RADIO-SILENT ISOLATED NEUTRON-STARS AS A NEW ASTRONOMICAL REALITY

As of today, seven X-ray sources have been tentatively identified as r adio-quiet, isolated neutron stars. The family appears to be a rapidly growing one, although not all the objects have been identified with t he same degree of certainty. The most convincing example of radio quie t pulsar is certainly Geminga, the neutron star nature of which, propo sed in 1983 on the basis of its similarity with the Vela pulsar, has b een firmly established with the discovery of its X and gamma pulsation . Four more neutron star candidates, originally found in the Einstein data, have been confirmed by ROSAT, which has added to the list two mo re entries. All this is not the result of an unbiased search. The seve n sources were not selected at random: four are inside supernova remna nts, an obvious place to search for isolated neutron stars, while the remaining three were singled out because of some peculiarity. Intense gamma-ray emission in the case of Geminga, very high X-ray counting ra te for RXJ185635-3754, or being the brightest unidentified source in t he Einstein medium sensitivity survey, MS 0317-6647. In spite of the l imited number of objects and of the observational biases, these seven radio quiet neutron star candidates add valuable pieces of information to the observational panorama of known pulsars. Their properties, inf erred from the X-ray emission, offer a coherent picture, pointing towa rds thermally emitting, cooling neutron stars.