ROSAT Observations of Isolated Neutron Stars.

We report on the serendipitous detection of a bright, very soft ROSAT X-ray source, RX J1856.5-3754, near the R CrA molecular cloud. Walter, Wolk, & Neuhäuser (1996) very recently have argued that this is a nearby isolated neutron star. As the source has also been detected with the Einstein Observatory slew survey, the mean X-ray count rate (3.6 cts/sec, as observed with ROSAT) remained steady over a 14 year interval. The X-ray spectrum is close to that of a blackbody with kT=57 eV. The absorption column places it foreground to the R CrA molecular cloud, i.e. at a distance of less than 120 pc. There is no optical counterpart to V≈23, meaning that the X-ray to optical flux ratio exceeds 7000. The emitting area is 480 km² at a distance of 100 pc. From pulsar birthrates and the inferred number of supernova required to account for the heavy element abundance in the Galaxy, it is likely that the Galaxy is populated with about 10⁸-10⁹ neutron stars, of which only a small fraction has been discovered, yet. It has been proposed that a few thousand old, isolated (i.e., those not in binary systems) neutron stars may be detectable in the ROSAT All-Sky Survey as hot thermal sources, either from a cooling surface or from accretion of interstellar material. However, this is only the second promising candidate.