They come from somewhere in the distant universe--probably some 6 billion to 11 billion light years away. They don't last very long, only about one-thousandth of a second. They happen all the time, up to 10,000 times a day. They create intense bursts of radio emission but nothing else--no light, no x-rays, no other visible evidence. And nobody knows what they are. Until now, nobody was even sure they existed.
Radio map of the sky highlights rapidly varying objects. Black dots are pulsars, the spinning remnants of dead stars. Red asterisks mark the four new mystery objects. (Credit: MPIfR/ C. Ng; Science/ D. Thornton et al.) Astronomers are calling these enigmatic signals "fast radio bursts" or "Lorimer bursts," after Duncan Lorimer, the researcher who detected the first one. The timing of the press releases made me momentarily suspicious--a fireworks-type story that breaks on the 4th of July?--but a quick look a the discovery paper, published today in the journal Science, quickly dispelled my cynicism. This is the real deal: a genuine new cosmic mystery. Hints have been accumulating for a while that something weird is going on out there. Dan Thornton, a doctoral candidate in astrophysics at the University of Manchester and at Australia's CSIRO, notes that the original Lorimer burst was reported a full six years ago. At the time, the observation attracted only minor attention, however; it was a single event that could have been many different things, including an erroneous measurement. For a young researcher like Thornton, looking to make his mark, this was a high-risk direction to explore. It also proved to be an irresistible one. Thornton ended up collaborating with 19 colleagues around the world (in the U.S., U.K., Italy, Germany, and Australia), working for four years at the Parkes radio telescope in Australia to explore the meaning of the Lorimer burst. Rather than training the telescope on one spot for a long period of time to build up a clear signal, they performed the more complicated task of listening continuously for sudden blips of radio noise. Their findings are presented in the paper, "A Population of Fast Radio Bursts at Cosmological Distances," in the latest issue of Science. At first glance, the payoff from all that labor looks modest: Thornton and his colleagues have detected only four new fast radio bursts. But that sample is large enough, and sufficiently well observed, to draw some sweeping conclusions. First, the Parkes survey definitively establishes that the bursts are real. Four very specific observations like this do not happen by chance. Second, this sample is large enough to say something meaningful about how common the bursts are. Thornton and company were examining only minuscule patches of sky. Assuming that the bursts occur randomly in all directions, they must be going off all the time, unnoticed, in many other locations. To a person with radio vision, the sky would light up several times a minute with strobe-like flashes originating from places unknown.
The Parkes radio telescope is superimposed in front of a radio-eye view of the night sky. The bright dot indicates one of the newfound fast radio bursts. (Credit: Swinburne Astronomy Productions/ CSIRO/ Harvard) Finally, the source of those flashes is not entirely unknown because there is yet another important piece of information embedded in the four observed bursts. The burst signals are blurred out in a way that looks different at high radio frequencies than at low ones. Such blurring occurs when radio waves pass through the thin smattering of atoms between galaxies, and gets stronger the farther the waves travel. The amount of blurring Thornton measured strongly indicates that the object causing the radio flashes must lie outside our home galaxy. Way, way outside our galaxy, in the far reaches of the cosmos. That helps narrow down where the fast radio bursts are, but the question of what they are is still wide open. Their extreme distance suggests they must be extremely powerful, or else they would not be detectable at all. Thornton's thoughts initially went to cataclysmic events like an exploding star collapsing into a black hole, or two ultradense neutron stars smashing into each other. But such extreme detonations should produce detectable flashes of light, x-rays, or gamma rays as well--and so far, searches for flashes associated with the four bursts has come up empty. Put in layman's terms: Scientists are stumped. Two astrophysicists, Heino Falke and Luciano Rezzolla, have put forward a proposal that fast radio bursts could be produced by a particularly exotic type of stellar explosion. In this scenario, a dying, massive star collapses first into an ultra-fast-rotating neutron star. Gradually the star slows down and undergoes another collapse into a black hole. As the neutron star dies, it emits a final, blazing burst of radio waves. Falke and Rezzolla call it a "blitzar," but for now it is entirely hypothetical. Other theorists are surely working on alternate explanations. One immediate, if somewhat obvious, lesson from the discovery of fast radio bursts is that the universe is still full of strange, unexplored phenomena. Astronomers are not about to run out of new frontiers anytime soon. More subtly, the work of Thornton and his team shows the incredible value of watching the universe in real time: viewing it as a movie rather than as a series of static snapshots, and watching attentively for things that change from moment to moment--even from millisecond to millisecond. In the 1960s, Jocelyn Bell took this approach and discovered pulsars, rapidly spinning neutron stars that emit precise pulses of radiation, like the beam from a lighthouse. In the 1990s, the Compton Gamma Ray Observatory watched the gamma-ray sky in real time and collected evidence for the most powerful explosions in the universe. Cosmic time may be measured in billions of years, but cosmic events can unfold literally in the blink of an eye. Thornton and others are pressing on with the movie-camera approach. Future high-speed radio studies at various observatories, including the powerful Square Kilometer Array, will gather a lot more information about fast radio bursts. The planned Large Synoptic Survey Telescope will perform the same kind of duty at visible wavelengths, looking for things that go flash in the night. And when the results come in, I will very likely be writing the words "cosmic mystery" all over again. Follow me on Twitter: @coreyspowell
