The Distance to the Isolated Neutron Star RX J0720.4−3125

We have used a set of dedicated astrometric data from the Hubble SpaceTelescope to measure the parallax and proper motion of the nearby neutron starRX J0720.4-3125. At each of eight epochs over two years, we used the HighResolution Camera of the Advanced Camera for Surveys to measure the position ofthe B=26.6 target to a precision of ~2 mas (~0.07 pix) relative to 22 otherstars. From these data we measure a parallax of pi=2.8+/-0.9 mas (for adistance of 360+170-90 pc) and a proper motion of mu=107.8+/-1.2 mas/yr.Exhaustive testing of every stage of our analysis suggests that it is robust,with a maximum systematic uncertainty on the parallax of 0.4 mas. The distanceis compatible with earlier estimates made from scaling the optical emission ofRX J0720.4-3125 relative to the even closer neutron star RX J1856.5-3754. Thedistance and proper motion imply a transverse velocity of 180+90-40 km/s,comparable to velocities observed for radio pulsars. The speed and directionsuggest an origin for RX J0720.4-3125 in the Trumpler 10 OB association ~0.7Myr ago, with a possible range of 0.5-1.0 Myr given by the uncertainty in thedistance.Comment: 17 pages, 8 figures. To be published in Ap