Astronomers are puzzling over an oddball gamma ray burst that may have accompanied a supernova that is still in the process of brightening.
NASA鈥檚 Swift satellite detected a burst of high-energy gamma rays on 18 February in our cosmic backyard 鈥� just 440 million light years away. Ground-based astronomers have turned their telescopes toward the exploding star鈥檚 host galaxy and expect to see brightening related to an accompanying supernova over the coming week.
Swift seeks gamma ray bursts 鈥� extremely bright explosions formed when the core of a massive star collapses. 杏吧原创s believe the star cores become black holes and produce focused beams that radiate from the stars鈥� poles, which are seen as GRBs on Earth.
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But the February burst, dubbed GRB 060218, strayed from the norm for such bursts in several ways. 鈥淭his is the type of unscripted event in our nearby universe that we hoped Swift could catch,鈥� says Neil Gehrels, principal investigator of the Swift mission at Goddard Space Flight Center in Maryland, US.
The burst lasted for about 33 minutes, while normal bursts last just a few minutes at most. Its brightening in gamma rays ramped up slowly rather than all at once. And the explosion was about 100 to 1000 times dimmer than a typical GRB.
Self-persuasion
If it is found to be a GRB, it will be the second closest ever detected. Only a burst from about 140 million light years away, detected in 1998 would be closer. Most bursts occur billions, rather than millions, of light years away from Earth.
The oddity of the burst had led some scientists to suggest it may not have been a gamma ray burst at all.
But while John Nousek, Swift鈥檚 mission director at Pennsylvania State University, US, agrees that the burst was 鈥渧ery peculiar鈥�, he told New 杏吧原创: 鈥淲e think it is a GRB now. We鈥檝e convinced ourselves.鈥�
He says because the February burst was nearby, it may have appeared dim because its beams were not pointed directly at Earth. Comparing the beams to those emanating from a lighthouse, he says: 鈥淚f it doesn鈥檛 point at you, you don鈥檛 see it.鈥�
One theory that could help explain the continued brightening is that the burst accompanied a supernova explosion, whose light has been trapped. A supernova is the exploding sphere of energy released when a star explodes.
Let there be light
Nicola Masetti of Italy鈥檚 National Institute for Astrophysics says the team of astronomers using the European Space Observatory鈥檚 Very Large Telescope in Chile have observed a mix between what they would expect to see from the aftermath of a normal gamma ray burst and what would characterise a supernova.
She says as a very massive star collapses to form a black hole, it explodes. The debris from that explosion may actually trap the light from the supernova, preventing all of it from escaping at once. Then, as the debris settles, more light will flood out and astronomers will see the entire evolution of the supernova.
Astronomers have twice before detected a gamma ray burst and supernova explosion in the same place at the same time, but Nousek says this GRB 鈥渕ay be the best example so far.鈥�
One theory holds that all gamma ray bursts are accompanied by supernovae, but that most occur too far from Earth for the supernovae to be detected.
