For the first time, a team of astronomers has placed a fast radio burst on the cosmic map, allowing them to better pinpoint its mysterious origin.
FRB 121102 is the gift that keeps on giving — literally. The radio source was first detected on November 2, 2012, in an event that placed it in a class of so-called fast radio bursts: ultrabright, ultrabrief bursts of radio waves with mysterious origins. But unlike its cousins, which have all burst precisely once before disappearing into the dark night sky, it has flared up several times, making it the only fast radio burst known to repeat.
That news made headlines last year. But now, those repetitions have helped astronomers finally tie the fast radio burst to its home galaxy.
It’s a long-anticipated discovery that will help shed light on these enigmatic bursts. “This detection really has broken open the gates on a new realm of science and discovery,” said Sarah Burke-Spolaor (West Virginia University) at a meeting of the American Astronomical Society on January 4th.
A Decade of Searching
The first of these puzzling events was announced in 2007, when Duncan Lorimer (West Virginia University) discovered one in archived data from the Parkes radio telescope in Australia. What made that burst (and all following bursts) truly spectacular was the fact that it was also smeared over a wide range of radio frequencies, with lower-frequency waves arriving later than their higher-frequency counterparts. This dispersion implied that the radio waves had traveled some 3 billion light-years to Earth, making the faraway source — whatever it was — unbelievably bright.
And if that really was the case, then these fast radio bursts might just be entirely new, previously undetected astronomical sources. That thought alone sparked astronomers’ wildest dreams, prompting more theoretical papers than the number of observed bursts. “Something that's truly not understood does not come along every day,” said Burke-Spolaor.
In the decade since, astronomers have detected exactly 18 additional fast radio bursts. It’s a small number when you consider that they might appear as often as 10,000 times per day. Although every one appears to be an extragalactic voyager, traversing great distances before reaching Earth, astronomers haven’t been able to precisely pinpoint where these bursts are coming from — until now.
Try, Try Again
One of the most popular explanations for fast radio bursts is that they’re one-off events, like collisions between neutron stars or collapsing supernovae. But the moment FRB 121102 was found to repeat, that scenario was crumbled up and thrown in the trash can.
“At one stroke, this discovery rules out all of those explosive cataclysmic models for fast radio bursts because we know that whatever it is that produced it has to have survived to produce the successive flashes,” said Shami Chatterjee (Cornell University). Or at least it does for this source.
Astronomers speculated that the culprit might instead be some sort of powerful outburst from a rotating neutron star or perhaps a pulsar. The trouble is that the burst doesn’t appear to follow the periodic pattern that you would expect for an object that regularly rotates.
In order to better narrow down the burst’s source, astronomers needed to find out where it was, not just on the sky but in the universe. Although the first set of observations allowed them to place it within the constellation Auriga, they needed to tie it to a galaxy. Only then could they better speculate what might cause such odd outbreaks.
So Chatterjee and his colleagues used the Karl G. Jansky Very Large Array in New Mexico with the hope of catching another one of its outbursts in a larger scope. They were lucky enough to detect not one, but nine additional bursts, allowing them to localize it to within one-tenth of an arcsecond. That’s 18,000 times smaller than the diameter of the full Moon.
But it wasn’t good enough. The team then used the European VLBI Network — an array of radio dishes spread across Europe — and Arecibo to further winnow down its location. That did the trick. Not only were they able to see that the bursts originated from a faint smudge (some 100 million times fainter than the faintest star you can see with your naked eye), but they also coincided with a persistent radio source at the same location.
An Unlikely Duo
Follow-up observations with the Gemini North Telescope on Mauna Kea, Hawai'i, revealed that the smudge was actually a dwarf galaxy 2.5 billion light-years away.
“The location is so precise that there's no question about the host galaxy,” says Emily Petroff (Netherlands Institute for Radio Astronomy), who was not involved in this study. “So we finally have some answers!”
One of those answers is the original brightness of the source (calculated for a fast radio burst for the first time). The results finally affirm what many astronomers had suspected all along: these bursts are so bright they might push the boundaries of known physics. “Just for an instant, when this burst flashes, the luminosity of that burst outshines all the stars in its own galaxy by far,” said Burke-Spolaor. “It rivals the luminosity of an active galactic nuclei, which are formed from the power accreted onto a supermassive black hole.”
Although the nature of FRB 121102’s remains unknown, hints can be gleaned from the fact that its host galaxy is a dwarf galaxy — disappointing news to those who argued for neutron stars. Because the galaxy doesn’t contain a high number of stars and because most of those stars seem relatively young, it likely doesn’t contain a high number of neutron stars, making this scenario less likely.
But if you’re looking for an intriguing culprit, don’t worry. Not only did the team pinpoint a host galaxy, but also a nearby persistent radio source within the galaxy. Although the exact relationship between the duo remains unclear, it’s likely that they’re somehow interacting. One scenario is that the persistent radio source is an active galactic nucleus that blows bubbles of plasma in space, which glow for a snapshot of time before they’re destroyed. That’s the fast radio burst. Since the galaxy will likely replenish those bubbles, this could happen again and again.
The authors are careful to point out that this is just speculation. The team plans to study the duo further with upcoming Hubble Space Telescope observations. In the meantime, they’re continuing to chase fast radio bursts with the hope that they’ll spot more repeating bursts and localize more in the coming years. “I would dare to say that every major astrophysical observatory is chasing this phenomenon,” said Burke-Spolaor.
S. Chatterjee et al. “A Direct Localization of a Fast Radio Burst and Its Host.” Nature, January 05, 2017.
This is just the latest development in the ongoing study of Fast Radio Bursts. Read all about these mysterious sources in the July 2016 issue of Sky & Telescope.