RX J1856.5−3754

RX J1856.5−3754 (also called RX J185635−3754, RX J185635−375, and various other designations) is a neutron star in the constellation Corona Australis. At approximately 400 light-years from Earth, it is the closest neutron star discovered to date.

RX J1856.5−3754

X-ray image of RX J1856.5−3754
Observation data Epoch 1996.7      Equinox J2000.0
Constellation	Corona Australis
Right ascension	18h 56m 35s
Declination	−37° 54′ 36″
Apparent magnitude (V)	~25.6
Details
Mass	$1.5\pm 0.2$ M☉
Radius	$12.1_{-1.6}^{+1.3}$ km
Age	1 Myr
Other designations
RX J185635−3754, 1ES 1853−37.9, 1RXS J185635.1−375433
Database references
SIMBAD	data

Discovery and location

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

Hubble image of RX J1856.5−3754—the first direct observation of an isolated neutron star in visible light

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 across the sky at 108 km/s. It was discovered in 1992, and observations in 1996 confirmed that it is a neutron star, the closest to Earth discovered to date.

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

RX J1856 is one of the Magnificent Seven, a group of young neutron stars at distances between 130 and 500 parsecs (420 and 1,630 light-years) of Earth.

Quark star hypothesis

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, and it was therefore suggested that it might be a quark star.

However, later refined analysis of improved Chandra and Hubble observations revealed that the surface temperature of the star is lower, only 434,000 °C, and, respectively, the radius is larger, about 14 km (the observed radius with account of the effects of general relativity appears about 17 km). Thus, RX J1856.5–3754 is now excluded from the list of quark star candidates. A subsequent more accurate parallax estimation has led to the correction of this result to $12.1_{-1.6}^{+1.3}$ km for the true radius (and about 15 km for the observed radius).

Vacuum birefringence

The polarization of the observed light in the extremely strong magnetic field suggests that the empty space around the neutron star is subject to the vacuum birefringence.

In 2016 a team of astronomers from Italy, Poland, and the U.K. using the Very Large Telescope reported observational indications of vacuum birefringence from RX J1856.5−3754. A degree of polarization of about 16% was measured from the visible spectrum being large enough to support evidence but not discovery due to the low accuracy of star model and the uncertain direction of the neutron magnetization axis.

Its inferred magnetic effect of 10¹³ G should produce a greater effect at X-ray wavelength which could be measured by future planned polarimeters such as NASA's Imaging X-ray Polarimeter Explorer (IXPE), NASA's Polarimetry of Relativistic X-ray Sources (PRAXYS) or ESA's X-ray Imaging Polarimetry Explorer (XIPE).

See also

• 3C 58, a possible quark star

References

• RX-J185635-375 at jumk.de
• RX J1856.5-3754 and 3C58: Cosmic X-rays May Reveal New Form of Matter Chandra X-ray Observatory. July 16, 2009.
• Walter, Frederick M.; Lattimer, James M., The Astrophysical Journal, 2002

External links

• Is RX J185635-375 a Quark Star?
• NASA Astronomy Picture of the Day: RX J185635-375: Candidate Quark Star (14 April 2002)
• Bare Quark Stars or Naked Neutron Stars? The Case of RX J1856.5-3754
• RX J185635-3754 - an Isolated Neutron Star
• News Release STScI-1997-32: Hubble Sees a Neutron Star Alone in Space