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X-Ray Evidence Suggests a New Form of Matter

3C58
Image: COURTESY OF NASA/SAO/CXC/P. SLANE ET AL.

Recent observations of two unusual stars have produced unexpected results, suggesting the existence of both a new form of matter and an exotic new type of star, scientists say. Previous research had led astronomers to believe that the objects, known as RXJ 1856.5-3754 and 3C58, were neutron stars--the extremely dense leftovers of supernova explosions. But according to the new results, one of them appears to be too small and the other, too cold. These inconsistencies, astronomers say, suggest that one or both of the stars could be composed of an odd combination of quarks, the tiny particles usually bound together inside protons and neutrons. In ordinary matter quarks come in two brands: up or down. But a heavier version, known as the strange quark, could be the secret ingredient in these stars. Stellar objects composed of a stable mix of up, down and strange quarks would supplant neutron stars as the densest stellar objects and could explain the size and behavior of RXJ 1856 and 3C58. A report detailing these findings will be published in the Astrophysical Journal in June.

Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics and colleagues used the Chandra X-Ray Observatory and the Hubble Space Telescope to study RXJ 1856. Whereas the minimum diameter for a neutron star is approximately 12 miles, the team's results indicate that RXJ 1856 is only seven miles wide. Drake notes that the observed x-ray radiation could be coming from a hot spot on the star, which would make it appear much smaller than it really is and return it to neutron star status. He says other teams are currently examining this possibility. But because a special orientation with respect to the earth would be required to explain the low incidence of pulsations detected from RXJ 1856, the likelihood of his group's results being overturned is small.

In the case of star 3C58 (see image), the circumstances are slightly different. Patrick Slane and Steven Murray of the Harvard-Smithsonian Center for Astrophysics, together with David Helfand of Columbia University, base their findings on the observations of Chinese and Japanese astronomers in 1181, who recorded the birth of the star in a supernova explosion. If those early astronomers were looking at 3C58 and 3C58 is a neutron star, the group says, then it should be much warmer than the observed temperature of less than one million degrees Celsius.

Both results, although unusual in their own way, lead to the same conclusion. According to Drake, "The combined observational evidence points to a star composed not of neutrons but of quarks in a form known as strange quark matter."

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