This Week

29 August 1998

Quasars pack a punch

COSMIC rays of exceptional power have reached Earth from the outermost edges
of the Universe, say an American physicist and a German astronomer. They have
studied the five most energetic cosmic rays ever observed and conclude that they
came from five extremely distant quasars.

This conclusion is important because, if true, it strengthens theories that
there may be a new subatomic particle
(This Week, 9 August 1997, p 18). “No
known particle could have survived such a long journey,” says Glennys Farrar,
one of the researchers, at Rutgers University in New Jersey.

When the particles that form ultra-high-energy cosmic rays—such as
protons, atomic nuclei or photons—travel through space they are sapped of
energy as they collide with the relic photons of the big bang. Because of this
gradual loss of energy, particles with an energy greater than 1020
electronvolts should not theoretically be able to travel more than 150 million
light years and any that originate farther away should not reach Earth
(see Diagram).

Number of cosmic rays reaching Earth

Yet Farrar and Peter Bierman of the Max Planck Institute for Radio Astronomy
in Bonn have studied data on the rays, which all have energies greater than
1020 electronvolts. All appear to be coming from quasars more than 150 million
light years away. “In fact,” says Farrar, “one quasar, known as 1204+281, is
about 12 000 million light years away—80 per cent of the way to the edge
of the Universe.”


All five quasars are of a type known as “compact radio-loud”. Such quasars
are the most powerful sources of energy in the Universe. “If any objects can
accelerate cosmic ray particles to 1020 electronvolts and beyond, these quasars
can,” says Farrar.

But it is difficult to explain how the cosmic rays could have crossed such
distances to Earth. Farrar believes that the rays may contain a new particle.
The particle is predicted by some so-called “supersymmetric” theories, which
attempt to unify gravity with other forces of nature.

The particle is called the S0—a peculiar hybrid consisting of a normal
up, down and strange quark bound to a gluino. In supersymmetric theories, every
known particle has a “superpartner” and the gluino is the superpartner of the
gluon. “An S0 would be neutral and less likely to interact with the big-bang
photons,” says Farrar. “It could easily travel across thousands of millions of
light years.”

“It’s a very exciting possibility,” says David Hough of Trinity University in
San Antonio. “We’re talking about pieces of quasars—objects at the very
edge of the Universe—actually falling to Earth.”

It could be a coincidence that the rays seem to be coming from the five
quasars, admits Farrar. “But we calculate that the chance of that happening is
less than half of one per cent due to the rarity of the type of quasar
involved.” Farrar and Bierman have submitted a paper to the journal Physical
Review Letters.