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Exploding star takes astronomers by surprise

October 11, 2002

A partially exploding star, known as a nova, has recovered more quickly than expected, say scientists who have analysed new data from the ESA`s XMM-Newton X-ray satellite.

Nova explosions are not completely destructive phenomena. In fact, after an explosion occurs, the star recovers and starts shining again. Until now, astronomers have not known how long this process takes. In this case, however, the exploding star recovered in less than three years. This is surprising, given the fact that the original explosion released about 100 000 times the energy given out by our Sun in a single year.
Exploding stars come in a number of guises, the largest being supernovae. In their case, nothing is left of the star after its immense detonation, except for the ultimate of all astronomical mysteries: a black hole. However, in the case of a nova, the explosion is not so destructive and the star lives to shine another day. How long does it take to return to normal after the explosion, or outburst as astronomers prefer to call it? XMM-Newton has provided the answer - a few years at most.

A nova is composed of two stars. Their `normal` state is for one star to be pulling the other to pieces. Originally starting as two ordinary stars, they are held together by the force of their gravity. Both shine steadily into space. However, one star ages faster than the other, becoming a small, hot core known as a white dwarf star. The pair become locked in a destructive cycle in which the white dwarf pulls matter from its companion, cloaking itself with the stolen gases. Once enough gas has built up, a catastrophic nuclear reaction begins, causing a massive explosion to engulf the white dwarf. Although this is not large enough to destroy the star, it causes a giant outburst of material from the surface that disrupts the flow of material from the larger star onto the white dwarf.

The nova V2487 Oph suffered just such an outburst in 1998. Observations with ESA`s XXM-Newton satellite show that the white dwarf star has resumed `eating` its neighbour in just 2.7 years. This is faster than astronomers had previously imagined. Margarida Hernanz is the principal investigator of the research at the Institut d`Estudis Espaciales de Catalunya and Spanish Research Council, Spain. She was peering through the expanding cloud of debris from the outburst to see whether nuclear reactions were still taking place on the surface of the white dwarf. She and fellow researcher, Gloria Sala, saw that particular signature in their data but they also discovered something unexpected. A much more energetic set of X-rays signalled V2487 Oph had returned to normal and was again pulling gas from its companion.

This signal matches that of a chance observation taken in 1990, by the ROSAT X-ray satellite, during an all-sky survey, before the system was known to be a nova. That makes it the first nova to have been observed in X-rays before and after the outburst.

Understanding the nature of novae is essential to understanding the details of how our Galaxy achieved its chemical composition. Hernanz says, "Although they are not as important as supernovae at influencing the chemical evolution of the Galaxy, novae are important because they produce certain chemicals that other celestial objects do not."

Astronomers can use novae to measure distances to other galaxies. All novae explode with about the same explosive force, so they always reach similar brightnesses. However, distant objects always look dimmer. Since astronomers know how bright the nova should be, they can calculate how far away it is.

As yet, novae have not really been observed at gamma-ray energies. With the launch of Integral next week, that could well change. "Some of the radioactive elements we think are created by novae, give out gamma rays. It would be good to use Integral to attempt their detection, testing out our ideas," says Hernanz.

In the meantime, Hernanz has XMM-Newton data for another nova that she is currently analysing. Talking about her work, Fred Jansen, XMM-Newton`s Project Scientist says, "Work of this quality proves that XMM-Newton is doing what it should be doing, pushing the limits of X-ray astronomy and making new discoveries possible."

European Space Agency (ESA)