June 12, 2012

Fermi Detects Highest-Energy Light Ever

Lee Rannals for redOrbit.com

NASA said that its Fermi Gamma-ray Space Telescope detected the highest-energy light ever associated with an eruption on the sun on March 7.

The latest discovery, according to the space agency, is echoing in Fermi's new role as a solar observatory, which is a tool that is being used to understand solar outbursts.

The March 7 flare was a class X5.4, and is the strongest eruption so far observed by Fermi's Large Area Telescope (LAT).

"For most of Fermi's four years in orbit, its LAT saw the sun as a faint, steady gamma-ray source thanks to the impacts of high-speed particles called cosmic rays," said Nicola Omodei, an astrophysicist at Stanford University in California. "Now we're beginning to see what the sun itself can do."

During the peak of the flare, Fermi detected gamma rays with two billion times the energy of visible light, or about four billion electron volts (GeV), setting a record for the highest-energy light ever detected during or immediately after a solar flare.

According to NASA, the flux of high-energy gamma rays was 1,000 times greater than the sun's output.

The space agency said the event marks the first time a greater-than-100 MeV gamma-ray source has been localized to the sun's disk.

Fermi's LAT scans the entire sky every 3 hours, looking for gamma rays with energies ranging from 20 MeV to more than 300 GeV. Its high sensitivity and wide field of view make the LAT a good tool for solar monitoring.

Fermi's Gamma-ray Burst Monitor (GBM) scans the sky at any given moment, detecting light at energies from 8,000 eV to 40 MeV. This instrument helps scientists access a lower, overlapping energy range.

"Seeing the rise and fall of this brief flare in both instruments allowed us to determine that some of these particles were accelerated to two-thirds of the speed of light in as little as 3 seconds," said Michael Briggs, a member of GBM team at the University of Alabama in Huntsville, said in a press release.

Solar eruptions are on the rise because the sun is progressing toward a peak of its 11-year-long activity cycle, which is expected to last until mid-2013.

"Merged with available theoretical models, Fermi observations will give us the ability to reconstruct the energies and types of particles that interact with the sun during flares, an understanding that will open up whole new avenues in solar research," Gerald Share, an astrophysicist at the University of Maryland in College Park, said in a press release.