Media Tip Sheets

Today, the LIGO Livingston detector and VIRGO detector captured another binary neutron star merger

Stefan Ballmer is an associate professor of physics at Syracuse University’s College of Arts and Sciences. Below, he answers four key questions about the LIGO/VIRGO detection, and what it means for the greater world of physics.

Q: What is most significant about these findings?

Ballmer: “What we are really looking forward to is to get a stronger signal from the collision phase of the two neutron stars. That will tell us how nuclear matter behaves under these extreme conditions.

“This event was a little too far away for that. But it gives us a much better handle on the rate of such collisions. The upshot: if we just observe a little longer we will get the strong signal we are hoping for.”

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Q: Can you break down what a binary neutron star merger is?

Ballmer: “A neutron star is an atomic nucleus the size of a city, but with the mass of a sun. We are observing the collision of two of these monsters at about half the speed of light.

“Since neutron stars still are made of matter (unlike black holes), we do expect to be able to see them optically as well, as was the case with GW170817, the first binary neutron star merger observed by LIGO.”

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Q: For someone not familiar with the physics world, why is this exciting?

Ballmer: “On a weekly basis we are now observing some of the most violent events in the universe, literally storms in space and time, shredding the remnants of stars. Black holes and neutron stars are no longer exotic hypothetical objects they once were, but the bread and butter of everyday science.

“This is also why the LIGO and Virgo collaborations now put out alerts as quickly as possible, in the hope that other astronomers can make complementary observations.

“In fact, you can download an app that alerts you within minutes whenever we see something interesting.” Link: https://itunes.apple.com/us/app/gravitational-wave-events/id1441897107

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Q: How does this tie in or connect with (if at all) with the first black hole image that was released just a short while ago?

Ballmer: “Since the latest run (O3) started we have observed 3 black hole mergers, and this neutron star merger. That brings the total observed in all runs to 13 black hole mergers and 2 neutron star mergers.

“It is amazing that what was considered to be impossible just three and a half years ago – and was worth the 2017 Nobel prize – is now happening weekly.”

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Syracuse University</s>In February of 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the first-ever detection of gravitational waves (GWs). Since then, multiple events have been detected, providing insight into a cosmic phenomena that was predicted over a century ago by Einstein’s Theory of General Relativity.

A little over a year ago, LIGO was taken offline so that upgrades could be made to its instruments, which would allow for detections to take place “weekly or even more often.” After completing the upgrades on April 1st, the observatory went back online and performed as expected, detecting two probable gravitational wave events in the space of two weeks.

LIGO announced the first of the two new GW events on April 8th, which was followed by a second announcement on April 12th. The signals were detected thanks to the three-facility collaboration between LIGO and the Virgo Observatory in Italy, and both are believed to have been the result of a pair of black holes merging.

Thanks to upgrades made to both LIGO and Virgo, this scientific collaboration has been able to increase the sensitivity of its instruments by about 40%. For their third observation run (dubbed O3), the astronomical community also benefited from a new public alert system, where the LIGO team sends out alerts the moment detections are made so that observatories around the world can point their telescopes at the source.

By observing the source in different wavelengths (optical, X-ray, ultraviolet, radio, etc.), scientists hope to learn more about what causes GW events and about the dynamics behind them. For these latest detections, a team of scientists from Penn State University – led by Chad Hanna, an associate professor of physics, astronomy and astrophysics – played a vital role.

As Cody Messick, a graduate student in physics at Penn State and member of the LIGO team, explained:

“Penn State is part of a small team of LIGO scientists that analyze the data in almost real-time. We are constantly comparing the data to hundreds of thousands of different possible gravitational waves and upload any significant candidates to a database as soon as possible. Although there are several different teams all performing similar analyses, the analysis ran by the Penn State team uploaded the candidates that were made public for both of these detections.”

Over the past nine months, Messick has been responsible for ensuring that newly-uploaded GW candidates contain information from all detectors running at the time of detection. This helps astronomers localize signals by narrowing down the predicted area of the sky that the signal is predicted to have come from.

LIGO public alerts also include a sky-map that shows the possible location of the source in the sky, the time of the event, and what kind of event it is believed to be. LIGO has also said that in the future, announcements of candidate events will be followed by more detailed information once they have had a chance to properly vet and study them.

As Ryan Magee, a graduate student in physics at Penn State and member of the LIGO team, put it:

“These are near real-time detections of gravitational waves produced from two probable black holes colliding. We detected the first signal within about 20 seconds of its arrival to earth. We can set up automatic alerts to get phone calls and texts when a significant candidate is identified. I thought I was getting a spam phone call at first!”

So far, astronomers have deduced that GW events can be the result of binary black hole mergers, a merger between a black hole and a neutron star, or a binary neutron star merger. Each of these events produce gravitational waves with very different signals, which allows astronomers to determine the cause.

In this case, the events are believed to be the result of binary black hole mergers, which will be tested with follow-up observations in the coming weeks and months. Surabhi Sachdev, a Eberly Postdoctoral Research Fellow in physics at Penn State and member of the LIGO team, explained the importance of these latest events:

“This is the first LIGO observation that was made public right away in an automated fashion. This is the new LIGO policy starting with this observing run. Events are instantly made public automatically. After human vetting, a confirmation or retraction is issued within hours.”

With the increased sensitivity of their detectors, the LIGO team hopes to not only make more detections, but detect a greater variety of signals. So far, events have been detected that were the result of mergers between two black holes or neutron stars. It is hoped that in the near future, the team might detect a signal produced by the merger of a black hole and a neutron star.

Whatever form the next events take, you can expect that we will be hearing about it! The public can keep track of public alerts at https://gracedb.ligo.org/latest/, or you can download the alert app at Gravitational Wave Events iPhone App.</s></s>
LIGO detects a second neutron star merger using its gravitational wave signal after the first such detection in August 2017.