Squirty star imitates black hole (Forwarded)

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Media Release: Jan 15 , 2004

Ref 2004/05

Squirty star imitates black hole

Scientists using CSIRO's Australia Telescope near Narrabri in northern NSW have
made a discovery that they hope will increase our understanding of a fundamental
cosmic process.

The team has seen a neutron star spitting out a jet of matter at very close to
the speed of light. This is the first time such a fast jet has been seen from
anything other than a black hole.

The discovery, reported in this week's issue of the journal Nature, challenges
the idea that only black holes can create the conditions needed to accelerate
jets of particles to extreme speeds.

"Making jets is a fundamental cosmic process, but one that is still not well
understood even after decades of work," says team leader Dr Rob Fender of the
University of Amsterdam.

"What we've seen should help us understand how much larger objects, such as
massive black holes, can produce jets that we can see half-way across the Universe."

The scientists, from The Netherlands, the UK and Australia, studied Circinus
X-1, a bright and variable source of cosmic X-rays, over a three-year period.

Circinus X-1 lies inside our Galaxy, about 20 000 light-years from Earth in the
constellation Circinus near the Southern Cross.

It consists of two stars: a 'regular' star, probably about 3 to 5 times the mass
of our Sun, and a small compact companion.

"We know that the companion's a neutron star from the kind of X-ray bursts it's
been seen to give off," says team member Dr Helen Johnston of the University of
Sydney.

"Those X-ray bursts are a sign of a star that has a surface. A black hole
doesn't have a surface. So the companion must be a neutron star."

A neutron star is a compressed, very dense ball of matter formed when a giant
star explodes after its nuclear fuel runs out. In the hierarchy of extreme
objects in the Universe, it is just one step away from a black hole.

The two stars in Circinus X-1 interact, with the neutron star's gravity pulling
matter off the larger star onto the neutron star's surface.

This 'accretion' process generates X-rays. The strength of the X-ray emission
varies with time, showing that the two stars of Circinus X-1 travel around each
other in a very elongated orbit with a 17 day period.

"At their point of closest approach, the two stars are almost touching," says Dr
Johnston.

Since the 1970s astronomers have known that Circinus X-1 produces radio waves as
well as X-rays. A large 'nebula' of radio emission lies around the X-ray source.
Within the nebula lies the new-found jet of radio-emitting material.

Jets are believed to emerge, not from black holes themselves, but from their
'accretion disk' -- the belt of dismembered stars and gas that a black hole
drags in towards it.

In Circinus X-1 it's likely that the accretion disk varies with the 17-day
cycle, being at its most intense when the stars are at their closest point in
the orbit.

The jet from Circinus X-1 is travelling at 99.8% of the speed of light. This is
the fastest outflow seen from any object in our Galaxy, and matches that of the
fastest jets being shot out of other complete galaxies. In those galaxies, the
jets come from supermassive black holes, millions or billions of times the mass
of the Sun, that lie at the centres of the galaxies.

Whatever process accelerates jets to near the speed of light, it does not rely
on the special properties of a black hole.

"The key process must be one common to both black holes and neutron stars, such
as accretion flow," says Dr Kinwah Wu, formerly of the University of Sydney, and
now at Unversity College London in the UK.

More Information:

Dr Bob Sault, Officer-in Charge
CSIRO Australia Telescope Compact Array
Tel: +61-2-6790-4000
Email:

Dr. Helen Johnston
University of Sydney
Tel: +61-(0)401-562-393
Email:

Dr Rob Fender
University of Amsterdam
Tel: +31 20 5257478
Email:

Dr. Kinwah Wu
University College London
Tel: +44 (0) 1483-204127
Email:

Media assistance:

Helen Sim
CSIRO Australia Telescope National Facility
Tel: +61-2-9372-4251 (office)
Email:

TEAM MEMBERS

Rob Fender (University of Amsterdam, NL)
Kinwah Wu (University College London, UK)
Helen Johnston (University of Sydney, Australia)
Tasso Tzioumis (Australia Telescope National Facility, Australia)
Peter Jonker (University of Cambridge, UK)
Ralph Spencer (Jodrell Bank / University of Manchester, UK)
Michiel van der Klis (University of Amsterdam, NL)

PUBLICATION

Rob Fender, Kinwah Wu, Helen Johnston, Tasso Tzioumis, Peter Jonker, Ralph
Spencer & Michiel van der Klis. "An ultra-relativistic outflow from a neutron
star accreting gas from a companion". Nature 427, 222-224 (2004).

Image:
http://www.atnf.csiro.au/news/press/nstarjet/