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FGST's New Gamma-Ray Catalog: Blazars and Pulsars and WIMPS, Oh My!

The Large Area Telescope team from the Fermi Gamma-ray Space Telescope released a full list of 205 bright objects online February 9, from which the sky map and top-ten list released today were drawn. This catalog of the most vivid gamma-ray sources found in the mission's first three months will help researchers from other institutions plan projects using FGST data.

For the first year of observations, the LAT team has sole access to the telescope's data. This group includes a team at SLAC, which managed the development of the LAT and runs data processing and instrument operations. But in August 2009, all the data and analysis tools will go public. NASA offers funding for other teams to use this data, and accepted project proposals until March 6. FGST released the bright source list halfway through the first year to allow other researchers to craft informed proposals.

"It will give astronomers some idea of what's coming out at the end of the year," said Seth Digel, an experimental physicist at SLAC. "But it's just the tip of the iceberg." The list represents the most statistically significant half of the gamma-ray sources the LAT has identified so far. The team expects hundreds more by August, and more than one thousand when they release the entire first-year data set in November.

More than half the list consists of blazars, unusually bright objects at the centers of galaxies. Astronomers think their radiation comes from material falling onto a supermassive black hole at the center of the galaxy. The resulting hot disc of gas and subatomic particles creates a magnetic field, which shoots jets of plasma at near-light speeds almost directly at Earth.

About 30 objects listed are gamma-ray pulsars, stellar leftovers from supernova explosions that spin rapidly and sweep the sky with a beam of electromagnetic energy. Some of them were already pegged as pulsars from radio observations, and FGST confirmed that they also emit gamma-rays. But about a dozen are previously unheard-of radio-quiet pulsars, which whisper in radio frequencies but shout in gamma-rays. "It wasn't always clear that we would see as many of these guys as we have," said Jim Chiang, a member of the LAT team at SLAC. These pulsars, discovered by LAT team members at the Naval Research Lab and UC Santa Cruz, were only detectable because of FGST's sensitivity. "It's a testament to the collecting area of the LAT, but also to the cleverness of the people on our teams," Digel said.

But of the 205 objects in the catalog, 37 have scientists guessing. "Those are the most interesting," Digel said. "It should be good hunting for people trying to track down what these unidentified sources are."

One exciting possibility is that they are composed of weakly interacting massive particles, or WIMPs, annihilating each other in a collision. WIMPs are hypothetical invisible particles that some hope will solve the mystery of dark matter. "If it's a dark matter particle colliding with another dark matter particle, that would suit many people just fine," Digel said.

óLisa Grossman
SLAC Today, March 11, 2009