What are fast radio bursts? Why are astronomers so excited about them? Find out in Everyday Einstein.
In 2007, astronomers digging through archival data from six years prior, found a very strong, very brief burst of radio emission coming from an unidentified source in space. The detection, sometimes called the Lorimer burst after the lead astronomer on the project, was the first discovery of its kind. It introduced us to a new class of objects dubbed fast radio bursts or FRBs for short. “Fast” because these blips are very short—less than 5 milliseconds in duration. So chop one second into a thousand parts and you’re looking at less than five of your pieces.
The “radio” portion of the moniker is due to the fact that the emission is detected by radio telescopes surveying the sky at radio wavelengths. They are called “bursts” because the signals disappear as quickly as they appeared, without warning and, so far, without explanation.
Since 2007, astronomers have added 17 more bursts to the list of known FRBs. However, their origins are still a bit of a mystery because their defining characteristics, the very reasons they are so interesting, also make them challenging to study.
Radio telescopes also usually have to make a choice: spatial resolution or field of view. In other words, single dish radio telescopes like Parkes and Arecibo can survey the sky more efficiently than arrays of dishes like the Very Large Array near Socorro, New Mexico. However, that large field of view comes with a price: the typical resolution of the Very Large Array is 150 times better than Arecibo and more than 600 times that of Parkes.
Thus, telescopes like the Very Large Array typically cannot survey the sky fast enough to reliably detect such a short event—you’d have to get really lucky in a right place, right time sort of way—but the blurred vision of the single dish telescopes that do detect the bursts aren’t able to pinpoint where exactly they come from. Locating the source of an FRB is especially challenging because they don’t repeat themselves making followup observations useless.
Assuming that you can convince yourself that your one-time, nonrepeating, extremely short signal is a real one—if that sounds difficult, it should!—then whatever explanation you have for its origin must explain why it doesn’t, in fact, repeat. Thus several astronomers have hypothesized that FRB phenomena arise from cataclysmic events like the violent death of a star or the merger of two black holes.
The First Repeating Fast Radio Burst
The world of FRBs was once again turned on its head in March of 2016 when a group of astronomers led by Laura Spitler published their discovery of the first ever repeating FRB in the journal Nature. Spitler and her team had monitored the location of a 3-millisecond burst that was first detected by the Arecibo telescope in 2012.
The astronomers had carefully monitored the spot for years hoping the emission would show up again, all the while knowing that repeating bursts were not known to exist. The risk paid off nearly three years later after 16 bursts were detected in 2015 and then 9 more in 2016 from the same location.
So the source of at least one of these incredibly powerful radio bursts was able to survive the act of producing an FRB. In other words, whatever physical mechanism produces an FRB, it doesn’t necessarily have to destroy its host in the process. However, the location of the source was still unknown, including whether or not it was even located outside of our Galaxy versus originating within the Milky Way.
Revealing the Source of a Fast Radio Burst
That was the case, at least, until last week. Fortunately, the repeating nature of the burst meant that an overlapping team of astronomers, led by Shami Chatterjee, were able to target the burst with higher resolution observations from the Very Large Array. Astronomers were once again abuzz as the results of their efforts were again published in Nature in the first week of January in 2017.
Chatterjee and his group were able to match up the location of the radio emission with a faint, unassuming smudge in a deep optical image taken with the Gemini telescope on Mauna Kea in Hawaii. That smudge is a dwarf galaxy, much smaller than our Milky Way, sitting three billion lightyears away. Thus, these observations show clearly not only that the source of the only known repeating FRB in the universe is a galaxy outside of our own, but also that it doesn’t take a big galaxy to host an FRB.
While these recent discoveries are a huge step toward unlocking the mysteries of FRB origins, large questions still remain. How common are bursts? What fraction of them repeat? Are they always or even usually hosted by dwarf galaxies? What physical mechanism causes the burst?
Any explanation for the bursts must explain why they are rare and not always observed to repeat. Chatterjee, along with other astronomers studying FRBs, have a few theories. In a press release from his home institution (and my alma mater) Cornell University, Chatterjee explains, “We think it may be a magnetar—a newborn neutron star with a huge magnetic field, inside a supernova remnant or a pulsar wind nebula – somehow producing these prodigious pulses. Or, it may be an active galactic nucleus of a dwarf galaxy. That would be novel. Or, it may be a combination of those two ideas.”
Sometimes questions in astronomy are long standing. The existence of dark matter was first suggested as early as the 1930s, and although much progress has been made, astronomers are still trying to understand what it is made of almost a century later. In other cases, progress can be rapid. The first exoplanet was discovered in 1992. Less than 30 years later, we already know of thousands more, including potentially habitable planets hosted by sun-like stars. How the FRB story will end remains to be seen, but luckily we get to be here for the exciting beginning.
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at email@example.com.