So far as we know, RX J1856.5-3754 is the neutron star closest to our solar system. This radio-quiet isolated neutron star can be found between 352 and 437 ly from our solar system, with its most likely distance being 401 ly. Directionally, it is located within the constellation Corona Australis, near the topside of the CrA circlet, just below the constellation Sagittarius. Its coordinates are:
α2000 = 18h 56m 35.11s, δ2000 = -37° 54′ 30.5″.
RX J1856.5-3754 was formed in a supernova explosion about 420,000 years ago. Today, this tiny 1.5 M☉ star about 15 miles across has a surface temperature of 1.6 million K and shines in visible light very feebly with an apparent visual magnitude of only 25.5. Its surface is so hot that its thermal emission is brightest in the soft X-ray part of the electromagnetic spectrum; this is how it was discovered in 1992.
Like all neutron stars, RX J1856.5-3754 has a very intense surface magnetic field (B ≈ 1013 G) which causes the electromagnetic radiation leaving it to exhibit a strong linear polarization. In the presence of such a strong magnetic field, the “empty” space through which the light travels behaves like a prism, linearly polarizing the outgoing light through a process known as vacuum birefringence.
An active area of neutron star research currently is a precise determination of their diameters. We do not yet know whether the extremely dense central regions of these stars contain neutrons, or an exotic form of matter such as a quark soup, hyperons, a Bose-Einstein condensate, or something else. Knowing the exact size and mass of a neutron star will allow us to infer what type of matter must exist in its interior. The majority of neutron stars are pulsars with active magnetospheres that make it difficult for us to see down to the surface. More “quiet” neutron stars such as RX J1856.5-3754 are the best candidates for precise size measurements of the neutron star itself. An accuracy of at least ± 1 mile is needed to begin to distinguish between the various models.
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