Summary of WFC identification assessment

The data used:

The results:

• Fraction of XMM sources with optical counterparts which are brighter than a certain X-ray flux level, e.g.
  (No. of ID >fx) / (No. of sourcs >fx).
  See the figures
  The curves represent the results using all the 5 optical bands (total) as well as using single optical band data.
  Different optical magnitude limits are also applied.
• The same as above but using X-ray sources detected by at least two detectors (i.e. 2 of PN, MOS1, and MOS2).
  See the figures
  
• Fraction of XMM sources with optical counterparts which are brighter than a certain optical magnitude, e.g.
  (No. of ID < mag) / (No. of all Xray sources).
  See the figures
  The curves represent the results using all the 5 bands (total) as well as using single band data.
  Different X-ray flux limits are also applied.
• The same as above but using X-ray sources detected by at least two detectors.
  See the figures
  

Summary of the results, notes and problems:

• For typical X-ray flux limits of 10^-15 erg/cm2/s of XMM fields, the apparent identification rate, by using the total 5 bands, reaches about 60% down to the current limiting magnitudes of around 23 (24 for g').
• The flattening of the curves in the ID rate vs. magnitude diagrams for the sample is mainly due to the field-by-field variation of the limiting magnitudes. (see histogram of 5-sigma magnitude limits for various bands for the whole XID imaging fields). The curves for individual fields show no flattening but cutoff at the limiting magnitudes. See results for individual fields (including fields with data in less than 5 bands.)
• i-band data give the highest overall identification rate, followed by r' and g'-band data. This can be seen in both ID-fraction vs. X-ray flux and vs. optical magnitude plots. It is interested to note that, in the bright magnitude range (< about 21 mag) of the ID-fraction vs. magnitude plot, the z-band curve follows most closely to the curve for the total bands (i.e. whichever the brighest band). This means that the z-band magnitudes of sources, if detected, tend to be the bightest in terms of the magnitude value amongest all the optical bands. (See the distribution of magnitudes -- cumulative [detected in any bands] [detected in all 5 bands] and differential)---for candidate optical counterparts. In general the z-band data do not go deep (see histogram of depth)
  
• Taking X-ray sources detected by at least two detectors raises the overall ID rate to 80% down to 1.E-15erg/s/cm^2 at the current magnitude limits, for the total band data as well as individual bands. This is mainly due to the exclusion of many sources at faint flux levels (low ID rate). See X-ray source counts plot (this is the plot for a larger sample. To be replaced later).
• We also note that the contribution of spurious optical object plays an increasing role towards the faint magnitude end, which is under further investigation. For example, about half of the IDs detected in U-band only are classified as noise (see figure). apparent gap between the ID fraction of the total band data and the i-band data There are also other minor problems such as that one source is treated as two in the merged catalogues. So the true ID fraction curves are expected to move slightly down. But the correction should not exceed 10%.

Other relevant information

• seeing distribution of all XID fields