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

Abstract

We have used a set of dedicated astrometric data from the Hubble Space Telescope to measure the parallax and proper motion of the nearby neutron star RX J0720.4-3125. At each of eight epochs over 2 years, we used the High Resolution Camera of the Advanced Camera for Surveys to measure the position of the B=26.6 target to a precision of ~2 mas (~0.07 pixels) relative to 22 other stars. From these data we measure a parallax of π=2.8+/-0.9 mas (for a distance of 360⁺¹⁷⁰_-90 pc) and a proper motion of μ=107.8+/-1.2 mas yr^-1. 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 distance is compatible with earlier estimates made from scaling the optical emission of RX J0720.4-3125 relative to the even closer neutron star RX J1856.5-3754. The distance and proper motion imply a transverse velocity of 180⁺⁹⁰_-40 km s^-1, comparable to velocities observed for radio pulsars. The speed and direction suggest an origin for RX J0720.4-3125 in the Trumpler 10 OB association ~0.7 Myr ago, with a possible range of 0.5-1.0 Myr given by the uncertainty in the distance.