The isolated neutron star candidate RBS 1223 ( 1 RXS

In the ROSAT Bright Survey (RBS) we have almost completely optically identified the brightest ∼2000 high-galactic latitude sources from the ROSAT All-Sky Survey Bright Source Catalogue (1RXS). A small number of sources has empty X-ray error circles on optical images. ROSAT HRI follow-up observations of RBS1223 (=1RXS J130848.6+212708), a soft object with extreme X-ray to optical flux ratio, have confirmed a relatively bright X-ray source, whose position could be determined to an accuracy of 1.6 arcsec (90%) due 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 ∼ 26 m. With an X-ray to optical flux ratio of log(f X /f opt) > 4.1 this object is almost certainly an isolated neutron star, similar to the two so far best-known examples RX J1856.4–3754 and RX J0720.4–3125. We discuss limits on the number of similar objects in the RBS catalogue.