Christmas gamma-ray mystery

Two teams of scientists develop two different theories to explain 2010's remarkable cosmic event

Was a comet impacting a neutron star responsible?
Was a comet impacting a neutron star responsible?

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An artist’s impression of a neutron star absorbing a comet – one of two rival explanations for a gamma-ray burst that was spotted on Christmas Day 2010

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Credit: A Simmonet, NASA, E/PO, Sonoma State University


On Christmas Day 2010, NASA’s Swift satellite detected something strange – an intense flurry of gamma-ray activity lasting over half an hour. Not only was this by far the longest-lived gamma-ray burst ever detected, but the distribution of wavelengths was also different from what has usually been seen previously.
 

Alerted by Swift, telescopes around the world were trained on the burst’s X-ray ‘afterglow’. Now two teams of scientists have come up with two different possible explanations for the startling event.

The facts

Gamma-rays are highly energetic photons and can have a number of origins. For gamma-ray bursts – intense bouts of gamma-ray activity – there are two main hypotheses. Bursts shorter than two seconds duration are believed to be the result of collisions between two neutron stars, the ultra-dense remnants of dead stars, while bursts longer than two seconds are thought to be caused by the collapse of a single massive star into a black hole or neutron star.

However the Christmas Day burst was unlike any previously seen, for the reasons stated above but also because its resultant X-ray ‘afterglow’ did not fade smoothly and steadily away, as is normally the case, but spiked numerous times over the next 10 hours. These differences have led scientists to seek a different explanation for its appearance.

The theories

A team led by Christina Thöne of Spain’s Institute of Physics for Andalusia has concluded that the burst could have been caused by the collision and merger of a neutron star and a red giant star in orbit around each other, taking place in a very distant galaxy. Based on observations by Korean astronomers using the 2.1m Otto Struve Telescope at the University of Texas, this theory posits that this collision must have taken place around 5,500 million lightyears from Earth. It cites as evidence the emergence, 10 days after the explosion, of what appears to be a Type Ic supernova. This would tally with the red giant-neutron star hypothesis, as a collision of this type would be likely to result in such a supernova.

A second team led by Sergio Campana of Italy’s Brera Astronomical Observatory, however, suggests a wildly different scenario: the gamma-ray burst may have been the result of a comet colliding with a neutron star. Neutron stars have extremely powerful gravitational fields, approaching those found around a black hole. If a comet were to be drawn into this field, it would be destroyed, and gamma-rays could be unleashed as fragments of the comet hit the star’s surface. This, argue Campana’s team, would also provide a satisfactory explanation for the X-ray spikes. 

 

 

NASA video explaining the two rival theories for the gamma-ray burst detected on 25 December 2010
 

Credit: NASA/Goddard Space Flight Center


Both teams have published their findings in Nature, leading Enrico Costa of Italy’s Space Astrophyics and Cosmic Physics Institute to comment in the same journal that “Whatever the case, it’s hard to escape the fascination of a possible comet death on Christmas Day.”
 
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Chryssa Kouveliotou of the supernova team at NASA’s Marshall Space Flight Centre is more guarded in her conclusions, saying that, “The beauty of the Christmas burst is that we must invoke two exotic scenarios to explain it. But such rare oddballs will help us advance the field.”