RX J1856.5-3754

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RX J1856.5-3754
RX J1856.5-3754.jpg
X-ray image of RX J1856.5-3754
Observation data
Epoch 1996.7      Equinox J2000.0[1]
Constellation Corona Australis
Right ascension 18h 56m 35s [1]
Declination −37° 54′ 36″[1]
Apparent magnitude (V) ~25.6[1]
Mass 0.9 M
Radius 1.9–4.1 km
Age 1 million years
Other designations
RX J185635-3754, 1ES 1853-37.9, 1RXS J185635.1-375433
Database references

RX J1856.5-3754 (also called RX J185635-3754, RX J185635-375, and various other designations) is a nearby neutron star in the constellation Corona Australis.

Discovery and location[edit]

Zooming in on the very faint neutron star RX J1856.5-3754

RX J1856.5-3754 is thought to have formed in a supernova explosion of its companion star about one million years ago and is moving at 108 km/s across the sky. It was discovered in 1992, and observations in 1996 confirmed that it is a neutron star, the closest to Earth discovered.[2]

It was originally thought to be about 150–200 light-years away,[3] but further observations using the Chandra X-ray Observatory in 2002 indicate that its distance is greater—about 400 light-years.[4][5]

RX J1856 is one of the Magnificent Seven, a group of young neutron stars at distances between 200 and 500 parsecs (652 and 1630 light years) of Earth.

Quark star hypothesis[edit]

By combining Chandra X-ray Observatory and Hubble Space Telescope data, astronomers previously estimated that RX J1856 radiates like a solid body with a temperature of 700,000 °C and has a diameter of about 4–8 km. This estimated size was too small to reconcile with the standard models of neutron stars, therefore it was suggested that it might be a quark star.[4]

However, later refined analysis[5][6] of improved Chandra and Hubble observations revealed that the surface temperature of the star is lower, only 434,000 °C, and respectively the diameter is larger, about 14 km (with account of the effects of general relativity, the observed radius appears about 17 km).[5] Thus, RX J1856.5-3754 is now excluded from the list of quark star candidates.[6]

Vacuum birefringence[edit]


Both Einstein's theory of special and general relativity state that light should pass freely through a vacuum without being altered, a principle known as Lorentz invariance. Yet, in theory, large nonlinear self-interaction of light due to quantum fluctuations should lead to this principle being measurably violated if the interactions are strong enough. Nearly all theories of quantum gravity predict that that Lorentz invariance is not an exact symmetry of nature. It is predicted the speed at which light travels through the vacuum depends on its direction, polarization and the local strength of the magnetic field. There have been a number of inconclusive results which claim to show evidence of a Lorentz violation by finding a rotation of the polarization plane of light coming from distant galaxies. The first concrete evidence for vacuum birefringence was published in 2017 when a team of astronomers looked at the light coming from the star RX J1856.5-3754.

Roberto Mignani at the National Institute for Astrophysics in Milan who led the team of astronomers has commented that "“When Einstein came up with the theory of general relativity 100 years ago, he had no idea that it would be used for navigational systems. The consequences of this discovery probably will also have to be realised on a longer timescale.” The team found that visible light from the star had undergone linear polarisation of around 16%. If the birefringence had been caused by light passing through interstellar gas or plasma, the effect should have been no more than 1%. Definitive proof would require repeating the observation at other wavelengths and on other neutron stars. At X-ray wavelengths the polarization from the quantum fluctuations should be near 100%. Although no telescope currently exists that can make such measurements, there are several proposed X-ray satellites that may soon be able to verify the result conclusively such as ESO's European Extremely Large Telescope (ELT), China's Hard X-ray Modulation Telescope (HXMT) and NASA's Imaging X-ray Polarimetry Explorer (IXPE).

See also[edit]

  • 3C 58, a possible quark star.


  1. ^ a b c d RX J185635-3754 - an Isolated Neutron Star, F. M. Walter, web page at the Department of Physics and Astronomy, State University of New York at Stony Brook. Accessed on line June 29, 2007.
  2. ^ Rees, Martin (2012) Universe. Dorling Kindersley. p. 528
  3. ^ "The Mystery of the Lonely Neutron Star". European Southern Observatory press release, September 11, 2000. Accessed online at spaceref.com May 20, 2007.
  4. ^ a b Drake J. J.; et al. (2002). "Is RX J1856.5-3754 a Quark Star?". Astrophys. J. 572 (2): 996–1001. Bibcode:2002ApJ...572..996D. arXiv:astro-ph/0204159Freely accessible. doi:10.1086/340368. 
  5. ^ a b c Ho W. C. G.; et al. (2007). "Magnetic hydrogen atmosphere models and the neutron star RX J1856.5–3754". Monthly Notices of the Royal Astronomical Society. 375 (2): 821–830. Bibcode:2007MNRAS.375..821H. arXiv:astro-ph/0612145v1Freely accessible. doi:10.1111/j.1365-2966.2006.11376.x. 
  6. ^ a b Truemper, J. E.; Burwitz, V.; Haberl, F.; Zavlin, V. E. (June 2004). "The puzzles of RX J1856.5-3754: neutron star or quark star?". Nuclear Physics B Proceedings Supplements. 132: 560–565. Bibcode:2004NuPhS.132..560T. arXiv:astro-ph/0312600Freely accessible. doi:10.1016/j.nuclphysbps.2004.04.094. 

External links[edit]