NASA recently observed a cosmic explosion, and nobody knows what it was. Actually, that's not exactly true: there are actually two equally good, utterly different explanations. It was either a collision in our galaxy...or a supernova billions of light-years away.

The Swift Observatory spotted the massive explosion on Christmas Day 2010. All we really know for sure about the "Christmas Burst" is that it lasted for an unusually long 28 minutes, and in that time it emitted a massive amount of gamma-rays comprising more energy in a few seconds than the Sun will produce in its entire lifetime. A year after the initial observation, astronomers have come up with these two equally valid models. And while the two models are almost nothing alike, we don't currently have a way of telling which one is correct.

That, says lead author Christina Thoene, is a tribute to just how bizarre gamma ray bursts can be, and how little we really know about them:


"What the Christmas burst seems to be telling us is that the family of gamma-ray bursts is more diverse than we fully appreciate. It's only by rapidly detecting hundreds of them, as Swift is doing, that we can catch some of the more eccentric siblings."

So just what are the two theories here? Both involve neutron stars, the ultra-dense remnants of giant stars that once went supernova. Thoene and her team believe the explosion happened in a binary star system some 5.5 billion light-years away. The system contained a neutron star and a Sun-like star at the end of its life, which was just entering its red giant phase. As the star expanded its outer atmosphere, it engulfed the neutron star.

This in turn caused the red giant's atmosphere to be suddenly ejected and the neutron star to tighten its orbit around the other star. And that's when all hell broke loose. Thoene says the neutron star may have spiraled into the other star and merged with it in just five orbits, taking only about 18 months. This merger of dense stellar mass created a black hole, a pair of particle jets emanating from the black hole, and a small supernova.


In this explanation, it was the particle jets that created the gamma-rays, and their unusual interaction with the gas previously ejected by the former red giant is what created the powerful burst. The only issue with this model is we don't actually know whether there's a galaxy where one would need to be for this explanation to work, though the team says they've found a faint trace of what may be a galaxy in just the right position.

So that's one explanation. But Italian astronomer Sergio Campana and his team offer up a rival solution. They believe the burst happened much closer to home somewhere in the Milky Way, roughly 10,000 light-years from here. In this model, a giant comet roughly half the size of the dwarf planet Ceres crashed into a neutron star. Such a large comet is unusual in our experience, but it's thought they might be common in the Kuiper Belt beyond Neptune, and the same might hold true for this neutron star's system.

In any event, the object broke up, with different clumps of debris forming orbits around the star, ultimately coalescing into a sort of disc around it. The Christmas Burst was the result of this debris falling onto the star, and some subsequent X-ray variations observed for several hours after the burst might be the result of late-arriving clumps passing through the disc before hitting the neutron star.

As NASA scientist Chryssa Kouveliotou observes, "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." The current hope is that deep observation by the Hubble Telescope could be sufficient to determine whether there really is a distant galaxy. If there is one there, that's probably just too big a coincidence to ignore the distant binary system model. But until then, we're left with two bizarre explanation, and both have a nearly equal chance of being right.

Original papers here and here via NASA/Goddard Space Flight Center. Rendering of Christmas Burst via NASA/GSFC.