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鈥檚 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鈥� rotation, but
didn鈥檛 know how. 鈥淭he question is, what would allow you to see the star
spinning?鈥� 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.
Advertisement
On Earth, the primary weather system is the westerly jet stream, resulting
from the Earth鈥檚 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. 鈥淎s the star rotates you see light and dark spots.鈥� 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. 鈥淚t鈥檚 a nice piece of work.鈥�