Scientists using CSIRO’s Australia TelescopeCompact Array, a radio synthesis
telescope in New South Wales, Australia, have 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 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

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

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

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 of Unversity College London, UK.


  • 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)


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).