They're smaller than Washington, D.C., spin around a few times per second, and are hundreds of light-years away. But that hasn't stopped astronomers from successfully mapping a trio of neutron stars affectionately known as the Three Musketeers. The remarkable feat also poses a new puzzle: In two cases, million-degree "hot spots" on the stars' surfaces turn out to be much larger than expected, indicating that popular models of neutron stars are wrong.
Neutron stars are extreme in every sense. These collapsed remains of stars that went supernova are very small, very dense, and very hot. Moreover, they spin extraordinarily fast and boast superstrong magnetic fields. According to theory, these fields can funnel part of the star's energy back onto the surface, creating hot spots near the magnetic poles. However, such polar hot spots have never been clearly detected.
Now, a team led by Patrizia Caraveo of the Istituto di Astrofisica Spaziale e Fisica Cosmica in Milan, Italy, has spotted the spots, using the European Space Agency's (ESA) sensitive x-ray satellite XMM-Newton. A few times per second, each of the Three Musketeers briefly pumps out high levels of high-energy x-rays, then quickly returns to normal. The rapid energy variations, the team reports in the 20 April issue of The Astrophysical Journal, are caused by hot spots on the stars' surfaces rotating in and out of view.
From their observations, Caraveo and her colleagues deduced the physical sizes of the hot spots. One spot is about 80 meters wide, which makes sense. But the hot spots on the other two neutron stars are much larger than expected--perhaps a few kilometers across. This indicates that the relatively simple models used to describe neutron stars need an overhaul. Since neutron stars are the densest known objects in the universe, the finding may ultimately help scientists better understand the behavior of matter under extreme circumstances.
Given the challenge of observing tiny structures on distant stars, "the observations are fantastic," says Mariano Méndez of the SRON National Institute for Space Research in Utrecht, the Netherlands. But according to Michiel van der Klis of the University of Amsterdam, the implications of the hot spot results are not yet clear. "Maybe they are not really surface structures, but tenuous clouds of hot gas at a certain altitude," he says. According to Méndez, the true appearance of the Three Musketeers may not be uncovered until future x-ray satellites like ESA's XEUS provide astronomers with even better maps.