Massive neutron star rules out exotic matterExotic states of matter such as free quarks do not arise inside neutron stars, according to a new analysis of one of the super-dense stellar corpses. The result contradicts previous theories and offers an unprecedented view into the behaviour of matter under extreme pressure.
Neutron stars are the dead cores of massive stars that exploded as supernovae. They are made of neutrons packed together so tightly that a teasthingyful of the material would weigh billions of tonnes.
Physicists have speculated that in the dense interiors of especially massive neutron stars, matter might be transformed into exotic states never seen elsewhere.
Some researchers believe the enormous pressure could cause the neutrons to break down, freeing the individual quarks of which they are made. Quarks are never found alone in nature. One group of astronomers reported tentative evidence of such a quark star in 2002 (see Exotic star is made entirely of quarks).
Another theory says the pressure might lead to a form of matter called a Bose-Einstein condensate (BEC). In this weird quantum state, the neutrons do not dissociate into quarks, but their individual identities blur and they behave as a single particle.
Stretched lightIn principle, it is possible to distinguish between the regular and exotic states of matter, because both free quarks and BECs would be more easily compressed than neutrons. So a star of a particular mass would have a smaller radius if it were made of squishy exotic matter.
But the exotic matter theories have received a blow from the study by Feryal Ozel of the University of Arizona, in Tucson, US. Using a new technique to analyse the mass and radius of a neutron star called EXO 0748-676, she finds that the star is probably made of ordinary neutrons.
The mass and size were determined from measurements of several other key properties of the star, taken by Europe's XMM-Newton and NASA's Rossi X-ray Timing Explorer space telescopes.
One of these properties was the amount of shifting in the wavelength of light emitted near the surface of the star. The powerful gravitational field near a neutron star stretches light out, towards longer wavelengths.
EXO 0748-676 is the only neutron star with enough detail in its spectrum for this gravitational redshift to be measured. The unprecedented detail allowed Ozel to apply a technique for calculating the mass that has never been practical before.
Massive surpriseOzel's calculations put the neutron star's radius at 13.8 kilometres. But the real surprise was its mass, which came out to 2.1 times the mass of the Sun.
That mass strongly suggests the star is made of normal neutrons. That is because as the mass of a neutron star increases, it must become more and more rigid to avoid collapsing into a black hole under the force of its own gravity. Most models of quark stars and BEC-containing neutron stars predict they would collapse into a black hole before reaching a mass as high as 2.1 solar masses.
"I think the physical measurement procedure is sound," says Frits Paerels of Columbia University in New York, US. "The number that comes out of it is interesting. The mass is surprisingly large."
Most neutron stars whose masses have been measured in other ways are relatively small, at just 1.4 to 1.5 times the mass of the Sun. But their physical sizes have been difficult to pin down, making it unclear what kind of matter lies inside them.
Ruled outOzel says the fact that squishy, exotic states of matter do not seem to occur in a star as massive as EXO 0748-676 suggests that these states do not occur in any neutron stars.
Paerels agrees. He told New Scientist that the study would rule out exotic states of matter in neutron stars, if the results are confirmed by future observations.
"It is interesting, it is titillating, it is suggestive," says Madappa Prakesh of Ohio University, US.
But Prakesh says the star's mass has not been pinned down precisely enough to exclude these exotic states completely. The data show the star's most likely mass is 2.1 solar masses but could be as low as 1.8 solar masses. The lower value would still be compatible with some models of star interiors made of free quarks or BECs.
Journal reference: Nature (vol 441, p 1115)
space.newscientist.com/article/dn9428-massive-neutron-star-rules-out-exotic-matter.html
Star Gobbles Up Its FriendTue, 06 Sep 2005 - The ESA's Integral space observatory and NASA's Rossi X-ray Timing Explorer spacecraft have found a rapidly spinning pulsar in the process of consuming a neighbour. This discovery supports the theory that pulsars spin so quickly because they pick up material from a companion, which increases their mass. Pulsars were once stars more than 8 times as large as our Sun, but their intense gravity compacted them down to a size of about 20 km (12 miles) across.
ESA's Integral space observatory, together with NASA's Rossi X-ray Timing Explorer spacecraft, has found a fast-spinning pulsar in the process of devouring its companion.
This finding supports the theory that the fastest-spinning isolated pulsars get that fast by cannibalising a nearby star. Gas ripped from the companion fuels the pulsar's acceleration. This is the sixth pulsar known in such an arrangement, and it represents a 'stepping stone' in the evolution of slower-spinning binary pulsars into faster-spinning isolated pulsars.
"We're getting to the point where we can look at any fast-spinning, isolated pulsar and say, 'That guy used to have a companion'," said Dr Maurizio Falanga, who led the Integral observations, at the Commissariat � l'Energie Atomique (CEA) in Saclay, France.
'Pulsars' are rotating neutron stars, which are created in stellar explosions. They are the remnants of stars that were once at least eight times more massive than the Sun. These stars still contain about the mass of our Sun compactified into a sphere of only about 20 kilometres across.
This pulsar, called IGR J00291+5934, belongs to a category of 'X-ray millisecond pulsars', which pulse with the X-ray light several hundred times a second, one of the fastest known. It has a period of 1.67 milliseconds which is much smaller that most other pulsars that rotate once every few seconds.
Neutron stars are born rapidly spinning in collapses of massive stars. They gradually slow down after a few hundred thousand years. Neutron stars in binary star systems, however, can reverse this trend and speed up with the help from the companion star.
For the first time ever, this speeding-up has been observed in the act. "We now have direct evidence for the star spinning faster whilst cannibalising its companion, something which no one had ever seen before for such a system," said Dr Lucien Kuiper from the Netherlands Institute for Space Research (SRON), in Utrecht.
A neutron star can remove gas from its companion star in a process called 'accretion'. The flow of gas onto the neutron star makes the star spin faster and faster. Both the flow of gas and its crashing upon the neutron star surface releases much energy in the form of X-ray and gamma radiation.
Neutron stars have such a strong gravitational field that light passing by the star changes its direction by almost 100 degrees (in comparison light passing by the Sun is deflected by an angle which is 200 thousands times smaller). "This 'gravitational bending' allows us to see the back side of the star," points out Prof. Juri Poutanen from the University of Oulu, Finland.
"This object was about ten times more energetic than what is usually observed for similar sources," said Falanga. "Only some kind of monster emits at these energies, which corresponds to a temperature of almost a billion degrees."
From a previous Integral result, scientists deduced that because the neutron star has a strong magnetic field, charged particles from its companion are channeled along the magnetic field lines until they slam into the neutron star surface at one of its magnetic poles, forming 'hot spots'. The very high temperatures seen by Integral arise from this very hot plasma over the accretion spots.
IGR J00291+5934 was discovered by Integral during a routine scan of the sky on 2 December 2004, in the outer reaches of our Milky Way galaxy, when it suddenly flared. On the day after, scientists accurately clocked the neutron star with the Rossi X-ray Timing Explorer.
Rossi observations revealed that the companion is already a fraction the size of our Sun, perhaps as small as 40 Jupiter masses. The binary orbit is 2.5 hours long (as opposed to the year long Earth-Sun orbit). The full system is very tight; both stars are so close that they will fit into the radius of the Sun. These details support the theory that the two stars are close enough for accretion to take place and that the companion star is being cannibalised.
"Accretion is expected to cease after a billion of years or so," said Dr Duncan Galloway of the Massachusetts Institute of Technology, USA, responsible for the Rossi observations. "This Integral-Rossi discovery provides more evidence of how pulsars evolve from one phase to another - from an initially slowly spinning binary neutron star emitting high energies, to a rapidly spinning isolated pulsar emitting in radio wavelengths."
The discovery is the first of its kind for Integral (four of the first five rapidly spinning X-ray pulsars were discovered by Rossi). This bodes well in the combined search for these rare objects. Integrals's sensitive detectors can identify relatively dim and distant sources and so, knowing where to look, Rossi can provide timing information through a dedicated observation extending over the entire two-week period of the typical outburst.
www.universetoday.com/am/publish/star_eats_companion.html?692005
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