Neutron stars are the collapsed cores of massive stars. They pack roughly one-and-a-half times the mass of our Sun into a space about the diameter of a city. In 1996, Tod Strohmayer of NASA's Goddard Space Flight Center in Maryland noticed that the X-ray bursts coming from some neutron stars flickered several hundred times a second.
Strohmayer reasoned that the effect was related to the stars' rotation, but didn't know how. "The question is, what would allow you to see the star spinning?" he says. Now Jeremy Heyl from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, suggests that weather systems on the surface of the stars could be causing the flicker.
On Earth, the primary weather system is the westerly jet stream, resulting from the Earth's rotation relative to its atmosphere. The rotation also causes planetary waves, called Rossby waves, which move westwards and modulate the jet stream.
A similar effect may occur on neutron stars, Heyl told astronomers attending the Chandra Symposium at the centre last week. The stars pull in matter from a normal neighbouring star, and when this pours onto the neutron core it creates an iron crust and an atmosphere of hydrogen and helium. The gases produce X-rays when they burn through nuclear fusion.
Rossby waves in the atmosphere would make some parts burn brighter, Heyl says. "As the star rotates you see light and dark spots." His calculations show that the spinning rate of a neutron star combined with the speed of the Rossby waves across its surface exactly reproduce the pattern of X-ray flicker that astronomers have observed.
This is the first real insight into what the surface of a neutron star might look like, Strohmayer says. "It's a nice piece of work."
Author: Eugenie Samuel, Boston
New Scientist issue: 27 October 2001
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