Strange flashes of radio waves, called fast radio bursts (or FRBs), have been confounding astronomers since the first one was detected in 2007. It鈥檚 tough to say what鈥檚 causing them because most FRBs are here and gone in a moment 鈥� only one has ever been seen to repeat 鈥� so it鈥檚 tricky to gather a lot of data about each one.
However, astronomers do have some ideas about where they might come from, ranging from the usual astronomical suspects bumping into one another, such as neutron stars and pulsars, to alien spacecraft powered by beams of light. So a new project aims to catalogue all of the likely candidates.
鈥淲hen we started the catalogue at the beginning of the year, we had more theories than we had observations, but we now we have more observations,鈥� says Emma Platts at the University of Cape Town.
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They鈥檒l be working to whittle down the list of explanations, which are grouped into the following seven categories.
1. Smash-ups
Mergers between two large astronomical objects could result in radio-wave emissions. When the magnetic fields of extremely dense neutron stars meet those of small white dwarf stars, supernovas ejecting their mass, or black holes, they can create streams of particles that result in radio waves. Pulsars, rapidly rotating neutron stars, or black holes hitting another black hole could have the same effect.
2. Collapses
As large celestial objects collapse, radiation in the form of radio waves can escape. FRBs could be a result of neutron stars collapsing into black holes or quark stars 鈥� a hypothetical type of star that contains such high pressure and temperature that its subatomic quarks are stripped from the neutrons that make up the star鈥檚 matter. Some theories suggest that dark matter, the mysterious substance thought to make up 85 per cent of the matter in the universe, could be sucked into a neutron star and set off a collapse that would spew out FRBs.
3. Giant flares and pulses
Rapidly rotating pulsars have been suggested as a source of FRBs, as their magnetic fields could accelerate clumps of particles that can give off radio waves. Or, a pulsar within a cloud of dust could create a wind bubble as it spins, triggering a burst that travels through the nebula it sits inside. Neutron stars with extremely powerful magnetic fields, known as magnetars, could produce a similar bubble.
4. Active Galactic Nuclei
When an enormous black hole sits in the centre of the galaxy, it can gobble up gas and dust that shine brightly under the forces of heat and friction as they are later shot out with intense radiation. This is an Active Galactic Nucleus (AGN), and some theories for the origins of FRBs suggest that several objects or particles may be interacting with AGNs to produce the characteristic radio waves.
5. Close encounters
A pulsar travelling through an asteroid belt could strip charged particles from an asteroid鈥檚 surface and accelerate them in a beam. Or a comet captured by a neutron star could break apart and emit radio waves as it is sucked into the larger star.
6. Axion interactions
One candidate for dark matter is a hypothetical subatomic particle called the axion, which could clump up and collapse or collide with a highly magnetised object such as a black hole or neutron star. The resulting interactions could produce FRBs.
7. Weird stuff
Maybe FRBs are created in exploding white holes, mirror opposites to black holes that spew matter out instead of sucking it in. Or maybe they come from lightning on a pulsar. String theorists suggest FRBs could occur when the cosmic strings they suppose make up our universe oscillate or collide.
鈥淪ome of these theories are more likely than others,鈥� says Shriharsh Tendulkar at McGill University, who worked on the project. 鈥淭hat鈥檚 why we set up a discussion board, so scientists can discuss the pros and cons of all the theories.鈥�
For any of these to be correct, they鈥檇 have to account for several oddities among the 60 or so FRBs that have been found. The high frequency of the signals suggests that they鈥檙e likely to come from a something compact. But that鈥檚 about where the similarities end. We鈥檝e seen light from FRBs that is both polarised and not, their pulses aren鈥檛 at precisely the same frequency, and we鈥檝e only been able to pin down a few to their home galaxy, so we鈥檙e not sure if certain environments produce FRBs more than others.
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