Nearest Neutron Star

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.

Mignani, R.P., Testa V., González Caniulef, D., et al. 2017, MNRAS 465, 1, 1
Özel, F., Sky & Telescope, July 2017, pp. 16-21
Yoneyama, T., Hayashida, K., Nakajima, H., Inoue, S., Tsunemi, H. 2017

Polarization of Starlight

The space between stars is not a perfect vacuum. It contains gas molecules and dust grains, although they are few and far between by any terrestrial standard. In the presence of a magnetic field, many types of interstellar dust grains line up in a way that is reminiscent of iron filings near a bar magnet. When light from a star passes through a region of space with magnetically-aligned dust grains (though in this case the short axis of the dust grains aligns with the local magnetic field), light with the electric field vector perpendicular to the long axis of the grains is less likely to be absorbed by the grains than light whose electric field vector is parallel to the long axis of the grains. This causes the light passing through such regions of space to become slightly polarized, and the polarization of starlight is something we can measure easily here on Earth. In this way, the strength and orientation of invisible interstellar or circumstellar magnetic fields can be determined at a distance.

Various astrophysical processes result in polarized electromagnetic radiation.  The differential absorption already mentioned polarizes the light from all stars to one degree or another.  Only the Sun—which is vastly nearer—offers us almost completely unpolarized light. Scattering of light off of interstellar clouds and planetary surfaces also results in polarization.  Finally, both synchrotron and cyclotron emission produce a characteristic polarization.

The polarization of starlight can be measured by the use of a polarimeter attached to the telescope.  Unlike standard photometry, polarization is simpler to measure with ground-based telescopes because the measurements are relative rather than absolute and, under normal circumstances, the Earth’s atmosphere does not affect the polarization state of incoming light.  Care must be taken, however, to ensure that the telescope itself does not create instrumental polarization due to oblique reflections.  Placing the polarimeter at the unfolded Cassegrain focus is one desirable configuration (Hough 2006).

Hough, J. 2006, A&G, 47, 3.31