February 29, 2020 0

Kilonova! Gravitational Waves AND Radiation Detected From a Neutron Star Collision

Kilonova! Gravitational Waves AND Radiation Detected From a Neutron Star Collision


Astronomers have made the groundbreaking detection
of two neutron stars colliding in a galaxy 130 million light years away. They detected
the gravitational waves AND the blast of radiation released from the collision, observing a kilonova
for the first time. I know we’ve been talking about gravitational
waves quite a bit here on the Guide to Space, and a few episodes ago, I hinted at the tantalizing
possibility that gravitational waves could, maybe, perhaps, be used to detect colliding
neutron stars. You know, once more sensitive instruments
came online, and physicists tweaked and tuned them. Well, it turns out such a detection
had already been made when I recorded that, now, hilariously out of date video. But it gets even better. At the same time
they detected the rippling distortion of spacetime produced by the gravitational waves, different
teams of astronomers were actually able to watch blast of radiation released by the neutron
star using every wavelength at their disposal. They detected the gravitational waves and
then saw the collision in the sky. I promise you, this is one of the biggest discoveries
of the year. So, let’s dig into the news. Late last week, the European Southern Observatory
teased us that they were going to announce something on Monday, October 16th. We didn’t
know what it was, but they called it “unprecedented”. Okay, that got my attention. On Monday we all learned what it was, and
the reality lived up to the hype. A 8:41 am EDT on the morning of August 17,
2017, researchers from the Laser Interferometer Gravitational Wave Observatory detected the
ripple of gravitational waves passing through the Earth. Unlike the previous colliding black hole observations
which lasted a couple of seconds, these ripples went on for 100 seconds, indicating that this
event wasn’t from black holes. Instead, it was tied to the collision of two neutron
stars; the remnants of massive stars with 8-20 times the mass of the Sun that went supernova. Two seconds later, NASA’s Fermi space telescope
detected a brief blast of gamma radiation, with the signature of a short gamma ray burst. The long gamma ray bursts are fairly well
understood, tied to supernova explosions. But the short bursts have long been a mystery,
although astronomers theorized that they had something to do with colliding neutron stars. In addition to LIGO, the VIRGO Interferometer
in Italy also detected the gravitational waves passing through the planet. With three separate
observations, the researchers were able to determine the rough direction that the neutron
star collision happened: an area in the constellation of Hydra covering several hundred full Moons
worth of area, with millions of stars. They encouraged the worldwide astronomy community
to go searching the region for the afterglow of the collision. From space, there was the
European INTEGRAL satellite as well as NASA’s Swift, Hubble, Chandra and Spitzer Observatories.
Ground-based observatories across the hemisphere scanned the region, searching for a visual
confirmation. The European Southern Observatory’s 1-meter
Swope Telescope in Cerro Las Campanas in Chile was the first to find a new point of light,
located in the galaxy NGC 4993. Then other observatories followed up, viewing
the spot across the entire electromagnetic spectrum, from radio waves through infrared
to ultraviolet, as well as the space-based observations in gamma rays. All told, 70 different
observatories were trained on the unfolding event using 100 different instruments. Until this point, LIGO had only detected merging
black holes. And the problem with black holes, of course, is that they don’t give off any
radiation when they collide. Nothing escapes them. But neutron stars aren’t massive and dense
enough, so radiation can escape them, so there was an opportunity to find the explosion with
the rest of the world’s observatories. These two neutron stars had already experienced
their share of catastrophe. In a previous lives, 11-12 billion years ago, they were
both hot massive stars that died as supernovae, one then the other. These neutron star remnants
orbited around one another ever since, getting closer and closer for billions of years. One neutron star was 10% more massive than
the Sun, and the other had 60% more mass than the Sun. But they were only 10 km across – the
size of cities. In their final moments, they were whirling
around one another, hundreds of times a second traveling at a third the speed of light, rippling
the fabric of spacetime with gravitational waves, until the their intense gravity couldn’t
withstand the forces. They broke apart, merged together and detonated, releasing the gamma
ray burst, seen 130 million light-years away. Colliding neutron stars detected by gravitational
waves and confirmed by other telescopes? That’s amazing, but it gets even better. And I’ll
get into that in a second, but first I’d like to thank: Andrew Ladzinski
Patricia Pipkin Joseph Salomone And the rest of our 793 patrons for their
generous support. If you love what we’re doing and want to get in on the action, head
over to patreon.com/universetoday. From previous observations, astronomers already
knew that NGC 4993 was approximately 130 million light-years away, confirming the predictions
made by the gravitational wave astronomers. This is amazingly close. The previous merging
black hole observations were billions of light-years away. The world’s observatories watched the explosion
brighten and then fade away within a few days. Astronomers realized that they were looking
at a very special kind of short gamma ray burst, one which had never been observed before:
a kilonova. You’ve probably heard that all the heavier
elements in the Universe, were formed in the hearts of supernovae explosions, when the
force of the collapsing layers compresses lighter elements into heavier ones. But the models of supernova-generated heavy
elements didn’t seem to match reality. There had to be another mechanism at work, and one
idea was colliding neutron stars. Only these events could generate and distribute gold,
platinum and other heavy elements into the Universe. In the cosmic wreckage blasting out of the
collision, astronomers detected the source of these heavier elements for the first time. The gold in your wedding ring, the precious
metals in your cell phone. They don’t come from supernovae, they come from colliding
neutron stars. In a fraction of a second, an amount of gold
equal to the mass of the Moon was created. In fact, just one of these events generates
the mass of the Earth in precious elements. So that’s where it all comes from. This was a very big day in the field of astronomy.
Astronomers were incredibly lucky to have two neutron stars collide so close to us.
I know, 130 million light-years doesn’t sound close, but it was. Thanks to the three separate detections, they
were able to determine its rough location to direct astronomers around the world. And by following up, they observed a kilonova
for the first time, watching how these unique collisions spread heavier elements into the
Universe. We now have confirmation that our precious metals are formed in events like
this. Since the gravitational waves and radiation
arrived here on Earth at the same time, it confirms Einstein’s prediction that gravity
moves at the speed of light. By seeing the same event in both gravity and
radiation, astronomers have a new way to confirm the speed of the expansion of the Universe. As a quick side note, one cool part about
this discovery is the massive collaboration that happened across countries, universities,
observatories and research agencies. In the end, 4,500 separate researchers were co-authors
on the paper announcing the discovery. Ask for Comments:
Well, what do you think about this discovery? Are there any aspects of gravitational waves,
gamma ray bursts or colliding neutron stars that you’d like me to dig into any further?
Let me know your thoughts in the comments. Of course, we’ll need a playlist, all about
this amazing discovery and the related science. First, let’s start with the original announcement
from the European Southern Observatory, and the announcement from NASA. Here’s a video
from Veritasium on the event. And of course, Scott Manley is all over this. Finally, and
overall explainer on gamma ray bursts from Crash Course Astronomy.

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