1995A&A...299L...9S

We report the discovery of two bright arcs in what turns out to be the brightest X-ray cluster in the ROSAT band ever observed, RX J1347.5-1145. Its luminosity is (6.2±0.6)x10⁴⁵erg/s (in the range 0.1-2.4keV). The arcs are most probably gravitationally lensed images of background galaxies. They were found serendipitously during our ongoing large-scale redshift survey of X-ray clusters detected by the ROSAT All Sky Survey. The arcs are almost opposite to each other with respect to the cluster centre, with a distance from it of about 35" (=240/h₅₀kpc) , a radius that enables the probing of a rather large cluster volume. In this Letter we limit ourselves to the discussion of the general optical and X-ray features of this cluster and to the potential implications of the gravitational arcs. A more detailed discussion of the different mass estimates and of the cosmological implications for this exceptional object are left for future work based on more accurate optical and X-ray data, which are currently being collected.