New photometry and astrometry of the isolated neutron star RX J0720.4-3125 using recent VLT/FORS observations

Since the first optical detection of RX J0720.4-3125 various observations have been performed to determine astrometric and photometric data. We present the first detection of the isolated neutron star in the V Bessel filter to study the spectral energy distribution and derive a new astrometric position. At ESO Paranal we obtained very deep images with FORS 1 (three hours exposure time) of RX J0720.4-3125 in the V Bessel filter in January 2008. We derive the visual magnitude by standard star aperture photometry. Using sophisticated resampling software we correct the images for field distortions. Then we derive an updated position and proper motion value by comparing its position with FORS 1 observations of December 2000. We calculate a visual magnitude of V=26.81±0.09 mag, which is seven times in excess of what is expected from X-ray data, but consistent with the extant U, B, and R data. Over about a seven year epoch difference we measured a proper motion of μ=105.1±7.4 mas yr^-1 towards θ=296.951^°±0.0063^° (NW), consistent with previous data.

Based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 080.D-0135(A). Based on observations with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA)....