Fermi's Large Area Telescope sees surprising flares in Crab Nebula
Scientists believe the flares were caused by supercharged electrons of up to 10 trillion electron volts.
January 10, 2011
The Crab Nebula (M1), one of our best-known and most stable neighbors in the winter sky, is shocking scientists with a propensity for fireworks — gamma-ray flares set off by the most energetic particles ever traced to a specific astronomical object. The discovery, reported January 6 by scientists working with two orbiting telescopes, is leading researchers to rethink their ideas of how cosmic particles accelerate.
Fermi's Large Area Telescope has recently detected two short-duration gamma-ray pulses coming from the Crab Nebula, which was previously believed to emit radiation at a very steady rate. The pulses were fueled by the most energetic particles ever traced to a discrete astronomical object.
Photo by NASA/ESA
“We were dumbfounded,” said Roger Blandford from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), jointly located at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University, California. “It’s an emblematic object,” he said; also known as M1, the Crab Nebula was the first astronomical object cataloged in 1771 by Charles Messier. “It’s a big deal historically, and we’re making an amazing discovery about it.”
Blandford was part of a KIPAC team led by scientists Rolf Buehler and Stefan Funk that used observations from the Large Area Telescope (LAT), one of two primary instruments aboard NASA’s Fermi Gamma-ray Space Telescope, to confirm one flare and discover another.
The Crab Nebula, and the rapidly spinning neutron star that powers it, is the remnant of a supernova explosion documented by Chinese and Middle Eastern astronomers in 1054. After shedding much of its outer gases and dust, the dying star collapsed into a pulsar, a superdense, rapidly spinning ball of neutrons that emits a pulse of radiation every 33 milliseconds.
Though it’s only 10 miles (16 kilometers) across, the amount of energy the pulsar releases is enormous, lighting up the Crab Nebula until it shines 75,000 times more brightly than the Sun. Most of this energy is contained in a particle wind of energetic electrons and positrons traveling close to the speed of light. These electrons and positrons interact with magnetic fields and low-energy photons to produce the famous glowing tendrils of dust and gas Messier mistook for a comet more than 300 years ago.
The particles are even forceful enough to produce the gamma rays the LAT normally observes during its regular surveys of the sky. But those particles did not cause the dramatic flares.
Each of the two flares the LAT observed lasted mere days before the Crab Nebula’s gamma-ray output returned to more normal levels. According to Funk, the short duration of the flares points to synchrotron radiation, or radiation emitted by electrons accelerating in the magnetic field of the nebula, as the cause. And not just any accelerated electrons — the flares were caused by supercharged electrons of up to 10 peta-electron volts, or 10 trillion electron volts, 1,000 times more energetic than anything the world’s most powerful man-made particle accelerator, the Large Hadron Collider in Europe, can produce, and more than 15 orders of magnitude more energetic than photons of visible light.
“The strength of the gamma-ray flares shows us they were emitted by the highest-energy particles we can associate with any discrete astrophysical object,” Funk said.
Not only are the electrons surprisingly energetic, said Buehler, but also, “the fact that the intensity is varying so rapidly means the acceleration has to happen extremely fast.” This challenges current theories about the way cosmic particles accelerate, which cannot easily account for the extreme energies of the electrons or the speed with which they’re accelerated.
The discovery of the Crab Nebula’s gamma-ray flares raises one obvious question: How can the nebula do that? The KIPAC scientists all agree they need a closer look at higher resolutions and in a variety of wavelengths before they can make any definitive statements. The next time the Crab Nebula flares, the Fermi LAT team will not be the only team gathering data, but they’ll need all the contributions they can get to decipher the nebula’s mysteries.
“We thought we knew the essential ingredients of the Crab Nebula,” Funk said, “but that’s no longer true. It’s still surprising us.”