  The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters
using all publicly available data in the XMM-Newton Science Archive. Its main
aims are to measure cosmological parameters and trace the evolution of X-ray
scaling relations. In this paper we present the first data release from the XMM
Cluster Survey (XCS-DR1). This consists of 503 optically confirmed,
serendipitously detected, X-ray clusters. Of these clusters, 255 are new to the
literature and 356 are new X-ray discoveries. We present 464 clusters with a
redshift estimate (0.06 < z < 1.46), including 261 clusters with spectroscopic
redshifts. In addition, we have measured X-ray temperatures (Tx) for 402
clusters (0.4 < Tx < 14.7 keV). We highlight seven interesting subsamples of
XCS-DR1 clusters: (i) 10 clusters at high redshift (z > 1.0, including a new
spectroscopically-confirmed cluster at z = 1.01); (ii) 67 clusters with high Tx
(> 5 keV); (iii) 131 clusters/groups with low Tx (< 2 keV); (iv) 27 clusters
with measured Tx values in the SDSS `Stripe 82' co-add region; (v) 78 clusters
with measured Tx values in the Dark Energy Survey region; (vi) 40 clusters
detected with sufficient counts to permit mass measurements (under the
assumption of hydrostatic equilibrium); (vii) 105 clusters that can be used for
applications such as the derivation of cosmological parameters and the
measurement of cluster scaling relations. The X-ray analysis methodology used
to construct and analyse the XCS-DR1 cluster sample has been presented in a
companion paper, Lloyd-Davies et al. (2010).
