When matching two source lists (on the basis of positional coincidence, as
described in section 
), one runs through each XMM source and
compares it with every WFC object in the field, computing the distance to it
along great circles. RA and Dec offsets and magnitude are stored for all WFC
candidates out to 50 arcsecs for each XMM source, and likelihood ratios are
computed as described in the previous section. We then evaluate reliabilities
according to equation .
This was done using WFC i--band source lists as this is usually the deepest band and data is consistently available for all fields with XMM data.
A database of cumulative magnitude counts was created for the WFC data: bins
are 0.25 magnitude wide and contain the number of objects per square arcsec
with magnitude less than or equal to the upper limit of the bin. The
area cover by one WFC pointing (4 CCD's) is taken to be around 0.2 square
degrees.
This was then used to estimate the background density as a function of magnitude for each candidate,