Fermi’s record breaking gamma-ray burst

ASTRONOMY NOW

Posted: 20 February, 2009

The latest gamma-ray burst to blast NASA’s Fermi space telescope arrived from a distance of 12 billion light years, had the greatest total energy, the fastest motions and the highest energy initial emissions seen to date.

"We were waiting for this one," says Peter Michelson, the principal investigator on Fermi's Large Area Telescope at Stanford University. "Burst emissions at these energies are still poorly understood, and Fermi is giving us the tools to understand them."

GRB 080916C's X-ray afterglow appears orange and yellow in this view that was captured by the Swift space telescope. Image: NASA/Swift/Stefan Immler.

Gamma-ray bursts (GRBs) are the Universe's most vibrant explosions, thought to result as massive exotic stars exhaust their supply of nuclear fuel. As their core collapses into a black hole, jets of material blast outward into space at nearly the speed of light where they interact with gas previously shed by the star and generate bright afterglows that fade with time.

The latest explosion, designated GRB 080916C, occurred on
15 September in the constellation Carina, bathing Fermi in gamma-ray emission from energies between 3,000 to more than five billion times that of visible light. The burst lasted for 23 minutes, almost 700 times as long as the two second average for high energy GRBs. Follow-up observations were made 32 hours after the blast using the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) on the 2.2 metre telescope at the European Southern Observatory in La Silla, Chile, allowing astronomers to pinpoint the blast’s distance to 12.2 billion light years.

"Already, this was an exciting burst," says Julie McEnery, a Fermi deputy project scientist at NASA's Goddard Space Flight Center. "But with the GROND team's distance, it went from exciting to extraordinary."

32 hours after GRB 080916C exploded, the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) on the 2.2m Max Planck Telescope at the European Southern Observatory, La Silla, Chile, began acquiring images of the blast's fading afterglow (circled). Image: MPE/GROND.

Combined with the GRB’s brightness, the extreme distance makes it the most powerful gamma ray event ever detected, with jets moving at 99.9999 percent the speed of light. "If the event that caused this blew out in every direction instead of being a focused beam, it would be equivalent to 4.9 times the mass of the Sun being converted to gamma rays in a matter of minutes," says Valerie Connaughton, Fermi Gamma-Ray Burst Monitor team member.

And accounting for the stretching of electromagnetic energy it would have experienced over the 12 billion light year journey means that the burst persisted for four minutes when it was first created. Astronomers are amazed that a central gamma-ray engine could be kept active for that period of time. The event is also defying standard theories of GRB formation when considering the way the energy is emitted. Typically, gamma-ray bursts start hot with high-energy gamma rays, then fade to progressively weaker rays. In contrast, GRB 080916C started cool, with the high-energy gamma rays showing up almost five seconds later. Curiously though, the high and low energy emissions overlapped for around 200 seconds, suggesting that everything that created both sets of rays happened in the same space at the same time.

The team's results appear online today in Science Express.