|
During
a powerful solar blast on March 7, NASA’s Fermi Gamma-ray Space
Telescope detected the highest-energy light ever associated with an
eruption on the sun. The discovery heralds Fermi’s new role as a solar
observatory, a powerful new tool for understanding solar outbursts
during the sun’s maximum period of activity.
A
solar flare is an explosive blast of light and charged particles. The
powerful March 7 flare, which earned a classification of X5.4 based on
the peak intensity of its X-rays, is the strongest eruption so far
observed by Fermi’s Large Area Telescope (LAT). The flare produced such
an outpouring of gamma rays—a form of light with even greater energy
than X-rays—that the sun briefly became the brightest object in the
gamma-ray sky.
“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.”
Omodei
described Fermi’s solar studies to journalists today at the 220th
meeting of the American Astronomical Society in Anchorage, Alaska.
At
the flare’s peak, the LAT detected gamma rays with two billion times
the energy of visible light, or about four billion electron volts (GeV),
easily setting a record for the highest-energy light ever detected
during or immediately after a solar flare. The flux of high-energy gamma
rays, defined as those with energies beyond 100 million electron volts
(MeV), was 1,000 times greater than the sun’s steady output.
The
March flare also is notable for the persistence of its gamma-ray
emission. Fermi’s LAT detected high-energy gamma rays for about 20
hours, two and a half times longer than any event on record.
Additionally,
the event marks the first time a greater-than-100-MeV gamma-ray source
has been localized to the sun’s disk, thanks to the LAT’s keen angular
resolution.
Flares
and other eruptive solar events produce gamma rays by accelerating
charged particles, which then collide with matter in the sun’s
atmosphere and visible surface. For instance, interactions among protons
result in short-lived subatomic particles called pions, which produce
high-energy gamma rays when they decay. Nuclei excited by collisions
with lower-energy ions give off characteristic gamma rays as they settle
down. Accelerated electrons emit gamma rays as they collide with
protons and atomic nuclei.
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 an excellent tool for solar
monitoring.
Another
Fermi instrument, the Gamma-ray Burst Monitor (GBM), observes the
entire sky not blocked by the Earth at any given moment. Designed to
detect light at energies from 8,000 eV to 40 MeV, the GBM’s
complementary capabilities give scientists access to a lower, but
overlapping energy range where solar phenomena produce interesting
features.
Both instruments observed a strong, but less powerful solar flare on June 12, 2010.
“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.
Solar
eruptions are on the rise as the sun progresses toward the peak of its
roughly 11-year-long activity cycle, now expected in 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,” said Gerald Share, an astrophysicist at
the University of Maryland in College Park.
NASA’s
Fermi Gamma-ray Space Telescope is an astrophysics and particle physics
partnership. Fermi is managed by NASA’s Goddard Space Flight Center,
Greenbelt, Md. It was developed in collaboration with the U.S.
Department of Energy, with contributions from academic institutions and
partners in France, Germany, Italy, Japan, Sweden and the United States.
Source: NASA
