This video is sponsored by Magellan TV.
700 million years ago, a supermassive black hole drew an unsuspecting star into its deadly
grasp. As the star drew closer, the black hole exerted ever-increasing tidal forces
on it. The star bulged and stretched until it was shredded in a tidal disruption event.
Half of the star's shredded material entered into a death spiral around the black hole,
crashing into itself and heating to millions of degrees.
In the ensuing maelstrom, powerful jets launched from the poles, accelerating particles to
nearly the speed of light. Among them was a rare, high-energy neutrino that scientists
traced back to its source across the Universe.
Welcome to Launch Pad Astronomy, I'm Christian Ready, your friendly neighborhood astronomer.
Every once in a while, a neutrino is detected that packs so much energy, it can only have
been generated in the most powerful particle accelerators in the Universe. However, neutrinos
are notoriously difficult to detect. They can pass right through planets, stars, and
even entire galaxies without ever touching any matter.
However, it's their elusive nature makes them great messengers for phenomena happening far
beyond our Galaxy. In 2018, astronomers determined that a high-energy neutrino came from a distant
blazar.
Now, astronomers have identified a cosmic neutrino's source for only the second time.
It turns out this one came from another supermassive black hole; this time, shredding a passing
star in a tidal disruption event, or TDE.
TDE's are among the most violent and energetic events in the Universe. As such, they've long
been thought to be a potential source of cosmic neutrinos. But weirdly, this neutrino wasn't
detected until five months after the TDE reached its peak.
That raises questions about when, where, and how cosmic neutrinos can form, and what they
can tell us about the phenomena that create them. So today, we're going talk about how
this neutrino was detected, how astronomers determined its source, and how they think
this neutrino might have been created. But first I'd like to thank Magellan TV who are
very kindly sponsoring today's video.
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Of all the particles that have mass, neutrinos are by far the most abundant in nature. They
carry no electric charge, and they're tiny, with barely any mass at all.
That means even a smidgen of energy will accelerate a neutrino to nearly the speed of light. But
Einstein's theory of relativity shows that it takes an ever-increasing amount of energy
to accelerate a mass - however tiny - incrementally closer to the actual speed of light.
So rather than characterize particles by how many decimal places closer to the speed of
light they're moving, we instead just characterize them by their energy.
For example, the Sun's core creates neutrinos with an energy of around 400,000 electron
volts. Solar flares can generate neutrinos up 18 million electron volts. Cosmic rays
smashing into the atmosphere generate neutrinos up to 10 trillion electron volts.
But some neutrinos have energies measured in the hundreds to millions of trillions electron
volts. That's an insane amount of energy, which that means these neutrinos had to have
been launched from cosmic accelerators like black holes, pulsars, gamma-ray bursts, and
active galactic nuclei.
But detecting neutrinos - cosmic or otherwise - is difficult because they're so small they
can pass right through objects without ever touching them.
In fact, 100 trillion neutrinos pass right through you every second of the day, and you
never once complained.
That's why the IceCube experiment strings detectors through a cubic kilometer of Antarctic
ice. Every now and then, it detects the extremely rare collision of a neutrino with a water
molecule.
In 2018, IceCube traced a cosmic neutrino back to a blazar. This is an energetic galaxy
powered by a supermassive black hole, whose jet happens to be aimed toward Earth.
However, it's unlikely that blazars are the only possible source of high-energy neutrinos.
Some theorists suspect that tidal disruption events - or TDE's - can generate cosmic neutrinos
as well.
TDEs occur when a star spirals into an ultra-dense object, like a black hole. As the star gets
closer, tidal forces increase until the star is ripped apart. About half of the star's
mass forms an accretion disk around the black hole, where temperatures reach millions of
degrees.
The erupting disk blazes to up to a billion times the Sun's luminosity! After a few years,
the disk gradually stabilizes and cools, and then. settles into a plateau of more or less
steady luminosity.
In certain cases, magnetic fields from the spinning disk can launch ultra high-speed
jets that act as particle accelerators. It's in these jets that particles collide at nearly
the speed of light and can produce the highest-energy neutrinos.
TDEs are very rare events. Our Galaxy might see one every 10,000 to 100,000 years. But
the Universe is a very big place, so rare events happen all the time. If they're sufficiently
powerful, and if we're lucky, we can spot them.
That's why the Zwicky Transit Facility scans the sky on a nightly basis, looking for just
these sort of events. ZTF is a camera with a 47 square degree field of view mounted to
the 48-inch Schmidt telescope on Mount Palomar, California. It scans the sky looking for transient
phenomena such as variable stars, supernovae, active galactic nuclei...anything going 'boom'
in the night.
On April 9, 2019 ZTF detected a tiny flash of light. The event was designated AT2019dsg,
or "DSG" for short. It may not look like much, but this flash of light is 690 million light-years
away in the galaxy 2MASX J20570298+1412165. Yeah, that took a few takes.
DSG was classified as a tidal disruption event based on its spectrum. A team of astronomers,
led by Dr. Robert Stein at the DESY research center in Germany, used NASA's Swift satellite
to make follow-up observations in visible, ultraviolet, and X-ray wavelengths. They made
additional X-ray measurements using ESA's XMM-Newton satellite and radio observations
with the Very Large Array in New Mexico, and the MeerKAT telescope in South Africa.
By May 2019, the TDE brightened to 100 billion times the Sun's luminosity! That's as much
light as an entire galaxy! This was peak TDE; if the black hole were to launch a relativistic
jet, now was the time.
Of course, At 700 light-years, the black hole is too far away to see the jet directly. But
if a jet had launched, it should generate a lot of X-ray emission as well as cosmic
neutrinos.
But instead of brightening, the X-ray emission faded by 98% in just 160 days. That's unprecedented
for a TDE. Oh, and by they way, no neutrinos, either.
Then, 5 months later, on October 1st, the IceCube neutrino detector in Antarctica detected
a neutrino packing 200 trillion electron volts! IceCube backtracked the neutrino's origin
to a region of the sky in the constellation Delphinus the Dolphin.
About 7 hours later, the Zwicky Transit Facility pointed in the same direction determined by
IceCube, and verified that DSG was still there, although it had faded a bit.
Stein and his team estimated that the chances of the neutrino coming from an origin other
than DSG's location is only about 0.5%. That's one chance in 500. So naturally, the question
is why wasn't the neutrino detected until 5 months after the TDE peaked?
Well, not only had the X-ray emission long dropped off by then, but the UV and optical
emission had already settled into a plateau. Such plateaus are common in TDEs and are thought
to be caused by emission coming from the outer part of an accretion disk after it had formed
around the black hole and settled in a bit. So in effect, the plateau symbolizes the establishment
of a proper accretion disk. However, TDEs typically don't plateau until a few years
after the disruption. DSG reached its plateau after just a few months.
However, an early plateau is expected if the disruption takes place around a supermassive
black hole, like the ones that dominate the centers of galaxies. A supermassive black
hole would establish its accretion disk much faster because of the black hole's stronger
gravitational field.
As it happens, the host galaxy's mass is in the top 10% of optical TDE hosts. As a rule
of thumb, a black hole's mass scales with the mass of its host galaxy. Given this mass-scaling
relationship, the team estimates the black hole at around 30 million solar masses. That's
an order of magnitude greater than the black hole at the center of the Milky Way Galaxy!
So on the one hand, we have an explanation for the rapid plateau in the ultraviolet,
but on the other, there was still no X-ray emission to signal a jet.
However, there's plenty of places high-energy neutrinos can still form, even when there's
no jet present. There's the inner disk where particles collide with X-rays, the outer disk
where particles collide with ultraviolet photons, and then there's the broad outflows, where
particles can collide with each other.
These broad outflows typically produce radio emission, which was steadily increasing through
the neutrino's arrival. That's consistent with the outflows expanding further away from
the black hole, leading to a steady increase in radio emission.
Still, the neutrino came in at 200 trillion electron volts. Given its energy, Stein's
team thinks it probably came from the outer ultraviolet disk. That would certainly be
consistent with the neutrino arriving as the UV radiation plateaued.
As for the lack of X-ray emission, Stein suggests that disk must have just cooled off rapidly,
cutting off the jet before it could form.
However, not everyone agrees with this interpretation. Walter Winter and Cecilia Lunardini published
an alternate scenario where the black hole did, in fact, launch a jet that produced the
neutrino.
In their model , about half the star's mass forms an accretion disk and launches an X-ray
jet. But the other half of the star's mass forms a kind of cocoon cuts off its X-ray
emission from our line of sight.
Inside the cocoon, particles fall into the jet and collide with X-rays, producing neutrinos.
This model would also explain why the X-ray emission fell off so rapidly and why the neutrino
arrived 5 months after peak brightness; the cocoon had simply covered up the jet which
later produced the neutrino.
Now, if astronomers are good at anything, it's coming up with different hypotheses to
explain observations. However, hypotheses can be tested with more observations. Not
only can we observe phenomena at multiple wavelengths of light, but we can now observe
them via multiple messengers as well, such as the neutrino that led us back to a blazar,
or the gravitational waves from merging black holes.
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