RX J1856.5-3754
(and the 3C58 Pulsar)

Just two years after neutrons were discovered in 1932, [Wilhelm Heinrich] Walter Baade (1893-1960) and Fritz Zwicky (1898-1974) proposed that "neutron stars" could be formed by supernovae. Although some estimate that the Milky Way galaxy may have a billion of them given the rate of supernovae over the eons, most older neutron stars are now invisible, having long since cooled down, become completely inactive, and faded out of sight (see an overview of isolated nearby neutron stars by astronomer Frederick Walter). Thus, none were detected until 1967 when Jocelyn Bell discovered the first young neutron star pulsing in radio wavelengths. Astronomers have since identified around 1,500 neutron stars.

According to what had become canonical theory, a neutron star can be formed when a star of 15 to 30 Solar-masses or so exhausts core fusion of elements lighter than iron. After the core creates iron (Fe 56), it can't fuse iron into heavier elements because iron has the highest binding energy per nucleon of any element and iron fusion or fission requires adding energy at the relatively low pressures available. Thus, the core accumulates iron until at some point beyond 1.4 Solar-masses (the "Chandrasekhar mass limit") gravity overcomes the electron degeneracy pressure that has been supporting the core, causing it to collapse inward ("implode") and rebound in a type-II supernova (possibly similar to Supernova 1987a). The supernova's thermonuclear shock wave races through its expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of billions of stars (more).

S. Lee, CXC, NASA

Larger illustration.


Core implosion rebounds into a
supernova explosion that blasts
its outer layers into interstellar
space (more).

Core implosions create such high pressures, in fact, that it becomes energetically easy to combine protons and electrons to form neutrons and neutrinos. Although the neutrinos escape after scattering a bit and helping to create the supernova explosion that blows off the outer layers of the star, the neutrons at the core can settle down and "pack closer together" if neutron degeneracy pressure is able to oppose further gravitationally-induced collapse. If the progenitor star's core contains more than 1.4 Solar-masses but less than three Solar-masses, then a superdense remnant composed predominantly of neutrons may be left behind that is even more compact that a white dwarf. Like one big atomic nucleus with a diameter of six to 12 miles (10 to 20 km) with a mass exceeding that of Sol, the resulting "neutron star" has such a high density that a spoonful would weigh more than a billion tons on Earth. Like Human ice-skaters that pull in their arms while twirling, compression of a stellar-sized, rotating mass into a city-sized object similarly causes most young neutron stars to rotate at a very high speed of about 10 to 100 times each second, which slows to about once each second over the course of some billions of years. (More discussion on neutron stars and maximum mass versus radii.)

Melissa Weiss, CXC, NASA -- larger illustration
(Unlike neutron stars, quark stars have free, strange and up and down quarks.)

Over the decades, however, theoretical astrophysicists have proposed some elaborations on the neutron star creation scenario. Although core collapse leading to a supernova can at first produce a hot neutron star, the collapse could continue (after a few seconds to a few days) to produce even greater densities and free up the quark matter confined inside individual neutrons inside a neutron shell until the even higher degeneracy pressure of the interior quark matter opposes further collapse (more discussion). The resulting "quark stars" would appear to be unusually small and cool. In 2002, x-ray and Hubble Space Telescope observations of RX J1856.5-3754 and the 3C58 Pulsar indicated that the matter in these collapsed stars may be denser than nuclear matter, the most dense matter found on Earth. This raises the possibility that these stars are composed of free quarks or crystals of sub-nuclear particles, rather than neutrons (see: Nature; NASA; CfA/CXC; and ESO). On the other hand, by November 2002, more accurate size estimates of another compact object named EXO 0748-676 (which has a stellar companion transferring mass to it that was leading to x-ray outbursts) left astronomers more certain that it was a neutron star (discussion and illustrations at NASA's Goddard Space Flight Center).

Melissa Weiss, CXC, NASA -- larger illustration
(RX J1856.5-3754 may be too small to be a neutron star -- more.)

Physicists know of six kinds of quarks, two of which -- up and down -- make up ordinary matter like protons and neutrons. Theorists, however, have long speculated that up and down quarks can be melded with a heavier kind known as the strange quark to form something called "strange matter," which could be fairly common in the universe because it should be stable and might grow like a crystal from the neutrons and protons it encounters. According to astronomer Jeremy J. Drake of the Harvard-Smithsonian Center for Astrophysics (CfA), it's possible that all neutron stars are, in fact, quark stars (more from Astronomy Picture of the Day and Drake et al, 2002). On June 28, 2006, some counter-evidence dependent on mass estimates of the neutron star in the EXO 0748-676 binary system was offered by astronomer Feryal Özel (more from NewScientist.com).

RX J1856.5-3754

Measurements finding a relatively high-density, interstellar medium (ISM) -- mostly hydrogen -- around this candidate quark star suggest that it is located around 460 ly (140 pc) away from Sol, farther than a previous estimate of around 200 ly or 62 pc (Drake et al, 2002; and Kaplan et al, 2002). It is located in the north central part (18:56:35.3-37:56:34.4, J2000; or 18:56:37-37:54.4, ICRS 2000.0) of Constellation Corona Australis, the Southern Crown -- north of Kappa Coronae Australis, west of Lambda Coronae Australis, southeast of Eta and Epsilon Sagittarii (Kaus Australis), south of globular clusters M69 and M70, and northwest of Mu and Theta Coronae Australis. Located in the outskirts of the R Coronae Australis dark molecular cloud, RX J1856.5-3754 (RXJ1986) was discovered as a bright X-ray source in 1992 with the ROSAT X-ray satellite observatory (Walter et al, 1996). The apparent visual magnitude of RX J1856 is 25.6, or nearly 100 million times fainter than what can be perceived with the naked eye from Earth's night sky.

STScI, NASA



Larger image



RX J1856.5-3754 is an
isolated neutron star,
without a binary
companion (more).

Since there has been sufficient time for an associated supernova remnant to disperse, however, RXJ1856 should be at least 100,000 years old. Moreover, unlike younger isolated neutron stars or neutron stars in binary stellar systems, RX J1856 does not show any signs of activity such as variability or pulsations. According to astronomer Frederick M. Walter of the University of New York at Stony Brook, the star appears to have left the Upper Scorpius Association at about the same time that a supernova ejected the runaway O star Zeta Ophiuchus. Hence, RX J1856 and Zeta Ophiuchus could have been members of a binary system, and RXJ1856 is the remnant of the star that exploded. Assuming that it originated in the Upper Scropius Association, RXJ1856 may be around 1.15 +/- 0.15 million years old.

X-ray observations of RXJ1856 and the 3C58 Pulsar (described below) suggest that the matter in these stars is even denser than nuclear matter found on Earth. Thus, these stars could be composed of pure quarks or contain crystals of sub-nuclear particles that normally have only a fleeting existence following high-energy collisions. By combining x-ray and optical observations, astronomers found that RX J1856 radiates like a solid body with a temperature of 1.2 million degrees Fahrenheit (700,000 degrees Celsius) and has a diameter of about seven miles (7.6 to 16.4 km), is too small to reconcile with standard models for neutron stars. According to some theoretical predictions, the neutrons in RXJ1856 may have dissolved at very high density into a soup of "up," "down," and "strange" quarks to form a "strange quark star," which would explain the smaller radius.

STScI, NASA



Larger image




RX J1856.5-3754 is
one of the fastest
moving stars in
Earth's night sky
(more).

Astronomer Jeremy Drake has cautioned that the observations of RXJ1856 could be interpreted as a more normal neutron star with a hot spot, so that the X-ray observations made it only appear to be a small surface. With a hot spot, such a neutron star would be expected to pulse. However, a team of scientists (led by Scott Ransom of McGill University) recently failed to detect pulsation (or any other variability) and concluded that no more than five percent of the X-radiation from RX J1856 could be in pulses (Ransom et al, 2002). To explain the paucity of pulsations would require a very special orientation with respect to the Earth, but the probability of this was considered small.

VLT, ESO


Larger image.



Although faint at optical
wavelengths RX J1856
is exceptionally bright
at which indicates that
it is extremely hot and
very small (more at ESO
and Astronomy Picture
of the Day for 4/14/02
and 10/25/00).

From positional measurements and its estimated distance, RXJ1856 was found to be moving at a very high velocity through space (exceeding 100 km per second -- much more if the new estimated distance of 460 ly is correct). Images and spectra obtained with the ESO Very Large Telescope (VLT) now show that the object creates a small nearby cone-shaped ("bowshock") nebula that shines in light from hydrogen atoms and is obviously a product of some kind of interaction with this strange star, which can be seen as a very faint, blue object very close to the top of the cone. Although similarly shaped cones have been found around fast-moving radio pulsars and massive stars, those bowshocks form from a strong outflow of particles from the star or the pulsar (a "stellar wind") that collides with interstellar matter. One possibility in the case of RXJ1856 is that, when the surrounding hydrogen atoms are ionized, their electrons and protons acquire substantial velocities which heat the interstellar gas near the passing neutron star. The heated gas expands and pushes aside the surrounding cooler gas and leads to a geometrical shape similar to that caused by a stellar wind (ESO; and van Kerkwijk and Kulkarni, 2001). Some useful catalogue numbers for this object are: 1ES 1853-37.9, RX J1856.4-3754, RX J1856.5-3754, RX J1856.6-3754, 1RXP J185635.1-375433, 1RXS J185635.1-375433, and 1WGA J1856.5-3754.

The 3C58 Pulsar

This candidate quark star is located 10,000 ly away from Sol. It is located in the north eastern part (2:5:37.0+64:49:48.0, J2000; or 2:5:37.9+64:49:42.8, ICRS 2000.0) of Constellation Cassiopeia, the Lady of the Chair -- southwest of Iota Cassiopeia, northeast of Epsilon and Cassiopeia (Ruchbah) and M103, southeast of Omega and Phi Cassiopeia, northwest of IC 1805 (sometime called the "Running Dog Nebula") and Eta, Tau, Delta, and Alpha Persei (Mirfak). Historical evidence strongly suggests an association of the remnant with supernova SN 1181, which would make 3C58 younger than the Crab Nebula, and x-ray observations have identified the young 65-millisecond pulsar J0205+6449 at the center of 3C58, embedded in a compact nebula that appears to be confined by the pulsar wind termination shock (Slane et al, 2002). Observations of 3C 58, the remnant of a supernova believed to have been seen by by Chinese and Japanese astronomers in CE 1181, reveal that the pulsar in the core has a temperature much lower than expected. This suggests that an exotic, denser state of matter might exist inside this object as well, which has cooled the young, compact supernova remnant faster than canonical neutron star would predict.

CXC, NASA



Larger x-ray image.



The 3C58 Pulsar
may be to be too
cool to be a
neutron star (more).

A team of astronomers (including Patrick Slane and Steven Murray of CfA and David Helfand of Columbia University) failed to detect the expected amount of X-rays from the hot surface of 3C58, which indicated that the compact object has a temperature of less than one million degrees Celsius (more than 1.8 million degrees F) -- far below the predicted value. According to Helfand, their observations of 3C58 offer the first compelling test of models for how neutron stars cool. Indeed, the results suggest that neutron stars are not composed of pure neutrons after all but require new forms of matter to explain the empirical findings. As the coordinates for the remnant object from that explosion are not known precisely, the temperature estimates actually may apply to another object. If it is the right object, however, even a neutron star's high density would not be enough to produce cooling particles fast enough to reduce its temperature to such a low level in the eight centuries that have elapsed since its birth. According to Helfand, the 3C58 pulsar would have to be as much as five times denser for this to occur, indicating that at least the core of the object is made of something other than neutrons (more from Slane et al, 2002).

X-ray images of 3C58 show a rapidly rotating, compact object embedded in a cloud of high energy particles. Observational data also reveal that the object is a pulsar that rotates about 15 times per second, although it is slowing down at the rate of about 10 microseconds per year. A comparison of the rate at which the pulsar is slowing down and its age indicate that the 3C58 pulsar is one of the youngest known pulsars, and it may be rotating just about as fast now as when it was formed. This is in contrast to the Crab pulsar, which was formed spinning much more rapidly and has slowed to about half its initial speed. Furthermore, the total X-ray luminosity of the 3C58 pulsar and its surrounding nebula is a thousand times weaker than that of the Crab and its surrounding nebula. Some useful catalogue numbers for the 3C58 Pulsar are: PSR J0205+6449 or J0205+6449. Some useful catalogue numbers for the 3C58 nebula around this object are: 2C 177, 3C 58, SN 1181, 4C 64.02, 2E 518, 2E 0201.8+6435, GRS 130.70 +03.10, SNR 130.7+03.1, 1RXS J020529.7+644934, RX J0201.8+6435, and RX J0201.8+6435.