Science: When stars get the shakes

A blast from an exceptionally powerful burst of cosmic energy disrupted the earth's ionosphere - the electrically charged part of the upper atmosphere - on 27 August. "It was as if night was briefly turned into day," Professor Umran Inan said.

To anyone watching without radio-sensitive instruments, the change wouldn't have been obvious - but to Professor Inan and his colleagues, the levels of electrical activity, comparable with those produced by the sun in the day, were a clue to a new type of star in the sky. Earlier this week, astronomers met at the Washington DC headquarters of the US space-agency Nasa to describe the "magnetars".

With crusts of solid iron and ultra-dense fluid-filled interiors, these stars harbour a weird secret. Every now and again they shudder with violent energy in a cosmic equivalent of a global earthquake. They possess enormous magnetic fields, some a thousand trillion times stronger than the earth's. Sometimes they experience forces so intense that their crusts are ripped apart - which may explain one of the greatest mysteries of deep space.

Astronomers first detected the repeating bursts of energy - equivalent, at their source, to the entire annual output of the sun being emitted in a few seconds - in 1979. Astronomers now think they have proof that magnetars are responsible for the unexplained bursts of energy of the past two decades.

As Dr Chryssa Kouveliotou, Nasa researcher and leading magnetar-hunter, says: "The importance of this discovery goes beyond adding a new oddity to the list of star types. It reveals that magnetars hold the key to a 19-year-old astronomical puzzle - repeated bursts of cosmic radiation, of unknown origin, that occur intermittently from certain directions in the sky."

Magnetars were first proposed six years ago, the dream-child of Dr Robert Duncan, at the University of Texas at Austin, and Dr Christopher Thompson, at the University of North Carolina at Chapel Hill. In the late Eighties, these two astronomers began a programme of research that would ultimately lead to a theory of "quaking stars", although they had no notion of that at the time.

"We were trying to understand a completely different issue - the origin of magnetic fields in radio pulsars," Dr Duncan says.

Radio pulsars are the ultra-dense relics of stars that were about five to 10 times heavier than the sun. Ordinary stars shine from the heat of nuclear reactions, but ultimately the nuclear fuel runs out, and the star collapses under its own weight. This collapse results in a spectacular explosion, called a supernova. Stars whose original mass was 10 times that of our sun then collapse into a black hole.

But the rest leave a small, iron-encrusted remnant, 10 miles across, which crushes itself until most of its protons and electrons turn into denser neutrons. This is a neutron star, an astronomical phoenix: if it is spinning rapidly (as is often the case) it is known as a radio pulsar, because it emits radio pulses.

Radio pulsars are strongly magnetic. And measurements reveal that their magnetic fields are always of about the same strength. This seemingly bland fact hides a wealth of intrigue. To their surprise, Dr Duncan and Dr Thompson calculated that magnetic fields 100 or 1,000 times stronger were possible. They christened these hypothetical, super-magnetic stars "magnetars". But why had none been found?

Maybe it was because the huge magnetism would somehow transform the stars' character and appearance, so that they would not resemble pulsars at all. After all, power has a tendency to corrupt. As Dr Duncan says: "We began to wonder what a magnetar would look like."

They realised that, when it comes to hunting out magnetars, radio signals are a non-starter: magnetars cannot emit them. Pulsar radio blips are caused by beams of charged particles that flow out along magnetic-field lines from each pole of the star. As the star rotates, these beams cross our line of sight and we receive a "lighthouse" flash of radio waves. But the generation of this radio emission requires a fast-spinning star, whereas theory predicts magnetars should rotate slowly.

So what is a magnetar's calling-card? Dr Duncan and Dr Thompson predicted that the colossal magnetism would affect the star violently. The magnetic fields would cause tremendous heating, making the star glow in the X- ray part of the electromagnetic spectrum. And, where the field passed through the magnetar surface, the crust would be stressed and buckled by magnetic forces so great that it would sometimes rupture.

So, to find a magnetar, look for a starquake. But how do you see one? Optical telescopes are useless. Magnetars would be much too small and faint to reveal themselves - let alone any surface detail - so easily. But, in theory, when a magnetar shakes, it produces one almighty disturbance. As the surface cracks, a powerful surge of magnetic energy will be released, producing a blast of radiation that will stream far out into the cosmos. These were ground-breaking ideas, but the problem was to prove them.

Magnetars would be more credible if these hypothetical energy outbursts could be detected. Then Dr Duncan and Dr Thompson had a revelation: the radiation flashes had already been seen for years. Astronomers even had a name for them: "soft gamma-ray repeaters".

In the late Sixties, the US launched its Vela satellites to watch out for illegal nuclear testing in space. The instruments were sensitive to gamma rays, a form of high-frequency radiation even more energetic than X-rays. Gamma rays can take a range of energies, and the term "soft" refers to the less energetic ones. Many brief unexpected flashes of gamma rays were detected, but fortunately the origin of the radiation was found to be cosmic rather than man-made.

But their exact source was a mystery. Over the years, it became clear that the bursts came in distinct types. One sort - the soft gamma ray repeaters - seemed to recur at irregular intervals from a handful of locations in the sky. Whatever was causing these flashes of radiation apparently had the power to do it over and over again. Moreover, the outbursts were huge, a single event radiating as much energy as the sun in a year.

Dr Duncan and Dr Thompson strongly suspected that magnetars were the culprits. Observations of soft gamma ray repeaters had shown that the bursts seemed to originate in the clouds of debris left over from supernova explosions, a natural place to find a magnetar. And the radiation produced by intermittent starquakes could explain the soft gamma ray bursts. Moreover, astronomers also had detected steady X-ray emission coming from the direction of the soft-gamma ray repeaters. Could this be the glow of a hot magnetar?

The pieces seemed to fit together. In late 1996, an old soft gamma ray repeater started bursting again. Dr Kouveliotou and her colleagues realised that this could be their chance to test the magnetar theory.

Using the Rossi X-ray Timing Explorer, a satellite designed to observe high energy, they found evidence of a star that was spinning slowly (once every 7.47 seconds) and slowing down rapidly. Its magnetic field was colossal, of magnetar proportions. So, after two decades, soft gamma ray repeaters have finally shed their aura of mystery, and astronomers have a new type of star to reckon with.

Karen Southwell is an assistant editor at `Nature'