-ray survey of the SMC covers a field of $8.95\ {\rm deg}^2$ considering the used 45' field of view of each pointed observation. From the merged (0.1-2.4) keV image corrected for exposure we derive a total count rate of 53.2 s^-1. The mean exposure of this image is 7500 s and the maximum 22000 s. We created a merged background image of the same field and subtracted the background rate image from the count rate image. We find an excess rate of $6.1~\pm~0.1$ s^-1. In comparison the total source count rate we derive from our catalog of 248 point-like and moderately extended sources is $6.9~\pm~0.3$ s^-1. Both rates are within the 2$\sigma$ errors identical and are in agreement with the finding from the ROSAT All-Sky Survey (RASS) data that the residual count rate of an extended emission component (a hot gas) is small. Kahabka & Pietsch (1993) find an excess count rate of $\sim 0.7$ s^-1 for the $8\times8\ {\rm deg}^2$ RASS field after subtracting a smooth background component and the source contribution. This result may be compared with the finding of Irwin & Sarazin (1998). They found that the integrated X-ray emission and colors of X-ray faint galaxies (group 1 galaxies) to which also M 31 and the SMC are belonging can be explained as the integrated emission from (low-mass) X-ray binaries.