The Distance to the Isolated Neutron Star Rx J 0720 . 4 − 3125

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 two 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 pix) 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. 1. INTRODUCTION One of the many interesting results from ROSAT All-Sky Survey (Voges et al. 1999) was the discovery of seven objects that appear to be nearby, thermally-emitting neutron stars that have little if any magnetospheric emission (for recent reviews, see Haberl 2004, 2006; van Kerkwijk & Kaplan 2006). These objects, known most commonly as " isolated neutron stars, " (INS) are distinguished by their long spin periods (3 s, when measured), largely thermal spectra with cool temperatures (kT 100 eV), faint optical counterparts, and lack of radio emission. Thermally-emitting neutron stars have been the targets of many observations, as they can potentially be used to constrain the equation of state (EOS) of neutron stars, and thereby explore nuclear physics in realms inaccessible from laboratories (e.g., Lattimer & Prakash 2000). Two main approaches are used. The first, using the spectrum, seems simple: determine the effective angular size from spectral fits, multiply by the distance (obtained by other means), and one has the apparent radius. This radius can be converted into the physical radius through use of mass. The radius is the crucial quantity in differentiating …