Neutron stars may have weather systems like those on Earth. This novel
idea may explain why some neutron stars emit mysterious flickering X-rays.

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.”

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Author: Eugenie Samuel, Boston

New Scientist issue: 27 October 2001

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