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? XXM-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.”

Note to editors
The result is published today in Science. The authors are Dr. Margarida
Hernanz and Dr. Gloria Sala from Institut de Ciencies de l’Espai (CSIC)
and Institut d’Estudis Espacials de Catalunya (IEEC), Spain.

XMM-Newton
XMM-Newton can detect more X-ray sources than any previous satellite and
is helping to solve many cosmic mysteries of the violent Universe, from
black holes to the formation of galaxies. It was launched on 10
December 1999, using an Ariane-5 rocket from French Guiana. It is
expected to return data for a decade. XMM-Newton’s high-tech design
uses over 170 wafer-thin cylindrical mirrors spread over three
telescopes. Its orbit takes it almost a third of the way to the Moon, so
that astronomers can enjoy long, uninterrupted views of celestial
objects.

Integral
The International Gamma Ray Astrophysics Laboratory (Integral) is the
first space observatory that can simultaneously observe celestial
objects in gamma rays, X-rays, and visible light. Integral will be
launched on a Russian Proton rocket on 17 October 2002 into a highly
elliptical orbit around Earth. Its principal targets will be the most
violent phenomena in the Universe like supernovae, the explosions known
as gamma-ray bursts and regions in the Universe thought to contain black
holes.

For more information please contact:
Dr. Fred Jansen – ESA XMM-Newton Project Scientist
Tel: +31 71 565 4426
E-mail: fjansen@rssd.esa.int

Dr. Margarida Hernanz
Institut de Ciencies de l’Espai/Institut d’Estudis Espacials de
Catalunya (CSIC /IEEC), Spain.
Tel: 34-93-2058528
E-mail: hernanz@ieec.fcr.es

ESA – Science Programme Communication Service
Tel : +31 71 565 3273
E-mail : irina.bruckner@esa.int

For more information on XMM, Integral and the ESA Science Programme
please visit : http://sci.esa.int
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