6 Brilliant Facts About LIGO and the Neutron Star Collision

“Wake up!” A simple shout out from one astronomer to another kicked off a fast-paced international collaboration which led to one of the most important leaps forward in our understanding of our universe in recent history.

Sabrina Stierwalt, PhD,
Episode #257

ligo discovery of colliding neutron stars and gravitational waves

In February of 2016, scientists made the historic announcement that they had observed the universe in an entirely new way with the first ever detection of gravitational waves—ripples in space produced by the catastrophic collision of two merging black holes.

What is LIGO?

Since then, the same telescope known as LIGO (short for Laser Interferometer Gravitational wave Observatory) has detected three additional black hole merger events which have confirmed major predictions from Einstein’s theory of general relativity. This detection was so important to our understanding of the universe that LIGO scientists Barry Barish, Kip Thorne, and Rainer Weiss earned the 2017 Nobel Prize in Physics earlier this month.

This week the scientists at LIGO, joined by astronomers from 70 other telescopes all around the world, are back again with big news. They have detected for the first time gravitational waves produced by the final collision of two neutron stars and observed the event across the electromagnetic spectrum kicking off the era of not just multi-wavelength but multi-messenger astronomy.

This news is so big that astronomers have been rumbling about it for months ever since the detection was made in August. It turns out, astronomers are terrible at keeping secrets. A few scientists leaked the information on Twitter, but, even more obvious was the fact that suddenly almost all of the world’s largest and most technologically advanced telescopes stared simultaneously at an otherwise normal-seeming galaxy about 130 million lightyears away. Many large observatories like Hubble, Green Bank, and Fermi post publicly where they are currently observing. There was so much off-the-record information flying around that the journal Nature published an article summarizing the rumors all the way back in August.

Here are six reasons astronomers are so excited about this new discovery and why you should be too.

1. The merger of two neutron stars has never before been observed.

On August 17th, 2017, the LIGO telescope detected a clear signal but one that was significantly longer—on the order of a minute—than the signals from merging black holes that last only a few seconds. LIGO scientists are constantly scanning their incoming signals and looking for matches in a library of hundreds of thousands of templates. In other words, the match-making software looks to see whether the detected signal matches what simulations predict we should observe for any of a variety of different kinds of merger events. Unlike all of LIGO’s previous detections which have all been signals from distant mergers of black holes, the August 17th signal the first ever detection of the final collision between two neutron stars with masses of 1.1 and 1.6 times the mass of our Sun at a distance of 130 million lightyears away off in the southern hemisphere.

Neutron stars are incredibly dense, dead stars produced by the supernova explosions that occur during stellar death. They are made up purely of neutrons and are three times denser than atomic nuclei. Neutron stars typically pack between one to two times the mass of our Sun into a space that’s 20-25 kilometers in diameter, or only about the length of Manhattan.

Models suggest the first neutron star was living a relatively quiet existence until the second neutron star arrived to form a binary system which then sent the pair careening throughout their host galaxy. As neutron stars orbit each other, general relativity predicts that the system will lose energy over time by radiating gravitational waves. This signal is exactly what LIGO detected.

The inevitable collision is then among the most violent and powerful events in the universe. But more on that later…

2. The event almost came too soon to find the host galaxy.

Even with its two detectors (in Louisiana and Washington in the US), LIGO can only pinpoint the source of one of its detections to within an area of 1,000 square degrees on the sky—a region far too large to be able to effectively narrow down the source of the signal.

However, just in the nick of time, LIGO’s sister observatory Virgo located in Cascina, Italy had turned back on earlier that month after a long break for an upgrade. When working together, LIGO and Virgo can localize mutual detections to within 30 square degrees.

Scientists were able to narrow down the source of the detection even further when they noticed a Fermi alert had been triggered at almost the exact same time as the LIGO and Virgo detections. A realization that led to the “Wake up!” message that then triggered a period of swift action and intense discovery. Julie McEnery, the Fermi Project Scientist called that first morning “the most exciting morning of the 9-year Fermi mission.”

The Fermi Gamma-Ray Space Telescope scans the sky for gamma rays, the highest energy form of light and triggers an alert to let scientists know when it has found something. INTEGRAL, the European gamma ray observatory, later also confirmed a detection of what is known as a short duration Gamma Ray Burst. Using the LIGO, Virgo, and Fermi observations together, scientists were able to narrow down the source of the neutron star collision to roughly 50 potential host galaxies.


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