This video is sponsored by MagellanTV.
Welcome back to Launch Pad, I'm Christian Ready your friendly neighborhood astronomer,
and today we're talking about Fast Radio Bursts, or FRBs, or Furbys. These are powerful bursts
of energy that were first discovered in 2007, and seem to be going off nearly every minute
of every day.
Most of these bursts are one-off events, but some of them repeat sporadically. They all
seem to originate in galaxies that are millions, even billions of light-years distant. So,
the fact that we can detect them at such high signal strength imply they're extremely energetic
events that can unleash the equivalent of half a billion suns in just a few milliseconds.
But even though more than a decade has passed since they were discovered, there was no smoking
gun observation of an actual FRB, until now.
In April 2020, a magnetar in our Galaxy unleashed a powerful burst of x-rays and radio energy.
This was our first detection of an FRB right here in our cosmic backyard.
We'll talk about what FRBs are and why this observation is so important, but first I'd
like to thank Magellan TV, who are very kindly sponsoring today's video.
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In 2007, Duncan Lorimer of West Virginia University was working on a survey of pulsars. Pulsars
are rapidly rotating neutron stars that emit powerful beams of radiation from their magnetic
poles.
If the beams sweep along our line of sight, we detect them as pulsars. [foreboding music]
They're often referred to as 'cosmic lighthouses' but in this case, the rotating lamp is an
ultra dense core of a dead star the size of a city spinning hundreds or even thousands
of times per second with a magnetic field capable of erasing all of the hard drives
on Earth from the Moon's distance, bringing about the end of civilization as we know it.
[foreboding music stops]
Lorimer's student David Narkevic was looking through archival data taken in 2001 by the
Parkes radio telescope in Australia. He noticed a powerful burst of radio energy that was
so bright it saturated the Parkes antenna!
The burst itself lasted all of 5 milliseconds and was gone, never to be seen again. But
its signal didn't arrive all at once. Instead, the highest frequency waves arrived first,
while the lower frequencies were delayed.
This is an effect called dispersion, and it's the reason light spreads out according to
its wavelength - or frequency - when it passes through a medium such as a prism or droplets
of water.
But there's another effect of dispersion that's less obvious: not only is light separated
into its component frequencies, but light at lower frequencies moves slower than light
at higher frequencies.
Now you might be wondering how that's possible, since light is supposed to travel at 300 million
meters per second. Well, it is true that light will move at that speed, but only if it's
moving in a vacuum. In a medium, it slows down according to its frequency. The lower
the frequency, the slower it moves.
Oh, and by the way, space is not vacuum. Sure, on small distance scales, yes it certainly
can be a vacuum. But on interstellar and intergalactic scales, space is filled with all kinds of
stuff, especially free electrons.
When light travels over cosmological distances, the occasional encounter with an electron
will start to add up in a big way. By the time radio light arrives at Earth, dispersion
sweeps the frequencies over hundreds of milliseconds. The longer the radio waves have to travel,
the greater the dispersion.
So, by measuring the sweep of the dispersion, or the delay between the highest and lowest
frequencies, you can work out how far away the source was.
And in this case, the burst had to be coming from outside of our Galaxy, up to a gigaparsec
or 3 billion light-years away!
The fact that such a strong signal arrived from so far away meant the burst had to unleash
something like 500 million Suns worth of energy in just 5 milliseconds!
For several years, the Lorimer Burst - as it came to be known - remained the only known
signal of its kind.
Then in 2013, another team reported four more FRBs in Parkes archival data.
Now, even though only a handful of these had been discovered, it was already very clear
these had to be going off everywhere all the time in order to be picked up by accident.
Estimates range from 2500 to as many as 10,000 bursts a day. That's more than one burst every
minute!
At first, it wasn't clear just what these events were. But after years of investigation,
astronomers were able to finally rule out...microwave ovens.
In 2010, astronomers at Parkes began noticing what looked like FRBs at first, but they were
kind of clumpy and seemed to radiate at some frequencies longer than others.
These weird bursts were named "perytons", after the mythological creature created by
Argentine short story writer Jorge Luis Borges. I know, right? THAT Jorge Luis Borges...I
am so ignorant of Argentinian fantasy...
The perytons had the all the markings of a terrestrial signal. But because they appeared
similar to FRBs, they also raised questions as to whether FRBs were real astronomical
phenomena or just another kind of peryton.
Eventually, astronomers at Parkes noticed that most of the perytons were only detected
when the antenna was pointed in the general direction of the visitors' residence, and
that most of the peryton events occurred at lunchtime.
It turns out that astronomers were opening the microwave oven before it was finished
heating up their lunch. If the oven is opened while it's still running, microwaves escape
and the frequency decreases as the magnetron turns off.
The results were even published in the Monthly Notices of the Royal Astronomical Society.
Science for the win!
But in its own way, the peryton result lent more credence to fast radio bursts because
unlike perytons, FRBs occurred at all times of the day regardless of where the telescope
was pointed. So fast radio bursts really were an astronomical phenomenon that needed investigation.
The problem was nobody knew just what these things were. Several ideas started popping
up in the literature, including a new kind of supernova, or colliding neutron stars,
or neutron stars falling into black holes, or comets or asteroids slamming into neutron
stars, or even exotic ideas such as primordial black holes exploding back out as white holes,
or kinks in superconducting cosmic strings.
There was even a proposal that FRBs were the light beams of alien civilizations to propel
sailcraft. Guess which explanation the New York Post decided to go with?
Meanwhile, astronomers tried to detect FRBs with other telescopes...preferably those without
microwave ovens nearby. And in 2012, a team detected an FRB at the Arecibo Observatory
in Puerto Rico. Arecibo is the second largest single-dish radio telescope in the world...or
it least it was before the accident. We'll talk about that later, or else I'll start
crying.
Not only was this the first FRB detected outside of Parkes, but many months later, Arecibo
detected 10 more bursts coming from the same source in a single day! Arecibo had found
the first FRB repeater!
Some of the bursts were very bright, others weak, some brighter in some frequencies but
not in others, and it was all just very weird and surprising.
But Arecibo showed that FRBs can and do repeat which in turn meant that FRBs aren't necessarily
things that would destroy its source like a supernova or neutron stars colliding or
collapsing into black holes.
The reason why it took Arecibo to find a repeating burst is because of its large aperture. Radio
waves are very long, on the order of centimeters, or even meters. The longer the wavelength,
the larger the telescope has to be resolve the source.
So even though Parkes is a 64 meter antenna, its angular resolution is so low it can only
localize the source to a general direction in the sky, which could contain up to thousands
of galaxies.
With a 300-meter dish, Arecibo gets significantly better resolution so it could narrow down
where in the sky the source is coming from.
But to really pinpoint the source, you need to use an interferometer of radio telescopes
like the Very Large Array in New Mexico. In 2016, VLA started hunting for the source.
The two circles in this diagram represent two positions of Arecibo was pointed at when
the bursts occurred. So they knew the source had to be somewhere in the overlap between
the two. After several months of scanning this region with VLA, the burst repeated again,
and the source was pinpointed.
Now that they had a precise location for the burst, astronomers could finally use the 8-meter
Gemini telescope to see what kind of galaxy it was coming from!
You might think that something capable of unleashing that much energy would have to
be coming from a big galaxy with lots of massive stars, black holes, and all sorts of things
that could go boom in the night. But Gemini instead found a tiny dwarf galaxy.
Now, they were able to use this galaxy's redshift to estimate its distance, which worked out
to about 3 billion light years. So this was confirmation that FRBs originate at cosmological
distances!
But like anything else in astronomy, answering one question raised new ones. Like, do all
FRBs repeat, are they all at cosmological distances, and are they only a thing in dwarf
galaxies or can they happen in larger galaxies?
To answer those questions, a lot more FRBs would need to be found. But FRBs are transient
events that could go off anywhere in the sky at any time. There needed to be a way of scanning
a large region of the sky at once, yet still be able to pinpoint an FRB's position.
Luckily, a new kind of radio telescope called the Canadian Hydrogen Intensity Mapping Experiment,
or CHIME, was designed to do just that. It's located at the Dominion Radio Astrophysical
Observatory in Penticton, British Columbia. It was built to map out the location of molecular
hydrogen over a good chunk of the observable universe, but it turned out to be an ideal
FRB hunter.
Unlike a conventional radio telescope, CHIME has no moving parts. It doesn't even have
an ordinary radio dish. Instead, it uses an array of four 100-meter cylinders.
The cylinders focus radio waves to 256 antennas running down the center of each pipe.
Each pipe is fixed in place so CHIME doesn't steer like an ordinary radio dish. Instead,
it scans a long rectangle of the sky stretching from north to south. Since Earth rotates,
CHIME sees essentially the entire northern sky every 24 hours. Any FRBs that go off in
its field of view are detected.
In fact, CHIME detected 13 FRBs while it was still being commissioned in 2018. Among them
was a second repeating source. In 2019, CHIME detected 8 new repeating FRBs. Then 9 more
repeaters were announced in March 2020. Keep in mind, those are just the repeating sources;
CHIME has detected hundreds of FRB events to date, and it's still got that new radio
telescope smell!
But all FRBs detected had one thing in common - they were all detected from beyond our Galaxy,
which means they must be extremely energetic phenomena.
But that finally changed in April 2020, when CHIME detected a bright radio burst coming
from the magnetar SGR 1935+2154.
Magnetars are ultra-magnetized neutron stars. Their magnetic fields are around a thousand
times stronger than a typical neutron star, and 10 trillion times stronger than a refrigerator
magnet. That's strong enough rip molecules apart from thousands of kilometers away, and
even distort individual protons that wander too close.
That means a flabbergasting amount of energy is stored up in a magnetar's magnetic field.
On April 27, SGR 1935 generated a storm of x-ray outbursts over several hours. It was
detected by NASA's Swift satellite, the Fermi gamma-ray observatory, and the NICER instrument
on the International Space Station.
During the storm, NICER recorded more than 200 x-ray bursts in just 20 minutes.
13 hours later, SGR 1935 underwent a second outburst. But this second outburst had powerful
radio component as well. The radio burst was picked up by CHIME and by Caltech's Survey
for Transient Astronomical Radio Emission 2 (STARE2) detector.
So this was the first-ever bonafide fast radio burst detected in our Galaxy, and we got the
X-ray data as well! But what exactly about the outburst caused the FRB?
One possible mechanism is that the magnetar produced a super-short flare that lasted less
than a millisecond. The particles unleashed in the flare, collided with particles that
had been emitted from the previous flares during the storm. The collision generates
an outward-moving shock front, which in turn produces strong magnetic fields. Electrons
gyrate around the magnetic field lines, and emit a burst of radio waves. The shock wave
also heats up the electrons, which causes them to emit X-rays.
Based on its distance and observed energy, SGR 1935 seems like a less energetic version
of the FRBs that we see in other galaxies. And that makes sense, because we should only
be able to detect the strongest FRBs in other galaxies.
But this leaves open the question - are FRB's "just" magnetars undergoing some kind of cataclysm,
or are they really something even more powerful? Well, I think the answer is probably gonna
be "both". I wouldn't be surprised if FRBs are either sub-classified or given entirely
different names depending on what they turn out to be.
But man, what an exciting time to be picking up new signals in the Universe! Now if you'd
like to learn more about neutron stars, pulsars, and magnetars, I made a video about them you'll
want to check out, and NASA created the first-ever map of a pulsar, only to find that it doesn't
look like we thought it should. So I'll see you over there in those videos when we're
done here.
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