RX J0720.4-3125 - STRONG EVIDENCE FOR AN ISOLATED PULSATING NEUTRON-STAR

We report the discovery of RX J0720.4-3125, a soft X-ray source showin g 8.391 spulsations, from the ROSAT all-sky survey. The relative const ant 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 50 0 are consistent with an isolated neutron star accreting from the inte rstellar medium (all. very similar to RX J1856.5-3754, so far the best candiaate). We estimate the magnetic field strength of the neutron st ar to be less than 10(10) G. RXJ0720.4-3125 may have emerged from comm on envelope evolution of a high mass X-ray binary. The final neutron s tar is expected to be close the galactic plane, have a low space veloc ity and a low magnetic field, accreting interstellar matter very effec tively. 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 de rived magnetic field strength and the pulse period are however also co mpatible with an old (10(9) yr) neutron star in which case we see a ne utron star from the very low end of the velocity distribution. The low number of isolated neutron star candidates from the ROSAT all-sky sur vey remains to be explained.