In this article, Ian O'Neill at news.discovery.com describes how new data from NASA's Fermi Gamma-ray Space Telescope have indicated that gammar ray bursts are too energetic to be generated by magnetars and must come from black holes.
A magnetar is a neutron star, a highly compact object made up of degenerate neutrons, with an extremely strong magnetic field.
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.
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.
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."