When the first repeating Fast Radio Burst (FRB) was discovered in 2016, astronomers thought it might be a coincidence or an anomaly. A further 24 had been reported prior to this week, ruling both those scenarios out. Now, a single paper has doubled the total to 50 thanks to the efforts of the Canadian Hydrogen Mapping Experiment (CHIME) wide-field radio telescope. We may still be unsure what is causing them, but it’s not rare.
As their name suggests, FRBs are spurts of radio waves lasting milliseconds or less. Most appear to pop up once, and not reoccur, at least in the 16 years since they were discovered. However, seven years ago FRB 121102 was found to repeat, overturning the assumption all FRBs were a side-effect of cataclysmic events.
The challenge in finding FRBs, repeating or not, is the sky is big, and without knowing what causes them we can’t know where to look. Technology has come to the rescue, however. Where once radio telescopes could only observe tiny portions of the sky at a time, some newer instruments have much wider fields of view, particularly CHIME, which can see the whole northern sky. A multi-institutional team put this to use and reported 25 new repeating FRBs.
The authors did not conduct their own observations. Instead, they utilized CHIME’s primary purpose; studying the distribution of the universe’s most common element, hydrogen. In the process, CHIME has amassed an immense amount of data, which could be searched for other things – FRBs included.
One obstacle to finding repeating FRBs is that the timelines on which they repeat are so varied. FRB 121102 burst ten times in a year. The paper’s discoveries included some with twelve bursts in a 20-month period, while others had just two. "We need a longer observation time because some repeaters could repeat every 10 years. We just don't know. They don't play by our time scales," said study co-author, University of British Columbia PhD student Adam Dong, in a statement
If FRBs came from within the Milky Way we could restrict our search for them to the galactic plane. However, with rare exceptions, they don’t. That means we have no preferential directions in which to look. Just following up on those that have been seen bursting once doesn’t help much. The authors found roughly 2 percent of the FRBs we have seen going off have repeated. "Many apparently one-off FRBs have simply not yet been observed long enough for a second burst from the source to be detected," said co-author Dr Ziggy Pleunis of the Dunlap Institute for Astronomy and Astrophysics.
The positive side of finding a repeater, particularly one that bursts frequently, is that the source can be watched using instruments that operate in the non-radio part of the electromagnetic spectrum. At a minimum, we will learn more about the galaxies they come from (currently a very diverse bunch) – but ideally, we may spot something at optical or infrared wavelengths coinciding with a burst. "In the long run, we hope to learn a lot about their origins," said co-author UBC Professor Ingrid Stairs.
Astronomers have already been trying to find patterns in the previously discovered repeating FRBs’ behavior, and the larger sample will definitely help. Magnetars – immensely magnetized neutron stars – are thought to be responsible for some repeating FRBs, but they may not all have the same cause. Complicating matters further, the team noticed repeating FRBs have lower dispersion measures than those that only burst once.
Besides wanting to know what they are, those same dispersion measures have meant FRBs have helped us measure the Milky Way’s halo. At least we now know how to distinguish them from prematurely opened microwave ovens.
The study is open access in The Astrophysical Journal