High Galactic Latitude

    The optimal energy band to be used to define the sample of sources should:

• give a fair representation of the sky as seen by XMM
• avoid the very soft photons to minimize non-uniformities introduced by the different values of N_HI along the line of sight to the different pointings
• avoid the very high energy channels where, for most of the sources the balance between signal and noise (background) is unfavourable
• compromise between broad passband to favour throughput and narrow passband to minimize non-uniformities in the selection function due to different source spectra

    After considerable thought and simulation we adopted the 0.5 - 4.5 keV as the band to be used for the XID programme. We also decided that the X-ray images to use for the core programme should be such that the detection limit is considerably below the faintest source used.
    This ensures that we will have X-ray colour and extent information and the possibility of studying variability. In particular, X-ray colour information is very important to find absorbed AGN (see Della Ceca et al 1999).

    In order to avoid severe vignetting and confusion due to an excessive degradation of the PSF, we resort to X-ray sources found within a radius of 12' from the optical axis. The area covered per pointing for survey purposes is therefore ~0.13deg².

    The high galactic latitude core programme consists of 1000 sources each in three different flux classes (>10^-15, >10^-14, >10^-13 erg cm^-2 s^-1) given in the above passband. The relevance of these surveys is highlighted in graph 3 against existing and planned surveys.

    For the logN-logS function prediction we used the Hasinger et al (1998) source counts in the 0.5-2 keV band and the prediction from Cagnoni et al (1997) (based on the Comastri et al 1995 model) for the 2-10 keV band, both approximately converted to the 0.5-4.5 keV band. We also compute the AGN content of the fields in the various samples by carrying out simulations of the Comastri et al (1995) AGN. Note that the Medium Sample counts, and to a much larger extent the Faint Sample counts, are extrapolated as opposed to measured.

Last modified 26^th August 1999
Text based on AXIS proposal by X.Barcons and the SSC consortium.
Page designed by J.Verdon
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