DISCOVERY OF AN ARC SYSTEM IN THE BRIGHTEST ROSAT CLUSTER OF GALAXIES

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, RWJ1347.5-11 45. Its luminosity is (6.2 +/- 0.6) 10(45)erg s(-1) (in the range 0.1- 2.4 keV). The arcs are most probably gravitationally lensed images of background galaxies. They were found serendipitously during our ongoin g large-scale redshift survey of X-ray clusters detected by the ROSAT All Sky Survey, The arcs are almost opposite to each other with respec t to the cluster centre, with a distance from it of about 35 '' (= 240 h(50)(-1) kpc), a radius that enables the probing of a rather large cl uster volume. In this Letter we limit ourselves to the discussion of t he general optical and X-ray features of this cluster and to the poten tial implications of the gravitational arcs. A more detailed discussio n of the different mass estimates and of the cosmological implications for this exceptional object are left for future work based on more ac curate optical and X-ray data, which are currently being collected.