The source of fast radio bursts (FRBs) – an extremely brief flash of radio waves emanating from space – remains unknown despite new observations. These events, of which only 17 have been observed, are largely mysterious. Until now, there’s been little indication of where in the universe they’re taking place, and so there’s been no way of knowing what physical process causes the flashes.
But in quick succession, two papers have suggested that the FRBs come from a single object that can cause repeated outbursts or that the outbursts come from the catastrophic destruction of a neutron star and thus impossible to repeat. Just as quickly, the second results, which placed the source outside our galaxy, were called into question.
When the first FRB was discovered in 2007, there were indications that the source was outside the Milky Way. There was a difference in arrival time at different wavelengths. The shortest radio waves arrived slightly earlier than their longer-wavelength counterparts.
Radio waves are a type of light (differing from the light your eyes see only because of their longer wavelength), so they travel at the speed of light. In principle, they should not arrive from the same source at different times, unless they are traveling through a medium that slows them down. While the speed of light never changes in a vacuum, light can be effectively slowed down as it passes through a medium because the individual photons are absorbed and then re-emitted by the particles in the medium. This process takes time, which adds to the length of the photons’ journey.
One medium that could slow down the radio waves is the intergalactic medium, a loose distribution of ionized gas that spreads out over the vast spaces between galaxies. Some wavelengths can travel through this medium better than others, which can cause the observed delay in arrival time. The more penetrating wavelengths are less interrupted by the intervening material, so they take less time to get here.
FRBs are considered an important discovery, both because of the interest in the phenomena themselves and because of the potential use of the light from these FRBs as a way to explore the cosmos. But learning more about the outbursts has been more difficult than originally thought, as the discovery of new FRBs depends on scanning the sky with radio telescopes, many of which have small fields of view.
As such, the FRBs discovered so far have not had their distances measured directly, making it difficult to do science with them. But the recent study, if it can handle the current challenges, could change that.
The seventeenth FRB
Unlike the first detection of an FRB, a new study led by Evan Keane of the Square Kilometer Array Organization in Jodrell, UK, does not rely on searching archive data. Instead, an FRB was detected in real time, spotted seconds after the radio waves reached the telescope. This provided a unique advantage: The researchers were able to quickly aim their telescopes at that region to try and find the source of the FRB.
The telescopes, collectively able to observe a wide range of wavelengths in the electromagnetic spectrum, usually yielded nothing. But one object did show up in a few of these observations, which looked a lot like fading radio light from the FRB. Over the next six days it slowly faded. After that time, the source dropped to the normal intensity of a distant galaxy, implying that FRB 150418 comes from such a galaxy.
Determining the distance of that galaxy was then a matter of determining how redshifted it is, as the redshift increases with distance. The researchers concluded that this FRB is about 1.8 billion parsecs away (about 5.8 billion light-years).
That information, combined with a measurement of the object’s energy production, could provide clues to its identity. The object in question, FRB 150418, radiated about as much energy in about 0.8 milliseconds as the sun does in two days. That’s a lot of energy, and whatever FRB 150418 is, it was about a billion times brighter than the Sun in that short time.
Interpretation
Many models regarding the identity of FRBs involve neutron stars. These are collapsed stars that are crushed so that masses greater than that of the sun are squeezed into an object about the size of Manhattan. One of those models believed that FRBs could be caused by a magnetar (a neutron star with a powerful magnetic field), as they can produce powerful bursts of radiation. Another suggested they could be the result of blazars (neutron stars collapsing into black holes). But the researchers argue that none of these explanations match the FRB 150418 data.
Instead, they argue that the most likely source of FRB 150418 is a merger between two neutron stars in that elliptical galaxy. These objects would be readily available in the old population of FRB 150418’s host system.
Duncan Lorimer of West Virginia University, who was not involved in the new study but was the principal investigator on the team that discovered the first FRB, further speculated on this premise in an article in the journal Nature.
“I speculate that it represents the electromagnetic emission released when a binary neutron star system coalesces,” he suggested. If so, it would produce gravitational waves in large quantities. Not enough for the LIGO observatory to notice, but enough for closer FRBs to be observed that way.