A deep ROSAT survey – VIII. X-ray detection of the evolved galaxy cluster 0055–279

On a deep (49-ks) ROSAT pointing near the South Galactic Pole, we detect an extended X-ray source, with hard X-ray colours, at a position corresponding to the 0055–279 galaxy cluster. Previous optical investigations indicated that this cluster is at a high redshift of z = 0.561 and possesses a much higher fraction of blue spiral galaxies than do local rich clusters. The observed X-ray luminosity of LX(0.5–2.0 keV)≃1044 erg s–1 is only slightly lower than the LX(0.5–2.0 keV)≃1.5x1044 erg s–1 luminosity expected for a local cluster of the same richness. Our results therefore support recent findings that Butcher–Oemler clusters, observed at earlier stages of evolution when they still contain blue spiral galaxies, produce high X-ray luminosities similar to those of present-day rich clusters. We also find that the spectral energy distribution of the 0055–279 cluster shows evidence of stronger absorption at soft energies than would be expected from Galactic hydrogen, suggesting that there is significant (NH> 1020 cm–2) neutral hydrogen absorption within the cluster itself. This may indicate the presence of a cooling flow. The detection of a high-redshift cluster with the observed flux of 0055–279 in our survey area and the numbers of z> 0.5 clusters detected on ROSAT images by Castander et al. are consistent with no evolution of the cluster X-ray luminosity function to z~0.6.