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

• Published in: The Astrophysical journal Vol. 660; no. 2; pp. 1428 - 1443
• Main Authors: Kaplan, D. L, van Kerkwijk, M. H, Anderson, J
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 10.05.2007; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; stars: individual (RX J0720.4-3125); Neutron stars; X-rays: individual (RX J0720.4-3125); stars: neutron; Astrometry
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/513309

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 similar to 2 mas ( similar to 0.07 pixels) relative to 22 other stars. From these data we measure a parallax of capital pi = 2.8 plus or minus 0.9 mas (for a distance of 360 super(+) sub(-) super(1) sub(9) super(7) sub(0) super(0)pc) and a proper motion of mu = 107.8 plus or minus 1.2 mas yr super(-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 super(+) sub(-) super(9) sub(1) super(0) sub(0) km s super(-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 similar to 0.7 Myr ago, with a possible range of 0.5-1.0 Myr given by the uncertainty in the distance.