Gamma-Ray Bursts: Spindown Of Cosmic Flywheels

December 03, 1998

Gamma-ray bursts (GRB) are short lived sources of energetic radiation observed at random positions in the sky at a rate of about 2 per day. Several of them have been identified with distant galaxies, roughly halfway between us and the end of the observable universe. This large distance makes them extremely energetic phenomena, corresponding to the complete conversion into energy of a mass of the order of a giant planet, in a time span of a few seconds. The extreme properties of these sources make them one of the most hotly debated mysteries in astrophysics.

By identifying two key elements of a puzzle, research at the Max Planck Institute for Astrophysics has shown how a rapidly spinning neutron star can quite naturally turn itself into a Gamma-ray burster. A neutron star rotating at a period of a millisecond or so contains the right amount of energy in rotation to power a GRB. Such spinning neutron stars occur in X-ray binary stars (of which there are a few hundred in our own galaxy). By mass transfer from its companion, the neutron star in such a system is spun up like a flywheel.

The main problem consists in finding a sufficiently powerful "brake" to extract the rotation energy from such a spinning star, since in an ordinary X-ray binary system the forces acting on the neutron star are only very feeble.

It has been known that a neutron star starts oscillating spontaneously if it rotates fast enough, and in the process radiates gravitational waves (as described by General Relativity). This oscillation is excited much like the squeaking of a glazed car brake, where the brake represents the neutron star and the brake lining the gravitational wave. It was also known that a magnetic field at the surface of the star can spin it down and radiate away ist rotation energy in the form of an electromagnetic wave (as observed in detail in the pulsar in the Crab nebula). But to brake the star within a few seconds by this process, a magnetic field strength of 1012 Tesla is needed, and so far there did not seem to be a very good reason why a neutron star would suddenly get magnetized to such a fantastic field strength.

Henk Spruit at the Max Planck Institute for Astrophysics in Garching, Germany, has now found the two missing pieces of the puzzle, and shown that a sudden magnetization is in fact quite likely for a rapidly rotating neutron star (Henrik Spruit, Gamma-ray bursts from X-ray binaries, Astronomy & Astrophysics, Vol. 341, Issue 1, L1 - L4, 1998). First, it turns out that due to the negative temperature dependence of neutron star matter, the gravitational wave instability is a runaway process. After a slow initial phase lasting a few hundred years, the amplitude of the oscillation grows explosively in hours or minutes. Secondly, the rotation of different parts of the star is braked to a different extent by the gravitational radiation they emit. The resulting differential rotation between interior and outer parts of the star strengthens the weak intial magnetic field of the star by "winding up" of the field lines.

When the field has become strong enough to float from the interior to the surface, at a field strength of about 1013 T, the entire remaining rotation energy of the star is radiated electromagnetically in a few seconds. The properties of this model are so far in good agreement with all characteristics of GRB.
Published: 3-12-98
Contact: Hendrik Spruit
Max Planck Institute for Astrophysics,
Phone: (+49 89) 32 99 - 32 20
Fax: (+49 89) 32 99 - 32 35


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