A joint Fermilab/SLAC publication

Pulsars or dark matter might be the source of high-energy cosmic electrons

May 02, 2009

Something in our galactic neighborhood seems to be producing large numbers of high-energy electrons, according to new data gathered by the Fermi Gamma-ray Space Telescope. The electrons could be coming from nearby pulsars-or they could be a longed-for signal of dark matter, the elusive, invisible material thought to make up nearly a quarter of the universe.

FGST's Large Area Telescope, a collaboration between NASA, the US Department of Energy, and multiple international partners, has been scanning the skies for gamma rays and particles since its launch last summer. The LAT, which was assembled at the SLAC National Accelerator Laboratory in Menlo Park, California, measured a strikingly high number of electrons with energies between 100 billion and one trillion electronvolts. It is not known from the LAT data alone if these electrons are coming from the distant background, or are the signal of a nearby source of high-energy particles.

"If these particles were emitted far away, they'd have lost a lot of their energy by the time they reached us," said LAT collaborator Luca Baldini of the Istituto Nazionale di Fisica Nucleare in Pisa, Italy.

When combined with other recent results, the LAT finding provides compelling evidence that something close by is churning out high-energy particles. The European satellite PAMELA, for example, last fall reported detecting surprisingly large quantities of high-energy positrons, the antimatter counterparts of electrons.

"Between the PAMELA results and our results, it's very hard to construct a conventional galactic cosmic-ray model" explaining these particle energies, said Elliott Bloom, a SLAC physicist who works on the LAT project. "You need relatively local sources of positrons and electrons."

These local sources could be pulsars, rapidly rotating neutron stars that emit intense electromagnetic radiation, positrons, and electrons. Alternatively, they could be bits of dark matter annihilating when they crash into each other or decaying because they are unstable. Such annihilations and decays also release high-energy particles, theorists think.

Physicists infer the existence of dark matter-which doesn't interact with any of the electromagnetic forces, making it invisible to our eyes and our instruments-from its gravitational effects on light and "normal matter" such as stars, planets and interstellar gas. Though studies suggest that dark matter is more than five times as abundant as normal matter, nobody has yet directly measured the strange material or characterized its nature. The LAT team isn't claiming they have detected dark matter.

"Occam's Razor says pulsars are the most prosaic, and therefore perhaps most likely, explanation," Bloom said. "But dark matter is also a possibility. This is particle astrophysics at its most exciting, trying to track down what's going on here."

A few other projects have recently mapped the spread of electron energies in space. One, the ATIC collaboration, found an even larger number of high-energy electrons than LAT did. However, the balloon-based ATIC must deal with atmospheric interference, which the orbiting LAT doesn't have to worry about. And LAT is a remarkably precise instrument.

"This measurement provides the definitive determination of the spectrum of electrons outside of Earth's atmosphere," said SLAC physicist Greg Madejski, another LAT team member. Without such an accurate spectrum, suppositions about pulsars, dark matter or any other source of high-energy particles are on shaky ground. The current measurement allows the Fermi LAT team to constrain astrophysical models but the young mission needs to collect further data to say definitively whether or not there is a signal due to dark matter.

The LAT measurements, presented in the opening plenary session at the American Physical Society meeting in Denver, Colorado on May 2 and published in the journal Physical Review Letters, are difficult to make. Luca Latronica of INFN, Pisa, Alex Moiseev of NASA's Goddard Space Flight Center, and Stefano Profumo of the University of California, Santa Cruz, will present further details of the results and their interpretation on behalf of the Fermi LAT collaboration at the APS meeting on Monday, May 4.

For each electron that hits LAT's detectors, 50 to 100 other charged particles, mainly protons, come through as well. "It's like finding a needle in a haystack," Baldini said. "It requires a lot of simulations, a lot of cross-checking and a lot of study about how electrons behave in the detector."

The LAT team is currently trying to pin down where exactly the electrons are coming from. The possibilities are that some electrons are coming from local sources, such as pulsars, supernova remnants, or from dark matter particle annihilations. They're hoping to correlate any significant departures from the background with positions of known pulsars. And the LAT team is extending measurements even further, to energies of a few trillion electronvolts, according to collaborator Igor Moskalenko of Stanford University.

"What we will see at higher energies can only come from local sources," he said. "If there are cosmic ray sources nearby, we may be able to find them."

By Mike Wall

Update: The scientific paper has now been published. You can get to it free by going to the viewpoint in APS' online publication Physics and clicking through free. If you go direct to the paper, you will need to be a subscriber to access it.

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