1998A&A...333L..59M

Deep optical B band images of the ROSAT HRI error region of RX J0720.4-3125 reveal the presence of two faint stellar-like objects with B=26.1±0.25 and B=26.5±0.30. Exposures obtained through U, V and I filters are not sensitive enough to detect the two candidates and provide upper limits of U=24.9, V=23.2 and I=21.9. These new observations virtually establish that RX J0720.4-3125 is a slowly rotating, probably completely isolated neutron star. The absence of an optical counterpart brighter than B=26.1 seems incompatible with a neutron star atmosphere having a chemical composition dominated by Hydrogen or Helium. UBI photometry of field stars shows astonishingly little interstellar reddening in the direction of the X-ray source. Together with the small column density detected by the ROSAT PSPC, this suggests a mean particle density in the range of n=0.1-0.4cm^–3. Such average densities would imply very low velocities relative to interstellar medium (v_rel≤10km/s) if the source were powered by accretion. These stringent constraints may be relaxed if the neutron star is presently crossing a small size structure of higher density or if the effective temperature of the heated atmosphere is overestimated by the blackbody approximation. Alternatively, RX J0720.4-3125 could be a young and highly magnetized cooling neutron star.