An international team of researchers has discovered a puzzling surplus of high-energy electrons bombarding Earth from space. The source of these cosmic rays is unknown, but it must be close to the solar system and it could be made of dark matter. Their results are being reported in the Nov. 20th issue of the journal Nature.
"This is a big discovery," says co-author John Wefel of Louisiana State University. "It's the first time we've seen a discrete source of accelerated cosmic rays standing out from the general galactic background."
Galactic cosmic rays are subatomic particles accelerated to almost light speed by distant supernova explosions and other violent events. They swarm through the Milky Way, forming a haze of high energy particles that enter the solar system from all directions. Cosmic rays consist mostly of protons and heavier atomic nuclei with a dash of electrons and photons spicing the mix.
To study the most powerful and interesting cosmic rays, Wefel and colleagues have spent the last eight years flying a series of balloons through the stratosphere over Antarctica. Each time the payload was a NASA-funded cosmic ray detector named ATIC, short for Advanced Thin Ionization Calorimeter. The team expected ATIC to tally the usual mix of particles, mainly protons and ions, but the calorimeter found something extra: an abundance of high-energy electrons.
Wefel likens it to driving down a freeway among family sedans, mini-vans and trucks—when suddenly a bunch of Lamborghinis bursts through the normal traffic. "You don't expect to see so many race cars on the road—or so many high-energy electrons in the mix of cosmic rays." During five weeks of ballooning in 2000 and 2003, ATIC counted 70 excess electrons in the energy range 300-800 GeV. ("Excess" means over and above the usual number expected from the galactic background.) Seventy electrons may not sound like a great number, but like seventy Lamborghinis on the freeway, it's a significant surplus.
"The source of these exotic electrons must be relatively close to the solar system—no more than a kiloparsec away," says co-author Jim Adams of the NASA Marshall Space Flight Center.
Why must the source be nearby? Adams explains: "High-energy electrons lose energy rapidly as they fly through the galaxy. They give up energy in two main ways: (1) when they collide with lower-energy photons, a process called inverse Compton scattering, and (2) when they radiate away some of their energy by spiraling through the galaxy's magnetic field." By the time an electron has traveled a whole kiloparsec, it isn't so 'high energy' any more.
High-energy electrons are therefore local. Some members of the research team believe the source could be less than a few hundred parsecs away. For comparison, the disk of the spiral Milky Way galaxy is about thirty thousand parsecs wide. (One parsec approximately equals three light years.)
"Unfortunately," says Wefel, "we can't pinpoint the source in the sky." Although ATIC does measure the direction of incoming particles, it's difficult to translate those arrival angles into celestial coordinates. For one thing, the detector was in the basket of a balloon bobbing around the South Pole in a turbulent vortex of high-altitude winds; that makes pointing tricky. Moreover, the incoming electrons have had their directions scrambled to some degree by galactic magnetic fields. "The best ATIC could hope to do is measure a general anisotropy—one side of the sky versus the other."
This uncertainty gives free rein to the imagination. The least exotic possibilities include, e.g., a nearby pulsar, a 'microquasar' or a stellar-mass black hole—all are capable of accelerating electrons to these energies. It is possible that such a source lurks undetected not far away. NASA's recently-launched Fermi Gamma-ray Space Telescope is only just beginning to survey the sky with sufficient sensitivity to reveal some of these objects.
An even more tantalizing possibility is dark matter.
There is a class of physical theories called "Kaluza-Klein theories" which seek to reconcile gravity with other fundamental forces by positing extra dimensions. In addition to the familiar 3D of human experience, there could be as many as eight more dimensions woven into the space around us. A popular yet unproven explanation for dark matter is that dark matter particles inhabit the extra dimensions. We feel their presence via the force of gravity, but do not sense them in any other way.
How does this produce excess cosmic rays? Kaluza-Klein particles have the curious property (one of many) that they are their own anti-particle. When two collide, they annihilate one another, producing a spray of high-energy photons and electrons. The electrons are not lost in hidden dimensions, however, they materialize in the 3-dimensions of the real world where ATIC can detect them as "cosmic rays."
"Our data could be explained by a cloud or clump of dark matter in the neighborhood of the solar system," says Wefel. "In particular, there is a hypothesized Kaluza-Klein particle with a mass near 620 GeV which, when annihilated, should produce electrons with the same spectrum of energies we observed."
Testing this possibility is nontrivial because dark matter is so, well, dark. But it may be possible to find the cloud by looking for other annihilation products, such as gamma-rays. Again, the Fermi Space Telescope may have the best chance of pinpointing the source.
"Whatever it is," says Adams, "it's going to be amazing."
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