The Serendipitous XMM-Newton Cluster Athens Survey (SEXCLAS): sample selection and the cluster logN-logS

In this paper we serendipitously identify X-ray cluster candidates using XMM-Newton archival observations complemented by five-band optical photometric follow-up observations (r~ 23 mag) as part of the X-ray Identification (XID) programme. Our sample covers an area of ~2.1 deg² (15 XMM-Newton fields) and comprises a total of 21 (19 serendipitous + two target) extended X-ray sources to the limit f_x (0.5-2 keV) ~ 6 × 10^-15 erg s^-1 cm^-2, with a high probability (>99.9 per cent) of being extended on the XMM-Newton images. Of the 21 X-ray clusters, 14 are detected for the first time while seven are spectroscopically confirmed in the literature. Exploiting the optical data available for these fields we discover that >~68 per cent of the X-ray cluster candidates are associated with optical galaxy overdensities. We also attempt to constrain the redshifts of our cluster candidates using photometric methods. We thus construct the photometric redshift distribution of galaxies in the vicinity of each X-ray selected cluster candidate and search for statistically significant redshift peaks against that of the background distribution of field galaxies. Most of our clusters have photometric or spectroscopic redshifts in the range 0.4 < z < 0.6. Comparison of photometric with spectroscopic redshift estimates for the confirmed clusters suggests that our simple method is robust out to z~ 0.5. For clusters at higher z, deeper optical data are required to estimate reliable photometric redshifts. Using the sample of the 19 serendipitous X-ray selected cluster candidates, we estimate their surface density down to f_x (0.5-2 keV) ~ 6 × 10^-15 erg s^-1 cm^-2 and find it to be in fair agreement with previous and recent studies.