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The most powerful explosions in the universe are caused by the births of black holes rather than dense neutron stars called magnetars, new evidence confirms.

Gamma-ray bursts are blasts of high-energy radiation that arrive at Earth from all directions.

Bursts lasting longer than 2 seconds are thought to be related to the collapse of massive stars that fire out jets of matter at near light speed, emitting copious radiation in the process. When the jets happen to be pointed at Earth, they send a volley of gamma rays our way.

But exactly how these jets are generated is still unclear. If the star collapses to form a rapidly spinning neutron star with a powerful magnetic field, called a magnetar, it could violently stir up matter around it to produce jets.

Alternatively, if the star collapses to form a black hole, the jets might arise from the interaction of the black hole with matter spiralling into it. Either way, the tremendous rotational energy of the black hole or neutron star is thought to power the jets.

One way to distinguish between the two possibilities is to measure the total energy of the blast. The energy of a rotating object depends on its mass, and there is a limit to how heavy neutron stars can get before they collapse to black holes. There is no limit on the mass of black holes, however, so they could provide more energy than neutron stars.

Now, of the University of California, Berkeley, and colleagues have analysed four of the most powerful gamma-ray bursts detected by NASA’s Fermi Gamma-ray Space Telescope.

“These are among the most energetic GRBs ever seen,” says team member Alex Filippenko, also of the University of California, Berkeley.

The most poweful of the bursts, called GRB 090926A, released about 1.4 x 10⁵² ergs in jets. Neutron stars should be able to produce no more than 3 x 10⁵² ergs, with only a fraction of their energy going into jets. So the researchers argue that this burst, along with the three others, must be due to black holes instead.

of the University of California, Santa Cruz, says this is an “exciting result” for pinning down the source of the most powerful gamma-ray bursts. But he adds: “This does not mean that magnetars don’t make some or even most GRBs – only that they have difficulty producing the most energetic.”

The researchers reported their findings on Wednesday at a in Annapolis, Maryland.