For each XMM source in a given field, the nearest WFC object was found using a rigorous great circle distance calculation and was declared a match if within a specified search radius.
Histograms of total, RA and DEC offsets out to a larger radius (30''), offset scatter plots and spatial distribution plots of XMM sources and offsets overlaid with the optical or IR image boundaries were produced. These are available in the following directory for each field and source_list:
http://www.ast.cam.ac.uk/
xmmssc/xidops/field/source_list/html/match/
(see document SSC--CAM--TN--0002 for a full description of the directory structure in use for SSC XID work).
Figure: Histograms of a) RA offset and b) Dec offset in arcsecs for the
Lockman Hole field XMM data matched with merged WFC data.
The horizontal dashed line shows the background level.
Fig. 
is a typical example of the results.
A Gaussian was fitted to each RA and Dec offset histogram and the optimal
search radius was taken to be 
.
For XMM / WFC or XMM / CIRSI matching this optimal radius was found to be 5
arcsecs, for XMM / APM matching 6 arcsecs.
is significantly reduced if only objects detected in the i--band are
included, which prompted us to perform the likelihood ratio analysis on the
unmerged WFC i--band data (see section ).