What is the unusual source of high-energy cosmic rays that has been discovered within 3000 light years of the Sun? Everyone loves a mystery, and this one has all the earmarks of a classic. The source was found by the Advanced Thin Ionization Calorimeter (ATIC) experiment, which was lofted to high altitude above Antarctica via helium-filled balloon. Behind the experiment was the goal of studying cosmic rays that are otherwise shielded from the surface by the Earth’s atmosphere, but among the results was an unexpected finding.
Cosmic ray electrons at 300 to 800 billion electron volts are simply too powerful to be regarded as standard fare, for these particles lose energy as they move through the galaxy. That means that a study like this should see fewer electrons at higher energies. Nearby sources, on the other hand, stand out, making it clear there is what principal investigator John Wefel (Louisiana State) calls “…a very interesting object near our solar system waiting to be studied by other instruments.” We’re talking about candidates ranging from a pulsar or a black hole to a supernova remnant or even a mini-quasar. See this news release for more.
Then again, could this be an indication of dark matter in the neighborhood? The annihilation of exotic particles caused by two of them colliding — some believe such particles are candidates for dark matter — could produce just what we are seeing, says Eun-Suk Seo (University of Maryland), who adds that the process would produce normal electrons and protons and their antimatter equivalents, positrons and antiprotons.
That latter possibility is highly theoretical, depending upon dark matter theories that invoke extra dimensions, but we still have to find out whether there is an unseen object that is accelerating electrons to these energy levels, surely the first step in characterizing this mysterious source. And here another mission may help us. For what ATIC has uncovered is roughly similar to data from the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) satellite, which likewise found a source of energetic particles in the same range. From the paper on the PAMELA results:
Our results clearly show an increase in the positron abundance at high energy that cannot be understood by standard models describing the secondary production of cosmic-rays. Either a signiﬁcant modiﬁcation in the acceleration and propagation models for cosmic-rays is needed, or a primary component is present. There are several interesting candidates for a primary component, including the annihilation of dark matter particles in the vicinity of our galaxy. There may also be a contribution from near-by astrophysical sources, such as pulsars.
So what are these exotic particles? Here I want to quote from an article by Geoff Brumfiel on this work in Nature, running in the same issue as the ATIC results:
The exact nature of the dark-matter particles that produce electrons is uncertain, but one idea is that they may be ordinary particles that spend part of their lives in a compact extra dimension of space. Whereas the particles would appear relatively stationary to observers trapped in three spatial dimensions, they could be moving at ultra-high speeds in a fourth spatial dimension. At high speeds, they would create a gravitational force that could be felt by matter trapped in three dimensions of space-time.
I’ll opt for a nearby pulsar, as discussed in the above article, one whose magnetic fields would create the needed acceleration of electrons, but we may just have to wait for the Fermi Gamma-ray Telescope (originally called GLAST) to verify both ATIC and PAMELA. The paper on ATIC is Chang et al., “An excess of cosmic ray electrons at energies of 300–800 GeV,” Nature 456 (20 November 2008), pp. 362-365 (abstract). The PAMELA work is Adriani et al., “Observation of an anomalous positron abundance in the cosmic radiation,” submitted to Nature and available online.