1997A&A...326..662H

We report the discovery of RX J0720.4-3125, a soft X-ray source showing 8.391s pulsations, from the ROSAT all-sky survey. The relative constant long-term X-ray intensity, the blackbody-like spectrum with little photo-electric absorption and the limit for f_x/f_v of more than 500 are consistent with an isolated neutron star accreting from the interstellar medium (all very similar to RX J1856.5-3754, so far the best candidate). We estimate the magnetic field strength of the neutron star to be less than 10¹⁰G. RX J0720.4-3125 may have emerged from common envelope evolution of a high mass X-ray binary. The final neutron star is expected to be close the galactic plane, have a low space velocity and a low magnetic field, accreting interstellar matter very effectively. In this case RX J0720.4-3125 would not belong to the expected large group of old neutron stars evolved from single stars. The low derived magnetic field strength and the pulse period are however also compatible with an old (10⁹yr) neutron star in which case we see a neutron star from the very low end of the velocity distribution. The low number of isolated neutron star candidates from the ROSAT all-sky survey remains to be explained.