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Brakes for Neutron Stars - Sources of Gravitational Waves

Exotic objects are likely to be sources of strong gravitational waves/search for "youthful speeders" among the pulsars

August 25, 1999

During certain phases of their life cycle, neutron stars probably emit large quantities of energy as gravitational waves. In any case, this is indicated by model calculations, which an international research team has undertaken together with scientists from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Golm near Potsdam. The results suggest that the super-compact stars radiate an energy equivalent of about one percent of the solar mass - approximately 5 x 1026 tons - in the form of gravitational waves within one year of their formation.

Neutron stars are formed when very massive stars explode at the end of their life cycles as supernovae. The centre of the star is so squeezed together that it essentially consists only of neutrons. Such remains of stars with a diameter of about 20 kilometres have the size of a small town and have been compressed to the density of an atomic nucleus. Because the abrupt shrinking process maintains the angular momentum, a newly formed neutron star can rotate extremely rapidly. It emits electromagnetic radiation which, like the light cone of a cosmic lighthouse, can strike the earth. Astronomers have recorded the pulsating radiation of about a thousand such objects, which are also called pulsars.

For astrophysicists, it is a mystery why there is not a single specimen among young neutron stars that requires less than 15 thousandths of a second to rotate around its own axis. "Theoretically, rotation times of less than one thousandth of a second are possible without having the centrifugal force to tear apart the super-compact neutron star," says Dr. Kostas Kokkotas from Aristotle University in Thessaloniki. He did the calculations together with Dr. Nils Anderson from the University of Southampton/USA and Prof. Bernard Schutz from the Max Planck Institute for Gravitational Physics.

A possible cause of the lack of "young speeders" among the pulsars could be gravitational radiation. Such radiation is especially high just a few weeks and months after a supernova star collapses into a highly compressed sphere of neutrons. Gravitational waves also amplify vibrations of the neutron gas, which has a heat of over a billion degrees. These vibrations are comparable to a wave phenomenon (Rossby waves) on the earth’s oceans and are described by astronomers as r-modes. This effect limits the rotation duration of new neutron stars. As a result, within one year the rotation period can drop from 1,000 to less than 100 revolutions per second, according to findings of the work group.

These research results provide an alternative explanation for models that explain the braking effect with a kind of magnetic coupling between the core area and the gaseous spout of the exploded precursor star, which has been put forth by the astrophysicists Dr. Hendrik Spruit from the Max Planck Institute for Astrophysics in Garching and E. Sterl Phinney from the California Institute of Technology (see also press release of the Max Planck Society of 30 November 1998).

"Some older neutron stars are promising sources of detectable gravitational waves," says Dr. Nikolaos Stergioulas of the Max Planck Institute for Gravitational Physics in Golm. Together with Nils Anderson and Kostas Kokkotas, he analysed compact model stars that absorbed gas from a close companion. Due to the impact of this matter, the neutron balls can get an additional "kick" that accelerates its rotation period. More than a dozen such objects in the universe have already been detected by astronomers. Some require barely more than two thousandths of a second for one orbit.

The most recent model calculations, which have been published in the Astrophysical Journal (Volume 516/1999 /pp. 307-314) show that these neutron stars are likely to emit gravitational waves at comparatively low temperatures of more than 200,000 degrees due to the "r-modes". The matter that comes from the companion star heats up this process and keeps it running. "These neutron stars are particularly interesting to us, because they are not only transmitting gravitational waves for months, but rather for many years, which increases our chances of finding them with instruments on earth," explains Dr. Kokkotas.

Researchers at the Max Planck Institute for Gravitational Physics are optimistic that, together with colleagues from other research institutes, they will be able to experimentally test the results of their mathematical analyses in the first decade of the 21st century. There are currently several instruments for the direct detection of gravitational waves under construction worldwide, so-called gravitational wave interferometers. Prof. Bernard Schutz, Managing Director of the Max Planck Institute for Gravitational Physics, is also a member of the "GEO 600" collaboration that is building a 600-meter interferometer at Hannover. "With the experiments it should be possible to detect gravitational waves for the first time and thus open a new window for astronomical observations," says Dr. Stergioulas. Astrophysical theories such as the brake effect in neutron stars could then be tested.


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