The Nearby Neutron Star Rx J0720.4−3125 from Radio to X-rays

We present radio, optical, ultraviolet, and X-ray observations of the isolated, thermally-emitting neutron star RX J0720.4−3125 using the Parkes radio telescope, the Very Large Array, the Hubble Space Telescope, and the Chandra X-ray Observatory. From these data we show that the optical/UV spectrum of RX J0720.4−3125 is not well fit by a Rayleigh-Jeans tail as previously thought, but is instead best fit by either a single non-thermal power-law or a combination of a Rayleigh-Jeans tail and a non-thermal power-law. Taken together with the X-ray spectrum, we find the best model for RX J0720.4−3125 to be two blackbodies plus a power-law, with the cool blackbody implying a radius of 11–13 km at an assumed distance of 300 pc. This is similar to many middle aged (10 yr) radio pulsars such as PSR B0656+14, evidence supporting the hypothesis that RX J0720.4−3125 is likely to be an off-beam radio pulsar. The radio data limit the flux at 1.4 GHz to be < 0.24 mJy, or a luminosity limit of 4πdF < 3× 10d300 ergs s, and we see no sign of extended nebulosity, consistent with expectations for a pulsar like RX J0720.4−3125. Subject headings: pulsars: individual (RX J0720.4−3125)—stars: neutron—X-rays: stars