The ROSAT-ESO Flux Limited X-ray Galaxy Cluster Survey (REFLEX II) I. Newly identified X-ray luminous clusters at z>=0.2

We report 19 intermediate redshift clusters newly detected in the ROSAT All-Sky survey that are spectroscopically confirmed. They form a part of 911 objects in the REFLEX II cluster catalogue with a limiting flux of 1.8\times10^12 erg/s/cm2 in the 0.1-2.4 keV ROSAT band at redshift z >= 0.2. In addition we report three clusters from the REFLEX III supplementary catalogue, which contains objects below the REFLEX II flux limit but satisfies the redshift constraint above. These clusters are spectroscopically followed-up by our ESO NTT-EFOSC2 campaigns for the redshift measurement. We describe our observing and data reduction methods. We show how X-ray properties such as spectral hardness ratio and source extent can be used as important diagnostics in selecting galaxy cluster candidates. Physical properties of the clusters are subsequently calculated from the X-ray observations. This sample contains the high mass and intermediate-redshift galaxy clusters for astrophysical and cosmological applications.