has observed the large Spitzer South Pole Telescope Deep Field (SSDF), with its Infrared Array Camera (IRAC) in channel 1, 3.6$\mu$m, and 2, 4.5$\mu$m. This field fully covers XXL-S. See @2014ApJS..212...16A. Similar observations (PI: M. Bremer) were acquired over XXL-N.

-   The Wide-field Infrared Survey Explorer (WISE) mission observed the whole sky in four mid-IR (MIR) bands: W1=3.4$\mu$m, W2=4.6$\mu$m, W3=12$\mu$m, W4=22$\mu$m [@2010AJ....140.1868W]. We used the ALLWISE data release.

-   The GALEX satellite surveyed the entire sky [@2005ApJ...619L...7M] in two ultraviolet bands, 1344-1786 Å (far-UV, FUV) and 1771-2831 Å (near-UV, NUV), with various surveys: All-Sky Imaging Survey (AIS), Medium Imaging Survey (MIS), and guest investigator (GI) programmes in both XXL areas, and the Deep Imaging Survey (DIS) in XXL-N only.

Counterpart association\[SecIdent\]
-----------------------------------

In order to associate X-ray sources with potential counterparts at other wavelengths, we first generated multiwavelength counterpart sets, matching the individual primary photometric catalogues (one per survey per band: 28 in XXL-N and 19 in XXL-S, see Table \[TabSurv\]) among them (using a matching radius of 0.7  for the optical and IR catalogues where the image PSF is small, and 2  for GALEX, IRAC, and WISE) as described in . For the further association with X-ray sources, we 