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The Parallax and Proper Motion of RX J1856.5-3754 Revisited

D. L. Kaplan·M. H. van Kerkwijk·J. Anderson
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Abstract

RX J1856.5-3754, a bright soft X-ray source believed to be the nearest thermally emitting neutron star, has commanded and continues to command intense interest from X-ray missions. One of the main goals is to determine the radius of this neutron star. An integral part of the determination is an accurate parallax. Walter (2001) analyzed Hubble Space Telescope (HST) data and derived a parallax, pi=16.5+/-2.3 mas. Combining this distance with the angular radius derived from blackbody fits to observations of RX J1856.5-3754 with ROSAT, EUVE, HST, Pons et al. (2001) derived an observed radius ("radiation radius"), R_{infty}= 7 km. This value is smaller than the radii calculated from all proposed equations-of-state (EOS) of dense baryonic matter (Haensel 2001). Here, we have analyzed the same HST data and find pi=7+/-2 mas. We have verified our result using a number of different, independent techniques, and find the result to be robust. Adopting our parallax, the radius of RX J1856.5-3754 is R_{infty}=15+/-6 km. This radius falls squarely in the range of radii, 12--16 km, expected from calculations of neutron star structure for different equations of state. With additional HST observations, the parallax estimate can be improved to the point by which the inferred radius can constrain the choice of EOS.

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D. L. Kaplan, M. H. van Kerkwijk, J. Anderson. 2002-01-31. The Parallax and Proper Motion of RX J1856.5-3754 Revisited. https://doi.org/10.1086/339879

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