domingo, 4 de diciembre de 2011

Astrophysics - Exploding Black Holes -- "Gamma Ray Burts Yield More Energy in a Second than the Sun in a Billion Years"


Gamma Ray Bursts are the brightest things to happen to the Universe since its beginning -- extraordinarily intense electromagnetic events releasing more energy per second than the sun does in a billion years, and basically an excuse for astronomers to use every awesome adjective they know.

GRBs are an incredible demonstration of just how big a universe is: they're extremely rare, only a few per galaxy per million years, and we see about one a day.  Think about what that implies (but stop when your head starts to spin -- we're trying to educate you here, not stick you in a Total Perspective Vortex).  They're so interesting NASA launched a satellite just for them, the Swift Gamma-Ray Burst Mission -- a mission so advanced that "Swift" isn't even an acronym.  They just liked the word.

GRBs are bursts of plasma ejected from the poles of dying stars but Swift observed that the central jet engine causing this emission lasted longer than the burst, and sometimes far longer than previous models could explain (almost three days).  Now scientists from the University of Leeds have put forward an explanation for how this intergalactically emitting dynamo could endure so long: it's eating a star.

By consuming a nearby star the black hole would be sucking up an enormous amount of matter, and if it's rapidly spinning (as many of these holes in spacetime are, a combination of concepts that really proves human language wasn't built for relativity) it can twist it all along the intense magnetic field lines being dragged around the event. The gigantic magnetic stresses involved are what give the burst its incredible energy, lasting as long as it takes for the entire star to be consumed.

Reminder:  this is not the plot from an episode of Voltron.  This is really happening, all over the place, and we have an actual robotic spacecraft up there watching them. 

The first gamma-ray burst to be seen in high-resolution from NASA's Fermi Gamma-ray Space Telescope was one for the record books. The blast had the greatest total energy, the fastest motions and the highest-energy initial emissions ever seen.

"We were waiting for this one," said 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."

This explosion, designated GRB 080916C, occurred at 7:13 p.m. EDT on Sept. 15, in the constellation Carina. Fermi's other instrument, the Gamma-ray Burst Monitor, simultaneously recorded the event. Together, the two instruments provide a view of the blast's initial, or prompt, gamma-ray emission from energies between 3,000 to more than 5 billion times that of visible light.

Nearly 32 hours after the blast, Jochen Greiner of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, led a group that searched for the explosion's fading afterglow. The team simultaneously captured the field in seven wavelengths using the Gamma-Ray Burst Optical/Near-Infrared Detector, or GROND, on the 2.2-meter telescope at the European Southern Observatory in La Silla, Chile. In certain colors, the brightness of a distant object shows a characteristic drop-off caused by intervening gas clouds. The farther away the object is, the redder the wavelength where this fade-out occurs. This gives astronomers a quick estimate of the object's distance. The team's follow-up observations established that the explosion took place 12.2 billion light-years away.

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

With the distance in hand, Fermi team members showed that the blast exceeded the power of approximately 9,000 ordinary supernovae, if the energy was emitted equally in all directions. This is a standard way for astronomers to compare events even though gamma-ray bursts emit most of their energy in tight jets.

Coupled with the Fermi measurements, the distance also helps astronomers determine the slowest speeds possible for material emitting the prompt gamma rays. Within the jet of this burst, gas bullets must have moved at 99.9999 percent the speed of light. This burst's tremendous power and speed make it the most extreme recorded to date.

One curious aspect of the burst is a five-second delay separating the highest-energy emissions from the lowest. Such a time lag has been seen clearly in only one earlier burst.

"It may mean that the highest-energy emissions are coming from different parts of the jet or created through a different mechanism," Michelson said.

Image Credit: image of GRB 080916C On Sept. 17, 31.7 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). Credit: MPE/GROND

Source: The Daily Galaxy via NASA/GLAST

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