The 2XMMi/SDSS Galaxy Cluster Survey I. The first cluster sample and X-ray luminosity-temperature relation

We present a catalogue of X-ray selected galaxy clusters and groups as a first release of the 2XMMi/SDSS Galaxy Cluster Survey. The survey is a search for galaxy clusters detected serendipitously in observations with XMM-Newton in the footprint of the Sloan Digital Sky Survey (SDSS). The main aims of the survey are to identify new X-ray galaxy clusters, investigate their X-ray scaling relations, identify distant cluster candidates, and study the correlation of the X-ray and optical properties. In this paper, we describe the basic strategy to identify and characterize the X-ray cluster candidates that currently comprise 1180 objects selected from the second XMM-Newton serendipitous source catalogue (2XMMi-DR3). Cross-correlation of the initial catalogue with recently published optically selected SDSS galaxy cluster catalogues yields photometric redshifts for 275 objects. Of these, 182 clusters have at least one member with a spectroscopic redshift from existing public data (SDSS-DR8). We developed an automated method to reprocess the XMM-Newton X-ray observations, determine the optimum source extraction radius, generate source and background spectra, and derive the temperatures and luminosities of the optically confirmed clusters. Here we present the X-ray properties of the first cluster sample, which comprises 175 clusters, among which 139 objects are new X-ray discoveries while the others were previously known as X-ray sources. For each cluster, the catalogue provides: two identifiers, coordinates, temperature, flux [0.5-2] keV, luminosity [0.5-2] keV extracted from an optimum aperture, bolometric luminosity L(500), total mass M(500), radius R(500), and the optical properties of the counterpart. The first cluster sample from the survey covers a wide range of redshifts from 0.09 to 0.61, bolometric luminosities L(500) = 1.9x10(42)-1.2x10(45) erg s(-1), and masses M(500) = 2.3x10(13)-4.9x10(14) M(circle dot). We extend the relation between the X-ray bolometric luminosity L(500) and the X-ray temperature towards significantly lower T and L and still find that the slope of the linear L - T relation is consistent with values published for high luminosities.