From CfA: “CfA Scientists Weigh in on Historic Gravitational Wave Discovery” and the Press Release

Harvard Smithsonian Center for Astrophysics

Center For Astrophysics

October 16, 2017

Scientists have detected gravitational waves and electromagnetic radiation, or light, from the same event for the first time, as described in our latest press release [see below].

Thousands of scientists around the world have worked on this result, with researchers at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., playing a pivotal role. A series of eight papers led by CfA astronomers and their colleagues detail the complete story of the aftermath of this event and reveal clues about its origin.

We conducted interviews with four CfA scientists about their work on this discovery: Professor Edo Berger, who led the work, postdoctoral fellow Matt Nicholl, and graduate students Kate Alexander and Philip Cowperthwaite. Here they describe their reactions to the exciting news that Advanced LIGO had detected gravitational waves from a neutron star merger, and they discuss unanswered questions and prospects for future work.

How did you hear about LIGO’s detection of a neutron star merger and what were your first thoughts?

Kate Alexander:

I saw the e-mail from the LIGO collaboration when I woke up in the morning, and no one was expecting it because LIGO was a week away from shutting down from its current observing run. We all just kind of went “Wow. Oh my goodness! This is actually happening.” Edo called a meeting and we all rushed into his office to prepare our plans for following it up.

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

ESA/eLISA the future of gravitational wave research

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

Edo Berger:

So, we first got an alert from LIGO on the morning of August 17th. I was actually in the middle of a boring committee meeting. My office phone started ringing, and I ignored it. Then my cell phone started ringing and I ignored it. Then text messages started coming in. At that point I knew that I couldn’t ignore it anymore, so I kicked everybody out of the office and started catching up on this new alert that came from the LIGO observatory saying that they detected the first merger of a neutron star binary system.

Matt Nicholl:

As soon as we started observing the sky in Chile we were transferring these images back to computers at Harvard as soon as they came in and we all frantically brought them up on our computer screens and looked for new sources that appeared. Really what we expected was that we wouldn’t find anything in real time and that we’d spend the whole day next day processing these images trying to find some sort of faint little detections of possible candidates. But what actually happened was that one of the first giant galaxies we looked had an obvious new source popping right out at us. This was an incredible moment. I think one of my collaborators saw it first and sent an email that I can’t quite repeat but I will never forget. After that our email inboxes exploded. Every team in the world was looking at this thing and trying to compete to say things first. It was a night unlike any other I’ve had in my career.

Phil Cowperthwaite:

I actually heard about it through a very informal email from a colleague. I just woke up that morning and it was there on my phone: “Oh we have a binary neutron star in LIGO with a coincident Fermi detection. It’s insane. It took a moment to process – it didn’t seem real because that was the goal we never expected to happen.”

What are some unanswered questions and the prospects for future work?

Kate Alexander:

The VLA has been invaluable to the science so far.

NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

This is going to continue to be a very interesting target for radio observations going forward. The radio emission that we’re observing is likely to continue to be observable with the VLA for the next several weeks to months, and we’ll be very eager to monitor the radio emission we’ve seen as it slowly fades away. We also predict that several years from now the source should brighten in the radio again as all of the slower moving material that produced the optical light eventually starts producing a shock wave [akin to a sonic boom] with the surrounding medium. Then we’ll have a completely independent second chance to figure out all of these properties of the environment around the neutron star. We don’t know exactly when this will happen, but we certainly will continue to look at it with the VLA for years to come.

Edo Berger:

In studying both the gravitational wave signal and the electromagnetic signal what we hope to do is understand the detailed composition of neutron stars. What are they exactly made of? What do they look like on the inside? The only way we get to see the inside of a neutron star is when it collides with another neutron star and then material from the inside spills out. This is what we see in our observations. We also want to understand how pairs of neutron stars actually come into being. How are they actually formed? How are these systems born? How was their life before they ended it in that final catastrophic collision?

One of the particularly exciting aspects of studying the collisions of neutron stars in both gravitational waves and electromagnetic radiation is that it gives us a completely new way of measuring the Hubble Constant, which is the measurement of how fast the Universe is expanding. So far, we’ve been studying the Hubble Constant using different techniques: supernova explosions or the cosmic microwave background [leftover radiation from the Big Bang].

CMB per ESA/Planck


But here, for the first time, we have a completely independent new way of measuring the Hubble Constant. We can measure the distance to the object from the gravitational wave signal and we can then measure the amount of redshifting which tells us how fast the universe is expanding from the electromagnetic signal. And by combining these two measurements we can directly measure the Hubble Constant.

Matt Nicholl:

I think the big outstanding questions now are first of all how typical was this event of the general population of neutron star mergers? Maybe we got lucky and we found a very bright one. Maybe the others aren’t going to be so great. But we’ll find this out in the next few years as LIGO detects more and more of these sources. By detecting more sources we can also measure the rate at which they occur. The combination of those two things is very powerful. If we know how diverse they are and how often they occur we can work out the total production of heavy elements in the universe. If we compare this production of heavy elements to the abundances that we measure in our local environment we can show definitively whether all heavy elements come from neutron star mergers.

Phil Cowperthwaite:

You can do all kinds of science that you could not do with just a gravitational wave detection. The gravitational wave detection is great for telling you about the binary, the objects that merged and their properties, but it can’t do other things. For instance, LIGO can’t give you a precise location on the sky. It can do very well, especially with Virgo, but once you have an optical counterpart you know exactly where that event occurred. And then you can do all kinds of other exciting science. We can associate the source with a galaxy. We can learn about where these objects come from. What are their homes like? Understanding all this information will help us understand the behavior of the merger: how much material is produced, which is important for understanding whether or not these events can truly be the source of heavy element production. So, it really is necessary to maximize the science goals.

Press release:
Astronomers See Light Show Associated With Gravitational Waves
October 16, 2017
Megan Watzke
Harvard-Smithsonian Center for Astrophysics
+1 617-496-7998

Marking the beginning of a new era in astrophysics, scientists have detected gravitational waves and electromagnetic radiation, or light, from the same event for the first time. This historic discovery reveals the merger of two neutron stars, the dense cores of dead stars, and resolves the debate about how the heaviest elements such as platinum and gold were created in the Universe.

To achieve this remarkable result, thousands of scientists around the world have worked feverishly using data from telescopes on the ground and in space. Researchers at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., have played a pivotal role. A series of eight papers led by CfA astronomers and their colleagues detail the complete story of the aftermath of this event and examine clues about its origin.

“It’s hard to describe our sense of excitement and historical purpose over the past couple of months,” said the leader of the team, CfA’s Edo Berger. “This is a once in a career moment — we have fulfilled a dream of scientists that has existed for decades.”

Gravitational waves are ripples in space-time caused by the accelerated motion of massive celestial objects. They were first predicted by Einstein’s General Theory of Relativity. The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first direct detection of gravitational waves in September 2015, when the merger of two stellar-mass black holes was discovered.

On 8:41 am EDT August 17, 2017, LIGO detected a new gravitational wave source, dubbed GW170817 to mark its discovery date. Just two seconds later NASA’s Fermi satellite detected a weak pulse of gamma rays from the same location of the sky. Later that morning, LIGO scientists announced that two merging neutron stars produced the gravitational waves from GW170817.

“Imagine that gravitational waves are like thunder. We’ve heard this thunder before, but this is the first time we’ve also been able to see the lightning that goes with it,” said Philip Cowperthwaite of the CfA. “The difference is that in this cosmic thunderstorm, we hear the thunder first and then get the light show afterwards.”

A few hours after the announcement, as night set in Chile, Berger’s team used the powerful Dark Energy Camera on the Blanco telescope to search the region of sky from which the gravitational waves emanated. In less than an hour they located a new source of visible light in the galaxy NGC 4993 at a distance of about 130 million light years.

“One of the first giant galaxies we looked at had an obvious new source of light popping right out at us, and this was an incredible moment,” said Matt Nicholl of the CfA. “We thought it would take days to locate the source but this was like X marks the spot.”

The CfA team and collaborators then launched a series of observations that spanned the electromagnetic spectrum from X-rays to radio waves to study the aftermath of the neutron star merger.

In their set of papers, the CfA scientists report their studies of the brightness and spectrum of the optical and infrared light and how it changed over time. They show that the light is caused by the radioactive glow when heavy elements in the material ejected by the neutron star merger are produced in a process called a kilonova.

“We’ve shown that the heaviest elements in the periodic table, whose origin was shrouded in mystery until today are made in the mergers of neutron stars,” said Edo Berger. “Each merger can produce more than an Earth’s mass of precious metals like gold and platinum and many of the rare elements found in our cellphones.”

The material observed in the kilonova is moving at high speeds, suggesting that it was expelled during the head-on collision of two neutron stars. This information, independent of the gravitational wave signature, suggests that two neutron stars were involved in GW170817, rather than a black hole and a neutron star.

Radio observations with the Very Large Array in New Mexico helped confirm that the merger of the two neutron stars triggered a short gamma ray burst (GRB), a brief burst of gamma rays in a jet of high-energy particles. The properties match those predicted by theoretical models of a short GRB that was viewed with the jet initially pointing at a large angle away from Earth. Combining the radio data with observations from NASA’s Chandra X-ray Observatory shows that the jet pointed about 30 degrees away from us.

“This object looks far more like the theories than we had any right to expect,” said the CfA’s Kate Alexander who led the teams’ VLA observations. “We will continue to track the radio emission for years to come as the material ejected from the collision slams into the surrounding medium,” she continued.

An analysis of the host galaxy, NGC 4993, and the environment of the cataclysmic merger shows that the neutron star binary most likely formed more than 11 billion years ago.

“The two neutron stars formed in supernova explosions when the universe was only two billion years old, and have spent the rest of cosmic history getting closer and closer to each other until they finally smashed together,” said Peter Blanchard of the CfA.

A long list of observatories were used to study the kilonova, including the SOAR and Magellan telescopes, the Hubble Space Telescope, the Dark Energy Camera on the Blanco, and the Gemini-South telescope.

The series of eight papers describing these results appeared in the Astrophysical Journal Letters on October 16th. The four papers with first authors from CfA are led by Philip Cowperthwaite about the changes with time of light from the kilonova, one led by Matt Nicholl about the changes with time of the kilonova’s spectrum, another led by Kate Alexander about the VLA observations, and another led by Peter Blanchard about how long the merger took to unfold and the properties of the host galaxy.

Completing the series of eight papers, Marcelle Soares-Santos from Brandeis University in Waltham MA led a paper about the discovery of the optical counterpart; Ryan Chornock from Ohio University in Athens, OH, led a paper about the kilonova’s infrared spectra, Raffaella Margutti from Northwestern University in Evanston, IL, led a paper about the Chandra observations of the jet, and Wen-fai Fong also from Northwestern led a paper about the comparison between GW170817 and previous short GRBs.

Graphics and other additional information on this result can be found at

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

See the full main article here .
See the press release here .

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