Pulsar or dark matter?
After Pamela, FERMI adds a new piece to the jigsaw
02 May 2009
Too many electrons, too high energy. That's what turned out in 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 U.S. Department of Energy, and multiple international partners (built with an important Italian contribution, coordinated by the Italian Space Agency with the National Institute of Nuclear Physics and the National Institute for Astrophysics), has been scanning the skies for gamma rays and particles since its launch last summer. The LAT 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.
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 believe.
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 measurements were presented at the American Physical Society meeting in Denver, Colorado on May 2 and published online in the journal Physical Review Letters, are difficult to make.
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.