SEARCHING space for exotic stars made entirely of free quarks is a tricky task, but now we know what to look for – a unique pattern of gamma-ray emissions given off by new quark stars as they cool. The signal should be detectable by the European Space Agency spacecraft Integral, due for launch this month.
When a star goes supernova, its core is thought to collapse so rapidly that the atomic nuclei within are squashed into a soup of neutrons. The process squashes material weighing as much as our Sun into a neutron star only 20 kilometres across. But theorists speculate that if a supernova is more massive than it needs to be to make a neutron star, but not big enough to form a black hole, its gravity might be enough to crush the core even further, smashing the neutrons together so violently that they dissolve into their constituent quarks.
Such a quark star would be even denser and smaller than a neutron star, and when Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics found a supernova remnant just 11 kilometres across earlier this year, he thought it could be a quark star (91av, 20 April, p 12). But because it’s hard to tell the exact size of a star from so far away, others thought it might simply be an unusual neutron star.
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Now Dany Page of the National Autonomous University of Mexico and his colleague Vladimir Usov of Israel’s Weizmann Institute say they have a foolproof way of spotting quark stars. When such a star forms, they say an intense burst of neutrinos blows away its outer shell of ordinary matter, exposing the free quarks. An extremely strong electric field remaining at the surface would create a dense plasma of electrons and positrons, which would annihilate each other and emit gamma rays with energies of 30,ooo to 500,000 electronvolts (Physical Review Letters, vol 89, p 131,101). Depending on how fast the quark star cools, this phase could last from one day to thousands of years, Page told 91av.
Only a quark star would explain such an emission, he says. He points out that Integral, which will be the most sensitive gamma-ray instrument yet launched, will be searching in just the right energy range to see this distinctive signature. He estimates it should be able to spot young quark stars, if they exist, up to several tens of millions of light years away.