Discovery of an Unexpected Surplus of Cosmic Ray Electrons at Very High Energy (300-800 GeV)
John Wefel [Photo courtesy: Louisianna State University]
In an article published in today's issue of 'Nature', a team of researchers from the Advanced Thin Ionization Calorimeter, or ATIC collaboration, have announced the discovery of an unexpected surplus of cosmic ray electrons at very high energy – 300-800 billion electron volts – which must come from a previously unidentified source near the Earth’s solar system.
John P. Wefel, principal investigator for the ATIC project and professor in the Department of Physics & Astronomy at Louisianna State University, said,“This electron excess cannot be explained by the standard model of cosmic ray origin, in which electrons are accelerated in sources such as supernova remnants and then propagate through the galaxy to us. Rather, there must be another source relatively near us that is producing these additional particles.”
According to the paper, this source would need to be within about 3,000 light years of the sun and could be an exotic object such as a pulsar, mini-quasar, supernova remnant or even an intermediate mass black hole. “The trouble with this explanation is that there are very few such objects close to the solar system and none have been observed with characteristics that could fit our results,” said Prof. Wefel. “However, we cannot rule out this possibility, and the ATIC results may be the first indication of a very interesting object near our solar system waiting to be studied by other instruments.”
An alternative explanation is that the surplus of high energy electrons might result from the annihilation of very exotic particles put forward to explain dark matter. Over the last several decades, scientists have learned that the kind of material making up the world around us only accounts for about 5% of the mass composition of the universe. Close to 70% of the universe is composed of dark energy – so called because its nature is unknown – which appears to be causing the universe’s expansion to accelerate. The remaining 25% acts gravitationally just like regular matter, but does nothing else so is normally not visible and consequently is referred to as dark matter.
T. Gregory Guzik [Photo courtesy: Louisianna State University]
The nature of dark matter is not understood, but several theories that attempt to describe how gravity works at very small, quantum distances predict exotic particles that could be good dark matter candidates and which, upon annihilation with each other, produce normal particles that scientists can observe. T. Gregory Guzik, ATIC co-investigator, said,“One such predicted particle has annihilation characteristics that would produce a very good fit for the ATIC results. If true, this would be a major advance in our understanding of dark matter and its role in the universe.”
However, the trouble with this model is that the ATIC result would require the dark matter annihilation rate to be 200 times larger than that calculated by some theoreticians. “This might be possible if dark matter is found in clumps, and one of these clumps is very close to our solar system", said Prof. Guzik.
It may take quite some time before having a full understanding of the nature and origin of these very high energy cosmic rays, but the current result certainly paves the way for a new exciting physics in the 100-500 GeV region of the cosmic-ray spectrum.
ATIC in external frame [Photo courtesy: ATIC Collaboration]
The 4,300-pound ATIC experiment was designed to be carried to an altitude of about 124,000 feet above Earth in order to study the cosmic rays that would otherwise be absorbed into the atmosphere. The original purpose of ATIC was to investigate where cosmic rays come from and how they are accelerated to such high speeds.
ATIC is an international collaboration of researchers from LSU, University of Maryland, Marshall Space Flight Center, Pruple Mountain Observatory in China, Moscow State University in Russia and Max-Planck Institute for Solar System Research in German. ATIC is supported in the United States by NASA and flights are conducted under the auspices of the Balloon Program Office at Wallops Flight Facility by the staff of the Columbia Scientific Balloon Facility.
"An excess of cosmic ray electrons at energies of 300–800 GeV"
J. Chang, J. H. Adams, H. S. Ahn, G. L. Bashindzhagyan, M. Christl, O. Ganel, T. G. Guzik, J. Isbert, K. C. Kim, E. N. Kuznetsov, M. I. Panasyuk, A. D. Panov, W. K. H. Schmidt, E. S. Seo, N. V. Sokolskaya, J. W. Watts, J. P. Wefel, J. Wu & V. I. Zatsepin,
Nature 456, 362-365 (20 November 2008), Abstract.