By JoEllen McBride, PhD
Humans have gazed at the stars since the beginning of recorded history. Astronomy was the first scientific field our distant ancestors recorded information about. Even now, after thousands of years of study, we’re still discovering new things about the cosmos.
Fast radio bursts (FRBs) are the most recent astronomical mystery. These short-lived, powerful signals from space occur at frequencies you can pick up with a ham radio. But don’t brush the dust off your amateur radio enthusiast kit just yet. Although they are powerful, they do not occur frequently and happen incredibly fast. Which is exactly why astronomers only recently noticed them. The first FRB was discovered in 2007 from data taken in 2001. The majority of FRBs are found in old data. Their short duration meant astronomers overlooked them as background signals but closer inspection revealed a property unique to radio signals originating from outside our galaxy.
Signal or Noise?
Radio signals are light waves with very long wavelengths and low frequencies. Visible light (the wavelengths of light that bounce off objects, hit our eyes and allow us to see) happens on wavelengths that are a few hundred times smaller than the thickness of your hair. The wavelength of radio waves can be anywhere from a centimeter to kilometers long. The longer the wavelength, the lower the frequency and more the signal is delayed by free-floating space particles. This is because space is not a perfect vacuum. There is dust, atoms, electrons and all kinds of small particles floating around out there. As light travels through space, it can be slowed down by these loitering particulates. Larger distances mean more chances for the light to interact with particles and these interactions are strongest at the lowest frequencies where radio waves happen.
Radio signals from within our own galaxy are close enough that they are not really affected by this delay. But sources far outside of the Milky Way have very large distances to travel so by the time the signal reaches our telescopes, it has interacted with many particles. This produces a streak or a ‘whistle’ where the higher radio frequencies in the signal reach our telescopes first and the lower ones arrive shortly afterwards.
When astronomers started noticing these whistles at unexpected frequencies, they no longer believed they were background noise but signals from the far reaches of space. They needed another piece to the puzzle though to determine exactly what was causing these interstellar calls.
It Takes Two to Find a FRB
The signals discovered in previous data appeared to be one-and-done events, which meant they could not be observed again with a bigger telescope to get a more precise location. Without a precise position on the sky, astronomers couldn’t tell where the signals were coming from, so had no idea what was producing them. What astronomers needed was a signal detected by two different telescopes at the same time. One telescope to broadly search for the signal and a second, much larger telescope to accurately determine its location. So they began to meticulously watch the sky for new FRBs. The first real-time observation of an FRB was in May of 2014. Although it was observed by only one telescope so its precise location was unknown, it gave astronomers a way to detect future ‘live’ bursts. In May and June of 2015 a search by another team of astronomers yielded the first ever repeating FRB.
The Arecibo radio telescope (yes the one from Goldeneye) detected the first signals then they requested follow-up observations from the Very Large Array to more precisely pin-down the location. Once they had a location, yet another team of astronomers could take pictures at visible frequencies to see what was lurking in that region of space. They found a teeny tiny galaxy, known as a dwarf galaxy, at a distance of 3 billion light years from Earth. This galaxy is full of the cold gas necessary to create new stars which means many stars are being born and the huge, bright ones are living quickly and dying.
Who or What is Calling Us?
Where the FRBs are coming from is important because it allows astronomers to pick between the two plausible theories for what causes FRBs. The energy produced by these bursts is impressive, so the most likely culprits take us into the realm of the small and massive: supermassive black holes (SMBH) and neutron stars. One idea suggests that FRBs could be the result of stars or gas falling into the SMBH at the center of every galaxy. If this were the case, we would expect the FRBs to occur in the central regions of a galaxy, not near the edges. Neutron stars, on the other hand, are formed after the death of massive stars. These stars are typically 10 to 30 times more massive than our Sun, so do not live for long. Astronomers expect a galaxy creating lots of new stars to also create lots of neutron stars as the most massive stars die first. Star formation can occur anywhere in a galaxy but is most commonly observed in the outer regions.
This repeat FRB is located pretty far from the center of a galaxy going through a period of intense star birth so this lends credence to neutron stars being the source. Of course, we are looking at a single data point here. There is no reason to suspect that there is a single cause for FRBs. We need more real-time observations of FRBs so we can figure out where they are located and whether or not they always come from dwarf galaxies. FRB searches have been added to three radio frequency surveys, known as CHIME, UTMOST and HIRAX, that will detect and locate these powerful signals with great precision.
It looks like we can continue to look forward to another few millennia of cosmic discoveries.