A Revised Parallax and Its Implications for RX J185635-3754

Abstract

New astrometric analysis of four Wide Field Planetary Camera 2 images of the isolated neutron star RX J185635-3754 shows that its distance is 117+/-12 pc, nearly double the originally published distance. We argue that the star's birthplace was in the Upper Scorpius association but that a connection with ζ Ophiuchi is now unlikely. Assuming birth in Upper Sco, the revised distance yields an age of 5×10⁵ yr and a space velocity of about 185 km s^-1. The radiation radius inferred from fitting the full spectral energy distribution lies between 12 and 26 km, with a best fit R_∞=16.4+/-0.3 km for a two-blackbody model. These radii are in the range of many equations of state, both with and without exotic matter, and remove the observational support for an extremely soft equation of state.