Black Hole 472

If only we could get close to it without being crushed…

A gamma-ray burst is an immensely powerful blast of high-energy light thought to be generated by a collapsing star in a distant galaxy, but what this collapse leaves behind has been a matter of debate.

A new analysis of four extremely bright bursts observed by NASA’s Fermi satellite suggests that the remnant from a long-duration gamma-ray burst is most likely a black hole — not a rapidly spinning, highly magnetized neutron star, or magnetar since such a burst emits more energy than is theoretically possible from a magnetar.

“Some of the events we have been finding seem to be pushing right up against this total limit for a neutron star progenitor system,” said S. Bradley Cenko, a post-doctoral fellow from the University of California, Berkeley.

Cenko is presenting these findings Nov. 3 at the Nov. 1-4 Gamma Ray Bursts 2010 conference in Annapolis, Md. Cenko is a member of an international team that includes astronomers from UC Berkeley and the National Radio Astronomy Observatory (NRAO) in New Mexico.

The group has submitted a paper detailing its analysis to The Astrophysical Journal.

Long-duration gamma-ray bursts (GRBs) are presumed to be created by the explosive collapse in distant galaxies of massive stars. The explosion is visible from Earth because the light is emitted in a narrow cone, like a beam from a lighthouse. First discovered in 1967 by satellites looking for nuclear blasts on Earth, gamma-ray bursts have been the focus of several satellite missions, most recently NASA’s Fermi gamma-ray space telescope, launched in 2008, and NASA’s Swift satellite, launched in 2004.

With accumulating observations, astronomers have been able to create models of how the collapse of a rapidly rotating, massive star can accelerate matter to nearly the speed of light and collimate it into two oppositely directed, tightly focused beams along the spin axis. They have also studied how these particles generate gamma rays and other emissions.

The two leading candidates for powering these long-duration bursts are a magnetar and a black hole, sometimes referred to as a collapsar. In both cases, material from the star falls inward and is catapulted out by the spinning neutron star or black hole. What distinguishes these models is that magnetar-powered bursts cannot be as powerful as black hole-powered bursts.

“The question we have been trying to answer is: What is the true energy release from these events?” Cenko said. “We can measure all the light emitted — very high energy gamma rays, and, at later times, X-ray, optical and radio afterglow emissions — but that doesn’t provide a very good estimate, because GRBs emit in relatively narrow jets. We have to get an idea of the geometry of this outflow, that is, how collimated the jets are.”

Previous studies have shown that light measured in the afterglow begins to drop steeply at a certain point, and the sooner this drop-off, called a jet break, the narrower the jet. Typically, the gamma-ray burst itself lasts from a few seconds to as long as 100 seconds, but the afterglow, produced when the jets interact with gas and dust surrounding the star, emits visible light for a couple of weeks and radio radiation for several months.

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