If you're a fan of over-the-top ridiculously huge violent explosions, then you won't do any better than gamma-ray bursts. With apologies to Douglas Adams and Eccentrica Gallumbits, GRBs are the Universe's largest bangs since The Big One. When they were first discovered, during the Cold War, it was unclear what caused them. There were more theories than there were observations of them! Now we've observed hundreds of these things, and we've learned quite a bit about them, like a) every one of them is different, 2) they have lots of different sources, and γ) even after five decades they can still surprise us.
Last year on Christmas, the light from a gamma-ray burst reached Earth and was detected by NASA's orbiting Swift satellite. Designated GRB 101225A, it was weird right off the bat: it lasted a staggering half hour, when most GRBs are over within seconds, or a few minutes at most. Followup observations came pouring in from telescopes on and above the Earth, and the next weird thing was found: the fading glow from the burst seemed to be coming from good old-fashioned heat: some type of material heated to unbelievable temperatures. Usually, the afterglow is dominated by other forces like rapidly moving super-intense magnetic fields that accelerate gigatons of subatomic particles to huge speeds, but in this case it looked like a regular-old explosion.
Both of these things are pretty dang weird. So what could have caused this burst?
Normally, we think GRBs are the birth cries of black holes. When a giant star explodes, or two tiny but ultra-dense neutrons stars merge, they can form a black hole and send vast amounts of gamma rays (super high-energy light) sleeting out into the Universe. In this case, though, something different happened, and two ideas of what was behind it are emerging.... but both involve neutron stars. And I'm not sure which idea is cooler.
First; neutron stars are extremely dense balls of matter only a few kilometers across, the collapsed remnants of the cores of stars that went supernova. By dense, I mean dense: imagine taking a mountain and crushing down in size to where it could fit in your hand! Or think of it this way: a cubic centimeter (roughly the size of a sugar cube or dice) of neutron star material would have about the same mass as all the cars in the US combined! So we're talking dense.
The gravity of a neutron star is nearly beyond comprehension. If you let something drop from a height onto the star's surface, that material will be moving at a large fraction of the speed of light upon impact! The energy release is monumental; a marshmallow traveling at that speed would explode like a nuclear weapon.
And that brings us to the first idea: a comet or other large chunk of material was orbiting a neutron star. It got too close, broke apart, and fell on the surface. As each piece hit it released far more energy than all the nukes on Earth combined -- by a factor of millions! -- sending out huge amounts of light into space. That explains both the flash of the GRB detected last year and the fact that the afterglow was in the form of heat; the vast energy of the repeated slamming impacts of the comet chunks would've heated the material (and the neutron star) to millions of degrees.
If this is the case, it happened in our own Milky Way galaxy. As huge as the explosions were, they would've faded to nothing had they been in another galaxy, so on a cosmic scale they must have been relatively nearby. But there are bigger bangs to be had...
... and the second idea to explain GRB 101225A is no less amazing. In that scenario, a neutron star was orbiting a normal, if older and somewhat bloated star. As the normal star expanded, it engulfed the neutron star, which then started eating up the material around it. Again, huge amounts of energy were released, heating up the normal star's outer layers so much they eventually blew off the star. In the meantime, though, friction with this material dropped the neutron star down to the normal star's core, which at this point was mostly helium (having had a normal life for billions of years fusing hydrogen into helium). At some point, the neutron star crashed right into this helium core, and that ginormous amount of material -- the mass of our own Sun or so -- slammed into the neutron star's über-dense matter.
Mind-numbingly powerful gravity would've squeezed that stuff as it fell on the neutron star, and the gravity became so intense not even the neutron star-stuff could resist: the neutron star itself collapsed into a black hole, releasing a flash of energy focused into two tightly-focused beams that lasted for a few seconds, equal to the Sun's total lifetime of energy release! This wave of energy slammed into the material previously ejected from the normal star, heating it and causing the long afterglow seen last year.
If this is the case, the cosmic bomb was farther away than the comet scenario; it would've been in a galaxy billions of light years away, far enough to dim even that titanic explosion to something we wouldn't even had noticed unless we had satellites orbiting the Earth, keeping a watchful eye on the sky.
So take your pick: giant ice cubes slamming repeatedly at near light-speed into an ultra-compact neutron star and releasing petaton yields of energy somewhere in our galaxy, or a 5.5 billion light-year-distant black widow neutron star consuming its companion star and instigating a supernova event that utterly destroyed both, leaving nothing but rapidly expanding vapor with a tiny black hole at its heart.
See? Like I said, both ideas are pretty cool.
NASA helpfully has created a nice video with animations for the two possible scenarios:
But which is correct? It's hard to say. Both ideas have their plus and minuses. Usually dueling ideas like this take more observations to figure out, but in this case it was a singular event, so that's not really possible. But better models, better understanding of physics, and better analysis of the original data may yet rule on this split decision. And who knows? We're still scanning the skies, and may yet see another event like this. If we do, that will help us understand such baffling phenomena. Heck, earlier this year we saw a black hole shred up and eat a star, so clearly the Universe has some pretty hefty tricks still left up its sleeve.
Images credits: NASA; NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and Z. Levay (STScI); NASA
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