Mystery solved: rare cosmic high energie particles come from outside our galaxy
The Pierre Auger Collaboration, in which ASTRON is a partner, reports
observational evidence demonstrating that cosmic rays with energies a
million times greater than that of the protons accelerated in the Large
Hadron Collider come from much further away than from our own Galaxy.
These findings
Ever since the existence of cosmic rays with individual energies of
several Joules was established in the 1960s, speculation has raged as to
whether such particles are created there or in distant extragalactic
objects. The 50 year-old mystery has been solved using cosmic particles
of mean energy of 2 Joules recorded with the largest cosmic-ray
observatory ever built, the Pierre Auger Observatory in Argentina. It is
found that at these energies the rate of arrival of cosmic rays is ~6%
greater from one side of the sky than from the opposite direction, with
the excess lying 120˚ away from the Galactic centre.
In the view of Professor Karl-Heinz Kampert (University of Wuppertal),
spokesperson for the Auger Collaboration, which involves over 400
scientists from 18 countries, "We are now considerably closer to solving
the mystery of where and how these extraordinary particles are created,
a question of great interest to astrophysicists. Our observation
provides compelling evidence that the sites of acceleration are outside
the Milky Way”. Professor Alan Watson (University of Leeds), emeritus
spokesperson, considers this result to be “one of the most exciting that
we have obtained and one which solves a problem targeted when the
Observatory was conceived by Jim Cronin and myself over 25 years ago”.
Rare particles, gigantic detector
Cosmic rays are the nuclei of elements from hydrogen (the proton) to
iron. Above 2 Joules the rate of their arrival at the top of the
atmosphere is only about 1 per sq km per year, equivalent to one hitting
the area of a football pitch about once per century. Such rare
particles are detectable because they create showers of electrons,
photons and muons through successive interactions with the nuclei in the
atmosphere. These showers spread out, sweeping through the atmosphere
at the speed of light in a disc-like structure, similar to a
dinner-plate, several kilometres in diameter. They contain over ten
billion particles and, at the Auger Observatory, are detected through
the Cherenkov light they produce in a few of 1600 detectors, each
containing 12 tonnes of water, spread over 3000 km2 of Western
Argentina, an area comparable to that of Rhode Island. The times of
arrival of the particles at the detectors, measured with GPS receivers,
are used to find the arrival directions of events to within ~1˚.
An extragalactic origin
By studying the distribution of the arrival directions of more than
30000 cosmic particles the Auger Collaboration has discovered an
anisotropy, significant at 5.2 standard deviations (a chance of about
two in ten million), in a direction where the distribution of galaxies
is relatively high. Although this discovery clearly indicates an
extragalactic origin for the particles, the actual sources have yet to
be pinned down. The direction of the excess points to a broad area of
sky rather than to specific sources as even particles as energetic as
these are deflected by a few 10s of degrees in the magnetic field of our
Galaxy. The direction, however, cannot be associated with putative
sources in the plane or centre of our Galaxy for any realistic
configuration of the Galactic magnetic field.
Cosmic rays of even higher energy than the bulk of those used in this
study exist, some even with the kinetic energy of well-struck tennis
ball. As the deflections of such particles are expected to be smaller,
the arrival directions should point closer to their birthplaces. These
cosmic rays are even rarer and further studies are underway using them
to try to pin down which extragalactic objects are the sources.
Knowledge of the nature of the particles will aid this identification
and work on this problem is targeted in the upgrade of the Auger
Observatory to be completed in 2018. Source: Radboud University
