Fermi telescope captures gamma-ray burst

NASA/Swift/Stefan Immler Gamma ray

The gamma ray burst's X-ray afterglow appears orange and yellow in this view that merges images from two different telescopes.

Just months after launching, the Fermi gamma-ray telescope has revealed the most massive gamma-ray blast ever detected, painting a new picture of the high-energy universe.

The orbiting observatory, whose design and assembly was directed by researchers at Stanford University and the SLAC National Accelerator Laboratory, features the most sensitive instruments capable of recording gamma rays, the highest energy photons in the universe. And now they've detected the most massive, fastest and highest energy gamma-ray burst ever recorded.

"You get a lot of little ones, a few medium sized ones and every once in a while you get a big whopper," said Patrick Nolan, a Fermi team member and senior research scientist with Stanford's Hansen Experimental Physics Laboratory. The Fermi telescope detects gamma-ray bursts almost daily, but this giant was roughly twice the size of any others.

The observation was published in the Feb. 19 issue of Sciencexpress.

The mind-boggling blast exceeded the power of nearly 9,000 ordinary supernovae—catastrophic star explosions—and the material emitting the initial gamma rays was moving at nearly the speed of light.

Due to its sheer power, scientists were able to detect the blast, which erupted more than 12.2 billion light years away, far outside the realms of our galaxy.

Gamma-ray bursts come in long form (lasting from two seconds to several minutes) and short form (less than two seconds). This most powerful burst was of the long variety, all of which originate in gigantic supernova explosions. Nolan compares these gamma-ray bursts to "shooting a pair of jets from the star's center, whose power rips through the whole mass of the star, erupting and blowing it to shreds."

The less energetic short-duration bursts originate when two black holes, trapped in orbit around each other, get closer and closer and then finally merge.

A new view

This latest observation is one of many for Fermi. Taken together, these observations are building a new view of the role and origin of gamma rays in the universe.

Just last month Fermi scientists announced the presence of 12 new gamma-ray-only pulsars, and detected gamma rays from 18 other pulsars. In the past, pulsars were identified almost exclusively via low-energy radio waves.

"Prior to Fermi, we had detected only a handful of gamma-ray pulsars and now we have a big population," said Roger Romani, professor of physics at Stanford and part of the Fermi team. "This demonstrates that the few we found were not just weird exceptions—they're fundamentally part of the way things work."

Pulsars are spinning remnants of large, exploded stars. When massive stars, over eight times larger than our sun, undergo dramatic supernova explosions, an equally remarkable implosion follows. During the implosion, a mass of roughly one and a half times our sun is squashed into a sphere only 20 miles in diameter, with a density that would equal one billion tons per teaspoon on Earth.

The energy of the imploded mass sets the star into a rapid spin and creates a magnetic field up to a trillion times stronger than those found on Earth. A powerful electric field is generated, accelerating charged particles that radiate electromagnetic energy in frequencies from radio waves to powerful gamma rays.

Searching for the source

For more than a decade, researchers have seen evidence of high-energy gamma-ray sources in the universe, but they couldn't tell what the source was, said Peter Michelson, professor of physics at Stanford and a principal investigator on the Fermi project. With the new discovery of gamma-ray-only pulsars, Michelson and colleagues have now solved at least part of that mystery.

Pulsars were first detected in the 1960s when researchers picked up radio waves emanating in beams from the magnetic poles of stars. But these radio waves account for only a tiny fraction of the energy coming off pulsars, according to Romani. Gamma rays, on the other hand, are the real powerhouses.

"The radio waves are like the hum of a Ferrari. It certainly doesn't tell the whole story about the energy that can be unleashed. But the gamma rays are like seeing the Ferrari take off," Romani said. "Now we have a real chance at understanding the power coming out of pulsars."

The new data also has changed the way researchers understand pulsars. Previous models suggested that pulsars emit energy in a relatively narrow cone comparable to a lighthouse beam sweeping across the sky. But the lighthouse picture is incomplete, according to Michelson. "It's now evident that in many cases, the gamma-ray radiation is more broadly beamed in the sky and in a different direction than the radio emission," he said.

In total, the Fermi team has detected more than 40 gamma-ray pulsars in just over four months of observations. In the next five years, Michelson and Romani expect to find upward of 100.

The Fermi gamma-ray space telescope is a partnership between astrophysicists and particle physicists at NASA, the U.S. Department of Energy and academic institutions in France, Germany, Italy, Japan, Sweden and the United States.

Cassandra Brooks is a science-writing intern at the Stanford News Service.