Scientists using the National Science Foundation’s Green Bank Telescope (GBT) and China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST) have teamed up to shed light on the origin of the thousands of mysterious fast radio bursts that hit the Earth each day from locations far beyond the Milky Way.
Fast Radio Bursts, called FRBs for short, were discovered by accident more than 15 years ago. These intense broadband flashes of radio emission last only a thousandth of a second. After the first one was discovered, radio telescopes spent hundreds of hours looking to see if it would flash again, but it was quiet. Astronomers now know that there are thousands to hundreds of thousands of these flashes hitting Earth every day, that they come from something so energetic that it can be detected across the Universe, and that a few of them actually repeat. But their origin is still a mystery.
Are the “repeaters” a different object from the others who give off a single flash and then are silent? The new data from FAST and the GBT focused on one particular aspect of the radio bursts: their polarization.
Radio waves can be broken down into a part that goes up and down, and a part that goes side to side. In many cases the two parts have the same intensity. But for others, and repeating FRBs in particular, one direction is favored over the other. Astronomers call this polarization. The emission from a FRB traverses an enormous distance before hitting Earth, passing through regions that can put their own particular twist on the radio polarization. For this reason, the study of the polarization of FRBs, and the changes it undergoes until it is detected by our telescopes on Earth, tells us about the environments where they are born and all the space in between.
A research team led by Dr. Di Li from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has analyzed the polarization properties of five repeating FRB sources using FAST to cover one set of radio frequencies and the GBT to cover another. They found that the polarization properties of FRBs depended on the observed frequency, and that the properties could evolve on relatively short times as well.
This can be understood if repeating FRB emission passes through a complex environment around the bursting sources, which could be a supernova remnant, a pulsar wind nebula, or a plasma near massive black holes. If FRBs are born in explosive events, as many theories predict, then the more complex environments can be easily explained as coming from more recent explosions, which would suggest a link between the activity level of an FRB and its age.
“These extremely active FRBs could be a distinct population. With these measurements we start to see the evolutionary trend in FRBs, with more active sources in more complex environments and larger polarization changes being younger explosions,” said Dr. Yi Feng, the first author of the paper, now a permanent scientist at the Zhejiang National Lab in Hangzhou, China.
“The key to this discovery”, said Dr. Ryan Lynch of the Green Bank Observatory, “is the combination of the data from two of the world’s largest radio telescopes. The picture would be incomplete with just one. It’s a great example of how different telescopes, with different strengths, can work together to advance science.”
The study was published in Science on March 18.