The proper motion and energy distribution of the isolated neutron star RX J0720.4-3125

ESO 4m class telescope and VLT deep imaging of the isolated neutron star RXJ0720.4-3125 reveals a proper motion of μ = 97 ± 12mas/yr and a blue U-B color index. We show that a neutron star atmosphere model modified to account for a limited amount of hydrogen on the star’s surface can well represent both the optical and X-ray data without invoking any additional components. The large proper motion almost completely excludes the possibility that accretion from interstellar medium is the powering mechanism of the X-ray emission. It also implies that the proposed spin down is entirely due to magnetic dipole losses. RXJ0720.4-3125 is thus a very likely middle aged cooling neutron star. Its overall properties are quite similar to some of the long period radio pulsars recently discovered, giving further support to the idea that RXJ0720.4-3125 may be a pulsar whose narrow radio beam does not cross the Earth.