The isolated neutron star candidate RBS1223 (1RXS J130848.6+212708)

In the ROSAT Bright Survey (RBS) we have almost completely optically identi fied the brightest similar to 2000 high-galactic latitude sources from the ROSAT All-Sky Survey Bright Source Catalogue (1RXS). A small number of sour ces has empty X-ray error circles on optical images. ROSAT HRI follow-up ob servations of RBS1223 (=1RXS J130848.6+212708), a soft object with extreme X-ray to optical flux ratio, have confirmed a relatively bright X-ray sourc e, whose position could be determined to an accuracy of 1.6 arcsec (90%) du e to the presence of a nearby, X-ray detected bright star. Deep Keck R- and B-band images of the field were taken, but the refined X-ray error circle remains empty to a limiting magnitude B similar to 26(m). With an X-ray to optical flux ratio of log(f(x)/f(opt)) > 4.1 this object is almost certainl y an isolated neutron star, similar to the two so far best-known examples R X J1856.4-3754 and RX J0720.4-3125. We discuss limits on the number of simi lar objects in the RES catalogue.