When a massive star dies, it collapses in on itself under the force of gravity. This happens when the star runs out of fuel to maintain fusion reactions in its core. Once fusion stops, outward radiation pressure diminishes and the star catastrophically implodes under the sheer gravitational pressure of its own mass. During the implosion, a shock wave rips through the stellar core, triggering a supernova.
The largest stars undergo the most energetic supernovae producing either a massive, magnetically dominated neutron star (known as a "magnetar") or a black hole. It is thought that young magnetars are the key driver behind GRBs.
When these inexplicable explosions were first observed in the 1960′s, scientists had little idea about what they were; they were simply way too energetic to be supernovae.
But a GRB is a very different creature to an ‘average' supernova. Via a mechanism that is poorly understood, intense narrow jets of hot plasma are blasted from the dead star's rotational axis. Intense radiation is also produced. If one of those jets are pointing directly at Earth, we'll see an explosion that seems too powerful to be a supernova. That's a gamma-ray burst.
But how is the energy concentrated into a beam - akin to the beam of light emitted by a lighthouse?
According to magnetar theory, when a massive star goes supernova and a magnetar is created, the rapidly spinning and powerful magnetic field of the object will churn up material nearby, drag it in and then blast powerful jets of hot plasma out from the poles.
However, there's another possible GRB mechanism that could generate these powerful jets. If a black hole is created from a collapsed star, it might also stir up surrounding matter, generate jets, firing intense radiation from its poles.
Although the magnetar and black hole GRB mechanisms sound similar, there's a limit to how powerful a magnetar-triggered GRB can be. As there's no limit on the mass of a black hole, it is thought that a black hole-generated GRB will be much more powerful than a magnetar GRB.
Collecting data from NASA's Fermi Gamma-ray Space Telescope, scientists from the University of California, Berkeley, analyzed the energy released by four powerful GRBs in the hope of understanding whether black holes or magnetars were the culprit.
It would appear that in all cases too much energy is generated for the magnetar model to be valid.
"The magnetar model is in serious trouble for such incredibly powerful events," said coauthor Alex Filippenko, UC Berkeley professor of astronomy. "Even if the magnetar energy limit is not strictly violated, the tremendous efficiency required by this process strains credulity."
The energies observed in the sample of GRBs may be below the threshold for the maximum energy allowed by the magnetar mechanism, but the magnetar would need to be extremely efficient at converting the majority of energy allowed into a GRB. As Filippenko points out, the efficiency required is not realistic.
Although the magnetars shouldn't be disregarded quite yet - after all, there are many more, less energetic GRBs that could be explained through the magnetar model - the researchers have found a way to tentatively identify what drives these mysterious cosmic explosions.
For this sample at least, it would seem the GRBs produced may be down to newborn black holes as they make their violent entrance into the Universe rather than the exotic, magnetically dominated magnetar.
Image credit: ESO/A. Roquette