Supermagnetic stars

Scientists discover stars with magnetic fields some million billion times that of the Earth

Published: Monday 31 August 1998

The Crab nebula was discovered (Credit: NASA)NEUTRON stars are rapidly rotating stars composed predominantly of closely packed neutrons. They emit rapid, regular pulses of radiation (usually at radio frequencies, and are known as pulsars). Since 1967, when the first pulsar was discovered accidentally, a lot of research has gone into neutron stars. One of the several end states of stars, these objects have very peculiar properties. They have enormous densities - some hundred thousand billion times that of the Sun - while their size is about 20 km. With such huge densities, they are also the most rapidly rotating objects in the Universe. Adding to the list of these extreme properties is the discovery of the highest magnetic fields ever observed. A team led by C Kouveliotou of the Space Science Laboratory, Huntsville, USA, has announced the discovery of a x-ray pulsar which has a superstrong magnetic field, some million billion times that of the Earth (Nature, Vol 393, No 6682).

To understand the significance of their discovery it is essential to back-track a little. Neutron stars, being so small, are impossible to see directly. However, they have several manifestations by which their presence can be inferred. The most common are pulsars or pulsating radio stars. When a star reaches the end of its life as a regular or main sequence star like the Sun, one of the ways it dies is through a massive explosion called supernova. Sometimes, the remnant of a supernova is a neutron star, which is rotating rapidly, at times as much as a 100 rotations per second. This rapid rotation and the presence of very high magnetic field generates particles moving at enormous speeds. These particles give out radio waves which are observed as pulses at regular intervals by radio telescopes. The source of the energy in a pulsar is the rotation of the star. Several thousand pulsars have been observed with periods ranging from a few seconds to milliseconds. One such pulsar is in the Crab nebula, itself a supernova, recorded in the 11th century by Chinese astronomers.

Scientists argue that neutron stars with bigger magnetic fields also exist, and have termed them magnetars. A pulsar's source of energy is the rotation, whereas in a magnetar, it is the decay of the magnetic field. Kouveliotou and his team have discovered one such candidate in another class of mysterious objects called gamma ray bursters. Gamma ray blasts are among the most enduring puzzles in astronomy, and are observed randomly in the sky about once a day. They are mysterious in their distance and their source of energy. A particular class of such bursts, soft gamma ray repeaters,emit multiple, brief and intense bursts of low energy gamma rays.

Soft gamma ray repeaters are very rare. Two have been associated with supernova remnants. A new repeater discovered by astronomers is one with a period of 7.47 seconds and a slowdown rate of about a thousandth of a second per year. It has an unimaginably high magnetic field. To top it all is the announcement of the discovery of a source with 89-second pulsation. If this report is also confirmed, then it would further boost the theory of magnetars having very high magnetic fields and longer periods.

The astronomers are now waiting to measure the changes in the periods of the two sources as these slowdowns will provide important clues to the nature of the source. They will also wait for the detailed x-ray spectra which will be soon obtained by a satellite. But it is clear that the presence of such high magnetic fields in objects in our Universe will force us to change the way we have understood the evolution of stars.

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