The proper motion of the neutron star RXJ1856.5-3754 as measured by optical and X-ray imaging

The proper motion of the faint (V=25.6 mag) neutron star RXJ1856.5-3754 as measured consistently with both the FORS1 optical imager on the ESO Very Large Telescope (VLT) as well as with the ROSAT High Resolution Imager is presented. With two VLT images obtained with a 0.95 year epoch difference in Apr 1999 and Apr 2000, a proper motion of mu_alpha = 0.326 \pm 0.064" per year to the east and no detectable motion in declination is found, consistent with simultaneous findings by Walter (2001) using three HST observations. Also, by comparing the positional differences between the strong X-ray source RXJ1856.5-3754 and other X-ray sources detected in the same ROSAT High Resolution Imager field (observed in Oct 1994 and Oct 1997), a proper motion of 0.34 \pm 0.12" to the east is detected. This may be the first star, where a proper motion is clearly detected in X-ray images. Additional VLT images taken in half-year intervalls give an upper limit to the parallax of about 51 mas. Furthermore, in archived VLT observations, the first B-band detection of this object is found with B=25.14 \pm 0.41 mag.