AT LAST we鈥檝e got a handle on how gamma-ray bursts are produced. The colossal outpourings of energy are triggered by twisted magnetic fields of unprecedented intensity, delegates heard last week at the American Astronomical Society鈥檚 meeting in Nashville, Tennessee.
Although GRBs are the most powerful explosions known in the universe since the big bang, they have kept astronomers guessing about their origins since their discovery about 30 years ago. In the last few years gamma-ray satellite missions have revealed that at least one class of GRB, those lasting longer than 2 seconds, come from sources as far away as the most distant galaxies, and that they are connected with exploding stars known as supernovae.
But that still left room for many different ideas about exactly what causes these intense spasms of energy. Some have suggested that the fireball-like shock wave of matter that radiates from a supernova is responsible, or the collision of blobs of that matter with other surrounding matter. Others propose that the trigger is a sudden release of energy from twisted magnetic field lines, analogous to a solar flare.
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When a GRB reached Earth on December 6 last year, Steven Boggs of the University of California at Berkeley and his collaborators measured the radiation with an orbiting gamma-ray observatory called RHESSI, which normally monitors the sun. They found that the electromagnetic waves were strongly oriented in the same direction, or polarised. The level of polarisation 鈥� 80 per cent 鈥� was the highest it is possible to have. 鈥淔or most observations in astronomy, you would never see polarisation levels that high,鈥� Boggs told 快猫短视频.
That means the burst must have been produced by a magnetic field rather than a simple explosion, which supports the flare idea. The field was probably associated with the black hole or neutron star left behind after the star exploded. But to produce such a strong signal, the field must be more intense than anyone ever imagined, says Boggs.
Meanwhile Greg Taylor of the National Radio Astronomy Observatory and his team have used radio observations of a GRB on March 29 to show that its source was stationary, centred on the original supernova. That independently rules out the idea that they are produced when fast-moving 鈥渃annonballs鈥� of matter from the dying star crash into surrounding matter. 鈥淲e have not seen that motion,鈥� says Taylor.