The Proper Motion, Parallax, and Origin of the Isolated Neutron Star RX J185635-3754

Abstract

The isolated neutron star RX J185635-3754 is the closest known neutron star to the Sun. Based on Hubble Space Telescope Wide Field Planetary Camera 2 observations over a 3 yr baseline, I report its proper motion (332+/-1 mas yr^-1 at a position angle of 100.3d+/-0.1d) and parallax (16.5+/-2.3 mas, 61 pc). This proper motion brings the neutron star from the general vicinity of the Sco-Cen OB association. For an assumed neutron star radial velocity between -55 and -60 km s^-1, the runaway O star ζ Oph, the Upper Sco OB association, and the neutron star come into spatial coincidence between 0.9×10⁶ and 1.0×10⁶ yr ago. RX J185635-3754 may be the remnant of the original primary of the ζ Oph system. If so, the space velocity suggests that the neutron star received a kick of about 200 km s^-1 at birth.