Posted: 21 November, 2008
The NASA-funded Advanced Thin Ionization Calorimeter (ATIC) balloon instrument has discovered a previously unidentified nearby source of high energy cosmic rays.
The 1,950 kilogram ATIC experiment was designed to be carried to an altitude of about 37,800 metres above Antarctica using a helium-filled balloon. The goal was to study cosmic rays – energetic particles originating from objects in space – that otherwise would be absorbed into the Earth’s atmosphere. Published in this week’s issue of the journal Nature, new results show an unexpected surplus of cosmic ray electrons at very high energy – 300-800 billion electron volts – that must come from a previously unidentified source or from the annihilation of very exotic theoretical particles used to explain dark matter.
"This electron excess cannot be explained by the standard model of cosmic ray origin," says John Wefel, ATIC project principal investigator. "There must be another source relatively near us that is producing these additional particles."
The crew from the Columbia Scientific Balloon Facility prepares to launch the Advanced Thin Ionization Calorimeter (ATIC) experiment from McMurdo Station, Antarctica. Image: NASA.
According to the research, this source would need to be within about 3,000 light years of the Sun. But because of their nature, charged particles are deflected in the galactic magnetic field between wherever they originate and when they are detected at the Earth. "While we do measure the arrival direction, it is not connected to the source location because of bending and scattering in the magnetic field," Wefel tells Astronomy Now.
The source of the cosmic rays could be an exotic object such as a pulsar, mini-quasar, supernova remnant or an intermediate mass black hole. "There are new objects and sources being discovered every day and week," says Wefel. "The ones we know about now do not seem to be able to do the job, but maybe next month one will be found to explain our excess electron data. But since we do not yet see such a source, maybe the alternate explanation – dark matter annihilation – should be considered."
Dark matter is thought to comprise around 23 percent of the Universe’s energy density, while ‘normal’ matter comprises just four percent. Dark energy, which is thought to play a dominant role in the expansion of the Universe, makes up 73 percent. The nature of dark matter is not understood, but its presence can be inferred from the gravitational effects it imposes on visible matter, such as distorting galaxy structure and influencing their rotation speeds. Several theories that describe how gravity works at very small, quantum distances predict exotic particles that could be good dark matter candidates.
"The annihilation of these exotic particles with each other would produce normal particles such as electrons, positrons, protons and antiprotons that can be observed by scientists," says Eun-Suk Seo, ATIC lead at the University of Maryland. "These results may be the first indication of a very interesting object near our Solar System waiting to be studied by other instruments," adds Wefel.
The science team are hoping that there will be new and continuing searches to try to locate a nearby source that can explain the data. "But, we also need to study the exact shape of this feature in more detail to see if it really does have the tell-tale shape expected for dark matter annihilation. That is the on-going experimental challenge," says Wefel.